Class 10th Science Chapter 10 – The Human Eye and the Colourful World | Formation of Rainbow

The Human Eye and the Colourful World

Formation of Rainbow | इन्द्रधनुष का निर्माण

Class 10 Science | CBSE + Foundation + Competitive

🌈 Formation of Rainbow | इन्द्रधनुष का निर्माण

1. Introduction | परिचय

A rainbow is a natural optical phenomenon in which sunlight appears as a multicoloured arc in the sky after or during rainfall when sunlight interacts with water droplets.

इन्द्रधनुष एक प्राकृतिक प्रकाशीय घटना है जिसमें वर्षा के बाद या वर्षा के दौरान सूर्य का प्रकाश जल की बूंदों से परस्पर क्रिया करके आकाश में बहुरंगी चाप के रूप में दिखाई देता है।

The formation of a primary rainbow mainly involves three important optical processes:

1. Refraction – अपवर्तन

2. Dispersion – विक्षेपण

3. Internal Reflection – आंतरिक परावर्तन

2. Principle of Rainbow Formation | इन्द्रधनुष बनने का सिद्धांत

When sunlight enters a spherical water droplet, it is refracted and dispersed into different colours. The separated light undergoes internal reflection inside the droplet and is refracted again when it emerges.

जब सूर्य का प्रकाश गोलाकार जल की बूंद में प्रवेश करता है, तो उसका अपवर्तन और विक्षेपण होता है। अलग हुए रंग बूंद के अंदर आंतरिक परावर्तन करते हैं और बाहर निकलते समय पुनः अपवर्तित होते हैं।

Thus, the formation of a primary rainbow can be summarized as:

Refraction → Dispersion → Internal Reflection → Refraction

3. Formation of Rainbow in a Water Droplet | जल की बूंद में इन्द्रधनुष

4. Step-by-Step Formation | चरणबद्ध निर्माण

Step 1 – Refraction | चरण 1 – अपवर्तन

When sunlight enters a water droplet from air, it changes its speed and bends due to refraction.

जब सूर्य का प्रकाश वायु से जल की बूंद में प्रवेश करता है, तो उसकी चाल बदलती है और वह अपवर्तित होता है।

Step 2 – Dispersion | चरण 2 – विक्षेपण

White sunlight contains different wavelengths. Water has slightly different refractive indices for different wavelengths, so the colours are deviated by different amounts.

श्वेत सूर्यप्रकाश में विभिन्न तरंगदैर्ध्य के रंग होते हैं। जल का अपवर्तनांक अलग-अलग तरंगदैर्ध्य के लिए अलग होता है, इसलिए रंग अलग-अलग मात्रा में विचलित होते हैं।

Step 3 – Internal Reflection | चरण 3 – आंतरिक परावर्तन

The dispersed light reaches the opposite inner surface of the water droplet and undergoes internal reflection.

विक्षेपित प्रकाश बूंद की दूसरी आंतरिक सतह तक पहुँचता है और वहाँ आंतरिक परावर्तन करता है।

Step 4 – Refraction Again | चरण 4 – पुनः अपवर्तन

The reflected light emerges from the droplet and is refracted again. Different colours emerge in different directions.

परावर्तित प्रकाश बूंद से बाहर निकलते समय पुनः अपवर्तित होता है। अलग-अलग रंग अलग दिशाओं में निकलते हैं।

5. Primary Rainbow | प्राथमिक इन्द्रधनुष

A primary rainbow is formed mainly after one internal reflection inside a water droplet.

प्राथमिक इन्द्रधनुष जल की बूंद के अंदर मुख्यतः एक आंतरिक परावर्तन के बाद बनता है।

Primary Rainbow: One internal reflection.

प्राथमिक इन्द्रधनुष: एक आंतरिक परावर्तन।

In a primary rainbow, red is generally seen on the outer side and violet on the inner side of the arc.

प्राथमिक इन्द्रधनुष में सामान्यतः लाल रंग बाहरी भाग में और बैंगनी रंग आंतरिक भाग में दिखाई देता है।

6. Secondary Rainbow | द्वितीयक इन्द्रधनुष

A secondary rainbow is formed when light undergoes two internal reflections inside a water droplet before emerging.

द्वितीयक इन्द्रधनुष तब बनता है जब प्रकाश जल की बूंद के अंदर दो आंतरिक परावर्तन करता है।

Its colour order is reversed compared with the primary rainbow.

इसका रंग क्रम प्राथमिक इन्द्रधनुष की तुलना में उल्टा होता है।

Primary Rainbow	Secondary Rainbow
One internal reflection	Two internal reflections
Brighter	Fainter
Red outer, violet inner	Violet outer, red inner
Smaller angular radius	Larger angular radius

7. Position of Rainbow | इन्द्रधनुष की स्थिति

A rainbow is usually seen when the observer has the Sun behind them and water droplets are present in the opposite part of the sky.

इन्द्रधनुष सामान्यतः तब दिखाई देता है जब सूर्य पर्यवेक्षक के पीछे हो और सामने के आकाश में जल की बूंदें मौजूद हों।

Memory Trick:

🌞 Sun Behind → Rainbow Ahead

सूर्य पीछे → इन्द्रधनुष सामने

8. Angular Position of Primary Rainbow | प्राथमिक इन्द्रधनुष का कोण

For a primary rainbow, the emerging rays reach the observer from a direction roughly 40°–42° from the antisolar direction, depending on colour and the observation conditions.

स्कूल स्तर पर प्राथमिक इन्द्रधनुष को सूर्य के विपरीत दिशा से लगभग 40°–42° के कोणीय क्षेत्र में देखा जाता है।

The approximate angular radius often quoted for the primary rainbow is about 42° for red light, with violet at a somewhat smaller angle.

9. Why Does a Rainbow Appear as an Arc? | इन्द्रधनुष चाप के रूप में क्यों?

A rainbow is formed by light emerging from many water droplets at suitable angles. From the observer's position, droplets satisfying the required geometry lie approximately on a cone around the line opposite the Sun. The intersection of this cone with the sky appears as an arc.

पर्यवेक्षक की स्थिति से उपयुक्त कोण पर प्रकाश भेजने वाली अनेक जल बूंदें एक शंकु जैसी ज्यामितीय व्यवस्था बनाती हैं। आकाश में इस व्यवस्था का भाग हमें चाप के रूप में दिखाई देता है।

10. Can a Rainbow Be a Complete Circle? | क्या इन्द्रधनुष पूरा वृत्त हो सकता है?

Yes. From an elevated position such as an aircraft, under suitable conditions, a rainbow may sometimes appear as a nearly complete circle. From the ground, the lower part is usually hidden below the horizon.

हाँ। विमान जैसी ऊँची स्थिति से उचित परिस्थितियों में इन्द्रधनुष लगभग पूर्ण वृत्त के रूप में दिखाई दे सकता है। भूमि से देखने पर इसका निचला भाग सामान्यतः क्षितिज के नीचे छिपा रहता है।

11. Colours of Rainbow | इन्द्रधनुष के रंग

Order	Colour	Relative Wavelength
1	Violet	Shortest
2	Indigo	Short
3	Blue	Short–medium
4	Green	Medium
5	Yellow	Medium–long
6	Orange	Long
7	Red	Longest

12. Role of Dispersion | विक्षेपण की भूमिका

Dispersion is essential for the formation of the colours of a rainbow. Without wavelength-dependent refraction, the colours would not separate in the same way.

इन्द्रधनुष में रंगों के अलग-अलग दिखाई देने के लिए विक्षेपण महत्वपूर्ण है। अलग-अलग तरंगदैर्ध्य के प्रकाश का अपवर्तन अलग-अलग मात्रा में होने के कारण रंग अलग होते हैं।

13. Role of Refraction | अपवर्तन की भूमिका

Refraction occurs when sunlight enters and leaves a water droplet. At these boundaries, the speed and direction of light change.

जल की बूंद में प्रकाश के प्रवेश और बाहर निकलने के समय अपवर्तन होता है। इस दौरान प्रकाश की चाल और दिशा बदलती है।

14. Role of Internal Reflection | आंतरिक परावर्तन की भूमिका

After entering the droplet and being dispersed, the light reaches the opposite surface and is internally reflected back towards the observer side.

विक्षेपण के बाद प्रकाश बूंद की दूसरी सतह तक पहुँचकर आंतरिक रूप से परावर्तित होता है और पर्यवेक्षक की दिशा में लौटता है।

15. Primary vs Secondary Rainbow | प्राथमिक और द्वितीयक इन्द्रधनुष

Feature	Primary Rainbow	Secondary Rainbow
Internal reflections	One	Two
Brightness	Brighter	Fainter
Colour order	Red outer, violet inner	Violet outer, red inner
Approximate radius	~42° for red	~51°–53°
Location	Closer to antisolar point	Outside primary rainbow

16. Double Rainbow | दोहरा इन्द्रधनुष

Sometimes both primary and secondary rainbows can be observed at the same time. The secondary rainbow appears outside the primary rainbow and is usually fainter.

कभी-कभी प्राथमिक और द्वितीयक दोनों इन्द्रधनुष एक साथ दिखाई दे सकते हैं। द्वितीयक इन्द्रधनुष प्राथमिक इन्द्रधनुष के बाहर और सामान्यतः अधिक हल्का दिखाई देता है।

17. Alexander's Dark Band | अलेक्जेंडर की डार्क बैंड

When both primary and secondary rainbows are visible, a relatively dark region may appear between them. This is called Alexander's dark band.

जब प्राथमिक और द्वितीयक इन्द्रधनुष दोनों दिखाई देते हैं, तो उनके बीच का क्षेत्र अपेक्षाकृत गहरा दिखाई दे सकता है। इसे Alexander's dark band कहा जाता है।

18. Why Is the Sun Behind the Observer? | सूर्य पीछे क्यों होता है?

For a primary rainbow, sunlight must enter droplets in front of the observer, undergo the required refraction, dispersion and internal reflection, and then return toward the observer. Therefore the Sun is on the opposite side of the sky.

प्राथमिक इन्द्रधनुष के लिए सूर्य का प्रकाश पर्यवेक्षक के सामने स्थित जल की बूंदों पर पड़ना चाहिए और परावर्तन के बाद पर्यवेक्षक की ओर लौटना चाहिए। इसलिए सूर्य पर्यवेक्षक के पीछे होता है।

19. Simple Demonstration of Rainbow | सरल प्रदर्शन

A rainbow-like spectrum can be demonstrated using a transparent glass prism or fine water spray under sunlight.

1. Stand with the Sun behind you.
2. Use a fine spray of water in front of you.
3. Observe the region opposite the Sun.
4. Under suitable conditions, coloured light may be visible.

यह प्रदर्शन केवल उपयुक्त प्रकाश, बूंदों और viewing geometry में स्पष्ट दिखाई देता है।

20. Important Relations | महत्वपूर्ण संबंध

v = νλ

μ = c/v

v = speed of light in the medium

ν = frequency

λ = wavelength

μ = refractive index

c = speed of light in vacuum

Exam Point:

Frequency of light does not change when light passes from one medium to another, whereas its speed and wavelength change.

21. 30 MCQs | बहुविकल्पीय प्रश्न

Q1. A rainbow is a:

(A) Natural optical phenomenon

(B) Chemical reaction

(C) Mechanical phenomenon

(D) Electrical phenomenon

Answer: A

Q2. Which three processes are mainly involved in primary rainbow formation?

(A) Reflection, absorption and diffraction

(B) Refraction, dispersion and internal reflection

(C) Diffraction, scattering and absorption

(D) Interference, reflection and absorption

Answer: B

Q3. The splitting of sunlight into colours inside a water droplet is called:

(A) Reflection

(B) Refraction

(C) Dispersion

(D) Absorption

Answer: C

Q4. In a primary rainbow, light undergoes:

(A) No internal reflection

(B) Three internal reflections

(C) Two internal reflections

(D) One internal reflection

Answer: D

Q5. In a primary rainbow, the outer colour is generally:

(A) Red

(B) Violet

(C) Blue

(D) Green

Answer: A

Q6. In a primary rainbow, violet is generally seen:

(A) Outside red

(B) On the inner side

(C) Above the Sun

(D) Nowhere

Answer: B

Q7. A secondary rainbow involves:

(A) No reflection

(B) One reflection

(C) Two internal reflections

(D) Only scattering

Answer: C

Q8. A secondary rainbow is generally:

(A) Brighter than primary

(B) Equal in brightness

(C) Invisible always

(D) Fainter than primary

Answer: D

Q9. For seeing a primary rainbow, the Sun is generally:

(A) Behind the observer

(B) Directly overhead only

(C) In front of the observer

(D) Below the ground

Answer: A

Q10. The approximate angular radius of the primary rainbow for red light is:

(A) 20°

(B) About 42°

(C) 90°

(D) 5°

Answer: B

Q11. The rainbow is formed by:

(A) Dust particles only

(B) Smoke only

(C) Water droplets

(D) Clouds only

Answer: C

Q12. Which colour has the longest wavelength?

(A) Violet

(B) Blue

(C) Green

(D) Red

Answer: D

Q13. Which colour has the shortest wavelength?

(A) Violet

(B) Red

(C) Yellow

(D) Orange

Answer: A

Q14. The colour order of the primary rainbow from outer to inner is:

(A) VIBGYOR

(B) ROYGBIV

(C) GYORVIB

(D) BIVGYOR

Answer: B

Q15. The secondary rainbow has colour order:

(A) Same as primary

(B) Random

(C) Reversed compared with primary

(D) Only red and violet

Answer: C

Q16. Which phenomenon causes separation of colours?

(A) Reflection

(B) Absorption

(C) Heating

(D) Dispersion

Answer: D

Q17. Internal reflection occurs inside:

(A) Water droplets

(B) Sun

(C) Air only

(D) Clouds only

Answer: A

Q18. The dark region between primary and secondary rainbows is called:

(A) Newton band

(B) Alexander's dark band

(C) Raman band

(D) Prism band

Answer: B

Q19. A primary rainbow is usually:

(A) Outside the secondary

(B) Below the ground

(C) Inside the secondary rainbow

(D) Above the Sun

Answer: C

Q20. The source of light responsible for a natural rainbow is:

(A) Moon only

(B) Stars

(C) Lightning

(D) Sun

Answer: D

Q21. A rainbow may appear after rainfall because the atmosphere contains:

(A) Water droplets

(B) Only dust

(C) Smoke

(D) Salt crystals only

Answer: A

Q22. Which process occurs when sunlight first enters a raindrop?

(A) Absorption only

(B) Refraction

(C) Reflection only

(D) Diffraction

Answer: B

Q23. The colours of rainbow arise because:

(A) Water is coloured

(B) Air is coloured

(C) Different wavelengths are refracted differently

(D) Sun emits only one colour

Answer: C

Q24. The secondary rainbow is located:

(A) Inside the primary

(B) At the Sun

(C) Underground

(D) Outside the primary

Answer: D

Q25. Which colour generally deviates least in water?

(A) Red

(B) Violet

(C) Blue

(D) Green

Answer: A

Q26. Which colour generally deviates most?

(A) Red

(B) Violet

(C) Yellow

(D) Orange

Answer: B

Q27. A complete circular rainbow is more likely to be observed:

(A) Underground

(B) Inside a closed room

(C) From an elevated position under suitable conditions

(D) At midnight always

Answer: C

Q28. The approximate angular radius of the secondary rainbow is:

(A) 10°

(B) 25°

(C) 35°

(D) About 51°–53°

Answer: D

Q29. The rainbow is an example of:

(A) Dispersion and reflection-related optical phenomena

(B) Chemical combustion

(C) Nuclear reaction

(D) Sound interference only

Answer: A

Q30. Which sequence best represents primary rainbow formation?

(A) Reflection → absorption → heating

(B) Refraction → dispersion → internal reflection → refraction

(C) Scattering → diffraction → absorption

(D) Heating → reflection → evaporation

Answer: B

22. 30 Subjective Questions with Answers | वर्णनात्मक प्रश्न

2 Marks Q1. What is a rainbow?

A rainbow is a natural optical phenomenon in which sunlight appears as a coloured arc due to its interaction with water droplets.

2 Marks Q2. Name the main phenomena involved in rainbow formation.

Refraction, dispersion and internal reflection are the main processes involved in the formation of a primary rainbow.

2 Marks Q3. What is a primary rainbow?

A primary rainbow is formed mainly after one internal reflection of light inside a water droplet.

2 Marks Q4. Why is a rainbow usually seen opposite the Sun?

Because the sunlight must enter water droplets in front of the observer, undergo internal reflection and emerge towards the observer.

2 Marks Q5. Which colour is seen on the outer side of a primary rainbow?

Red is generally seen on the outer side of a primary rainbow.

3 Marks Q6. Explain the role of refraction in rainbow formation.

Light is refracted when it enters and leaves a water droplet. This changes its direction and contributes to the angular separation required for the rainbow.

3 Marks Q7. What is the role of dispersion?

Different colours have different wavelengths and experience different refractive indices in water. Hence they are deviated by different amounts and become separated.

3 Marks Q8. What is internal reflection?

Internal reflection is the reflection of light back into a medium when light travelling inside the medium reaches its boundary under conditions suitable for internal reflection.

3 Marks Q9. Why does the Sun need to be behind the observer?

The sunlight must fall on water droplets in front of the observer and return towards the observer after internal reflection.

3 Marks Q10. Write the colour sequence of a primary rainbow.

From outer to inner: Red, Orange, Yellow, Green, Blue, Indigo and Violet.

4 Marks Q11. Explain the formation of a primary rainbow.

Sunlight enters a water droplet and is refracted and dispersed. The separated colours undergo one internal reflection and then emerge after another refraction. The observer sees the separated colours as a rainbow.

4 Marks Q12. Why is violet inside and red outside in a primary rainbow?

Different colours have different refractive indices and therefore different deviations in water. The resulting geometry places red at the outer edge and violet at the inner edge of the primary arc.

4 Marks Q13. What is the difference between primary and secondary rainbow?

A primary rainbow involves one internal reflection and is brighter, with red outside. A secondary rainbow involves two internal reflections, is fainter, and has reversed colour order.

4 Marks Q14. Why is the secondary rainbow fainter?

The light undergoes an additional internal reflection, so less light reaches the observer compared with the primary rainbow.

4 Marks Q15. What is Alexander's dark band?

It is the relatively dark region between the primary and secondary rainbows when both are visible.

5 Marks Q16. Describe rainbow formation step by step.

1. Sunlight enters the water droplet and is refracted. 2. White light is dispersed into colours. 3. The colours travel inside the droplet. 4. The light undergoes internal reflection. 5. The reflected colours leave the droplet after another refraction. 6. The observer sees the coloured arc.

5 Marks Q17. Explain why a rainbow has seven commonly identified colours.

White sunlight contains a continuous range of visible wavelengths. Due to dispersion, different wavelengths are separated. These colours are commonly grouped and named as VIBGYOR in school-level description.

5 Marks Q18. Explain the role of water droplets.

Water droplets act like tiny optical systems. They refract sunlight, disperse its colours, internally reflect the separated light and refract it again on emergence.

5 Marks Q19. Explain why rainbow is not visible whenever it rains.

Rain alone is not sufficient. Suitable positions of the Sun, observer and water droplets are required. Usually the Sun should be behind the observer and the droplets should be in the opposite part of the sky.

5 Marks Q20. Explain the approximate angular position of a primary rainbow.

The primary rainbow is seen at an angular radius of roughly 40°–42° from the antisolar direction. The commonly quoted value is about 42° for red light, with violet appearing at a somewhat smaller angle.

6 Marks Q21. Explain the complete formation of a primary rainbow with a labelled diagram.

Sunlight enters a water droplet and undergoes refraction and dispersion. The separated colours reach the opposite surface and undergo one internal reflection. They then leave the droplet after another refraction. Due to different deviations of different colours, red appears on the outer side and violet on the inner side of the primary rainbow.

6 Marks Q22. Explain the formation of a secondary rainbow.

In a secondary rainbow, sunlight enters the water droplet and is refracted and dispersed. The separated light undergoes two internal reflections before emerging after refraction. The additional reflection makes it fainter and reverses the colour order compared with the primary rainbow.

6 Marks Q23. Compare primary and secondary rainbows.

Primary: one internal reflection, brighter, red outer and violet inner, smaller angular radius.

Secondary: two internal reflections, fainter, violet outer and red inner, larger angular radius.

6 Marks Q24. Explain why a rainbow appears as an arc.

Only droplets at suitable angles send the separated light towards the observer's eye. These droplets form an approximate cone around the antisolar direction. The visible intersection of this geometry with the sky appears as an arc.

6 Marks Q25. Explain why a complete rainbow may sometimes be observed from an aircraft.

From an elevated position, the horizon may not block the lower portion of the rainbow. Under suitable Sun, observer and droplet geometry, the observer can see much more of the circular cone of rainbow light, sometimes nearly a complete circle.

6 Marks Q26. Why is the secondary rainbow outside the primary rainbow?

The additional internal reflection changes the direction in which the light emerges. Consequently, the secondary rainbow has a larger angular radius and appears outside the primary rainbow.

6 Marks Q27. Explain the importance of dispersion in rainbow formation.

Dispersion separates the wavelengths present in white sunlight. Because different colours are refracted differently by water, they emerge at different angles and become visible as separate colours.

6 Marks Q28. Why does the colour sequence reverse in a secondary rainbow?

The extra internal reflection changes the geometry of the rays inside the droplet before they emerge. This reverses the order in which the colours are seen compared with the primary rainbow.

6 Marks Q29. Explain the role of sunlight in rainbow formation.

Sunlight provides the white light that contains a continuous range of visible wavelengths. These wavelengths are refracted, dispersed and internally reflected by water droplets to form the rainbow.

6 Marks Q30. Explain the complete optical sequence involved in a rainbow.

The sequence is:

Refraction → Dispersion → Internal Reflection → Refraction

This sequence occurs in water droplets and produces the coloured light observed as a rainbow.

23. Assertion–Reason Questions

Q1.
Assertion: A primary rainbow involves one internal reflection inside a water droplet.
Reason: The dispersed light is reflected from the inner surface of the droplet.

Answer: Both Assertion and Reason are true, and the Reason correctly explains the Assertion.

Q2.
Assertion: The Sun is generally behind the observer when a primary rainbow is observed.
Reason: The light returning from the droplets travels towards the observer from the opposite part of the sky.

Answer: Both are true, and the Reason correctly explains the Assertion.

Q3.
Assertion: The secondary rainbow is usually fainter than the primary rainbow.
Reason: Light undergoes an additional internal reflection in the secondary rainbow.

Answer: Both are true, and the Reason correctly explains the Assertion.

Q4.
Assertion: Red is generally on the outer edge of the primary rainbow.
Reason: Red undergoes greater deviation than violet in water.

Answer: Assertion is true, but Reason is false.

Q5.
Assertion: Dispersion is important in rainbow formation.
Reason: Different wavelengths are refracted by different amounts.

Answer: Both are true, and the Reason correctly explains the Assertion.

24. HOTS Questions | उच्च स्तरीय चिंतन

HOTS 1: Why can two people standing at different positions not see exactly the same set of rainbow-producing droplets?

The required geometry depends on the observer's position. Different observers receive light from different droplets at the required angles. Therefore each observer effectively sees their own rainbow.

HOTS 2: Why does moving the observer change the position of the rainbow?

The rainbow is a geometrical optical phenomenon determined by the relative positions of the Sun, observer and water droplets. When the observer moves, the set of droplets satisfying the required geometry changes.

HOTS 3: Why is the rainbow not a physical object located at a fixed distance?

A rainbow is produced by light reaching the observer from suitable droplets at particular angles. It has no fixed physical surface or location like a solid object.

HOTS 4: Why is a secondary rainbow wider and fainter than the primary rainbow?

The secondary rainbow results from two internal reflections, changing the emergence geometry and giving a larger angular radius. The extra reflection also reduces the amount of light reaching the observer.

HOTS 5: Why does a rainbow usually disappear when the Sun becomes too high in the sky for an observer on the ground?

The required rainbow geometry may place the rainbow below the horizon when the Sun is sufficiently high. Hence the observer cannot see the necessary portion of the rainbow.

25. Diagram Labelling Practice

Label the following:

• Sunlight
• Water droplet
• Refraction
• Dispersion
• Internal reflection
• Emerging rays
• Red colour
• Violet colour

26. CBSE Golden Points | परीक्षा के लिए महत्वपूर्ण बिंदु

✔ Rainbow is a natural optical phenomenon.

✔ Water droplets act as tiny optical systems.

✔ Primary rainbow involves one internal reflection.

✔ Secondary rainbow involves two internal reflections.

✔ Refraction occurs when light enters and leaves a droplet.

✔ Dispersion separates white sunlight into colours.

✔ Primary rainbow: red outer, violet inner.

✔ Secondary rainbow: violet outer, red inner.

✔ Secondary rainbow is generally fainter.

✔ The Sun is generally behind the observer.

✔ Primary rainbow angular radius is roughly 40°–42°.

✔ Secondary rainbow is seen outside the primary rainbow.

✔ Alexander's dark band lies between primary and secondary rainbows.

✔ A complete circular rainbow may be visible from an elevated position under suitable conditions.

27. Memory Tricks | याद रखने की ट्रिक

🌈 Rainbow Formula:

Refraction → Dispersion → Internal Reflection → Refraction

☀️ Position Trick:

Sun Behind → Rainbow Ahead

🔴 Primary Rainbow:

Red → Outer
Violet → Inner

🔄 Secondary Rainbow:

Violet → Outer
Red → Inner

1 Reflection = Primary

2 Reflections = Secondary

28. One-Minute Revision | एक मिनट में पूरा टॉपिक

🌞 Sunlight enters water droplet.

↘ Refraction occurs.

🌈 White light disperses into colours.

↩ One internal reflection produces the primary rainbow.

↘ Light emerges after another refraction.

🔴 Red appears on outer side of primary rainbow.

🟣 Violet appears on inner side.

🌈 Primary rainbow is brighter.

🌈 Secondary rainbow involves two internal reflections.

🔄 Secondary rainbow has reversed colour order.

☀️ Sun is generally behind the observer.

29. Final Summary | अंतिम सारांश

A rainbow is produced when sunlight interacts with water droplets. The formation of a primary rainbow involves refraction, dispersion, one internal reflection and refraction again. The different wavelengths of sunlight are separated because they undergo different amounts of refraction in water.

प्राथमिक इन्द्रधनुष के निर्माण में सूर्य का प्रकाश जल की बूंद में प्रवेश करते समय अपवर्तित और विक्षेपित होता है। इसके बाद प्रकाश एक बार आंतरिक परावर्तन करता है और बाहर निकलते समय पुनः अपवर्तित होता है। परिणामस्वरूप विभिन्न रंग अलग-अलग दिशाओं में पर्यवेक्षक तक पहुँचते हैं।

🌞 Sunlight → 💧 Water Droplet → 🔺 Refraction → 🌈 Dispersion → ↩ Internal Reflection → 🌈 Rainbow

Class 10th Science Chapter 10 – The Human Eye and the Colourful World | Dispersion of White Light

The Human Eye and the Colourful World

Dispersion of White Light | श्वेत प्रकाश का विक्षेपण

Class 10 Science | CBSE + Foundation + Competitive

🌈 Dispersion of White Light | श्वेत प्रकाश का विक्षेपण

1. Introduction | परिचय

Dispersion of light is the phenomenon in which white light splits into its constituent colours when it passes through a transparent medium such as a glass prism.

प्रकाश का विक्षेपण वह घटना है जिसमें श्वेत प्रकाश किसी पारदर्शी माध्यम, जैसे काँच के प्रिज्म, से गुजरने पर अपने विभिन्न घटक रंगों में विभाजित हो जाता है।

The seven commonly identified colours are represented by VIBGYOR:

V – Violet | I – Indigo | B – Blue | G – Green | Y – Yellow | O – Orange | R – Red

2. What is Dispersion? | विक्षेपण क्या है?

When a narrow beam of white light is allowed to pass through a glass prism, it emerges as a band of different colours called the spectrum.

जब श्वेत प्रकाश की एक संकीर्ण किरण को काँच के प्रिज्म से गुजारा जाता है, तो बाहर निकलने पर विभिन्न रंगों की एक पट्टी प्राप्त होती है, जिसे वर्णक्रम (Spectrum) कहते हैं।

The splitting of white light into its constituent colours is called dispersion of white light.

3. Dispersion Through a Glass Prism | काँच के प्रिज्म द्वारा विक्षेपण

4. Why Does Dispersion Occur? | विक्षेपण क्यों होता है?

Dispersion occurs because the refractive index of a transparent material is different for different wavelengths (colours) of light.

अलग-अलग रंगों के प्रकाश की तरंगदैर्ध्य अलग होती है और काँच का अपवर्तनांक विभिन्न तरंगदैर्ध्यों के लिए अलग-अलग होता है। इसलिए प्रत्येक रंग का अपवर्तन अलग मात्रा में होता है।

Thus, different colours travel with different speeds in glass and are deviated by different amounts.

Key Concept:

Different wavelength → Different refractive index → Different refraction → Different deviation → Dispersion

अलग तरंगदैर्ध्य → अलग अपवर्तनांक → अलग अपवर्तन → अलग विचलन → विक्षेपण

5. Spectrum of White Light | श्वेत प्रकाश का वर्णक्रम

The band of colours obtained after dispersion of white light is called the visible spectrum.

श्वेत प्रकाश के विक्षेपण के बाद प्राप्त रंगों की पट्टी को दृश्य वर्णक्रम कहते हैं।

Colour	Approximate Wavelength	Deviation in Ordinary Glass
Violet	~400 nm	Maximum
Indigo	~445 nm	High
Blue	~470 nm	High
Green	~510 nm	Medium
Yellow	~570 nm	Medium
Orange	~590 nm	Low
Red	~650–700 nm	Minimum

6. Violet and Red Light | बैंगनी और लाल प्रकाश

In ordinary glass:

• Violet light has the shortest wavelength among the visible colours and undergoes the greatest deviation.
• Red light has the longest wavelength among the visible colours and undergoes the least deviation.

सामान्य काँच में:

• बैंगनी प्रकाश का विचलन सबसे अधिक होता है।
• लाल प्रकाश का विचलन सबसे कम होता है।

Exam Point: In the usual school-level treatment of dispersion by a glass prism, Violet deviates most and Red deviates least.

7. Wavelength and Dispersion | तरंगदैर्ध्य और विक्षेपण

Visible light has different wavelengths. In ordinary glass, shorter wavelength light generally has a higher refractive index than longer wavelength light. Therefore, violet is deviated more than red.

Property	Violet	Red
Wavelength	Shortest	Longest
Frequency	Highest	Lowest
Speed in vacuum	Same
Speed in glass	Lower than red	Higher than violet
Deviation through ordinary glass prism	Maximum	Minimum

8. Newton's Experiment | न्यूटन का प्रयोग

Sir Isaac Newton investigated the dispersion of white light using a glass prism.

उन्होंने श्वेत प्रकाश को प्रिज्म से गुजारकर प्राप्त रंगों का अध्ययन किया। उन्होंने यह भी दिखाया कि ये रंग पुनः एकत्र होकर श्वेत प्रकाश बना सकते हैं।

When the dispersed colours are passed through another suitable prism or combined appropriately, they can be recombined to produce white light.

9. Refraction vs Dispersion | अपवर्तन और विक्षेपण में अंतर

Refraction	Dispersion
Bending of light when it changes medium.	Splitting of white light into constituent colours.
Can occur for a single colour.	Requires different colours/wavelengths to separate.
Direction changes because speed changes.	Different colours undergo different amounts of refraction.
Example: Light through glass slab.	Example: White light through glass prism.

10. Dispersion vs Scattering | विक्षेपण और प्रकीर्णन में अंतर

Dispersion	Scattering
Separation of colours due to wavelength-dependent refraction.	Redirection of light by particles.
Commonly demonstrated with a prism.	Occurs in atmosphere and colloidal systems.
Produces a spectrum from white light.	Explains phenomena such as blue sky and reddish sunrise/sunset.

11. Rainbow and Dispersion | इन्द्रधनुष और विक्षेपण

A rainbow is a natural optical phenomenon involving the interaction of sunlight with water droplets. It involves:

1. Refraction of light when it enters a water droplet.
2. Dispersion of white light into colours.
3. Internal reflection inside the droplet.
4. Refraction again when light emerges.

इन्द्रधनुष में जल की बूंदों द्वारा प्रकाश का अपवर्तन, विक्षेपण और आंतरिक परावर्तन शामिल होता है।

12. Important Formulae and Relations | महत्वपूर्ण सूत्र

c = νλ

Where: c = speed of light in vacuum, ν = frequency, λ = wavelength.

μ = c / v

Where μ is refractive index, c is speed in vacuum and v is speed of light in the medium.

v = c / μ

Important: When light enters a transparent medium, its frequency remains unchanged, while its speed and wavelength change.

13. Solved Examples | हल किए गए उदाहरण

Example 1:

The wavelength of red light is 700 nm. Express it in metres.

Solution:
1 nm = 10⁻⁹ m

700 nm = 700 × 10⁻⁹ m
= 7 × 10⁻⁷ m

Example 2:

A medium has refractive index 1.5. If the speed of light in vacuum is 3 × 10⁸ m/s, find the speed of light in the medium.

μ = c/v

v = c/μ
v = (3 × 10⁸)/1.5
v = 2 × 10⁸ m/s

Example 3:

Which colour is deviated most and which is deviated least by an ordinary glass prism?

Answer:
Violet → Maximum deviation
Red → Minimum deviation

14. 30 MCQs | बहुविकल्पीय प्रश्न

Q1. The splitting of white light into its constituent colours is called:

(A) Dispersion

(B) Reflection

(C) Diffraction

(D) Absorption

Answer: A — Dispersion

Q2. The band of colours obtained by dispersion is called:

(A) Image

(B) Spectrum

(C) Shadow

(D) Beam

Answer: B — Spectrum

Q3. Which colour suffers maximum deviation in an ordinary glass prism?

(A) Red

(B) Green

(C) Violet

(D) Yellow

Answer: C — Violet

Q4. Which colour suffers minimum deviation?

(A) Violet

(B) Blue

(C) Green

(D) Red

Answer: D — Red

Q5. The visible spectrum is commonly represented by:

(A) VIBGYOR

(B) ABCDEFG

(C) RYBGCIV

(D) UVIR

Answer: A — VIBGYOR

Q6. Dispersion occurs mainly because:

(A) All colours have the same refractive index

(B) Refractive index depends on wavelength

(C) Light stops inside glass

(D) Glass produces light

Answer: B

Q7. Which colour has the longest wavelength among visible colours?

(A) Violet

(B) Blue

(C) Red

(D) Green

Answer: C — Red

Q8. Which colour has the shortest wavelength among visible colours?

(A) Red

(B) Yellow

(C) Green

(D) Violet

Answer: D — Violet

Q9. Which quantity remains unchanged when light enters glass from air?

(A) Frequency

(B) Speed

(C) Wavelength

(D) Direction

Answer: A — Frequency

Q10. The refractive index of a medium is given by:

(A) v/c

(B) c/v

(C) cv

(D) c+v

Answer: B — c/v

Q11. The colour corresponding to the greatest wavelength is:

(A) Violet

(B) Blue

(C) Red

(D) Indigo

Answer: C — Red

Q12. A prism separates colours because they:

(A) Have identical speeds in glass

(B) Have identical wavelengths

(C) Are all absorbed equally

(D) Are refracted by different amounts

Answer: D

Q13. Newton studied the dispersion of white light using:

(A) Glass prism

(B) Plane mirror

(C) Convex lens only

(D) Concave mirror

Answer: A

Q14. Which colour has the highest frequency among visible colours?

(A) Red

(B) Violet

(C) Yellow

(D) Orange

Answer: B — Violet

Q15. Which colour has the lowest frequency among visible colours?

(A) Violet

(B) Blue

(C) Red

(D) Green

Answer: C — Red

Q16. The phenomenon responsible for the formation of a spectrum by a prism is:

(A) Reflection only

(B) Absorption

(C) Heating

(D) Dispersion

Answer: D

Q17. In ordinary glass, the refractive index is generally greater for:

(A) Violet light

(B) Red light

(C) Orange light

(D) Yellow light

Answer: A

Q18. The speed of light in vacuum is approximately:

(A) 3 × 10⁶ m/s

(B) 3 × 10⁸ m/s

(C) 3 × 10⁴ m/s

(D) 3 × 10¹⁰ m/s

Answer: B

Q19. The wavelength of visible light is usually expressed in:

(A) metre only

(B) kilometre

(C) nanometre

(D) centimetre only

Answer: C

Q20. The colour sequence from shorter to longer wavelength is:

(A) ROYGBIV

(B) RYOGBIV

(C) GYORVIB

(D) VIBGYOR

Answer: D — VIBGYOR

Q21. A rainbow involves:

(A) Refraction, dispersion and internal reflection

(B) Reflection only

(C) Absorption only

(D) Diffraction only

Answer: A

Q22. Which statement is correct?

(A) Red has shorter wavelength than violet

(B) Red has longer wavelength than violet

(C) Both have equal wavelength

(D) Violet has no frequency

Answer: B

Q23. If wavelength decreases, frequency:

(A) Decreases

(B) Becomes zero

(C) Increases

(D) Remains unrelated

Answer: C

Q24. The relation between speed, frequency and wavelength is:

(A) v=λ/ν

(B) v=ν/λ

(C) v=λ−ν

(D) v=νλ

Answer: D — v=νλ

Q25. The phenomenon of splitting of white light is best demonstrated using:

(A) Prism

(B) Plane mirror

(C) Black sheet

(D) Metal rod

Answer: A

Q26. In glass, red light generally travels:

(A) Slower than violet

(B) Faster than violet

(C) At zero speed

(D) At infinite speed

Answer: B

Q27. The visible colour with maximum deviation in ordinary glass is:

(A) Red

(B) Green

(C) Violet

(D) Yellow

Answer: C

Q28. Which phenomenon explains the separation of colours by a prism?

(A) Reflection

(B) Scattering

(C) Interference

(D) Dispersion

Answer: D

Q29. White light is:

(A) A combination of visible colours

(B) Only red light

(C) Only blue light

(D) Only violet light

Answer: A

Q30. The order of colours from violet to red is:

(A) R-O-Y-G-B-I-V

(B) V-I-B-G-Y-O-R

(C) V-B-I-G-O-Y-R

(D) I-V-B-G-Y-R-O

Answer: B — V-I-B-G-Y-O-R

15. 30 Subjective Questions with Answers | वर्णनात्मक प्रश्न

2 Marks Q1. What is dispersion of white light?

Dispersion is the splitting of white light into its constituent colours when it passes through a suitable transparent medium such as a glass prism.

2 Marks Q2. What is a spectrum?

The band of colours obtained after dispersion of white light is called a spectrum.

2 Marks Q3. Write the sequence of colours in VIBGYOR.

Violet, Indigo, Blue, Green, Yellow, Orange and Red.

2 Marks Q4. Which colour deviates most and which deviates least?

Violet deviates most and red deviates least in an ordinary glass prism.

2 Marks Q5. Why does white light split in a prism?

Because different colours have different wavelengths and the refractive index of glass depends on wavelength. Hence they undergo different refractions.

3 Marks Q6. Define dispersion and explain it briefly.

Dispersion is the splitting of white light into constituent colours due to wavelength-dependent refraction in a transparent medium.

3 Marks Q7. Why is violet deviated more than red?

Violet has shorter wavelength. In ordinary glass its refractive index is higher than that for red light, so violet undergoes greater refraction and deviation.

3 Marks Q8. What is meant by visible spectrum?

The range of electromagnetic radiation detectable by the human eye is called visible light. Its colours form the visible spectrum.

3 Marks Q9. State the relation between speed, frequency and wavelength.

The relation is:
v = νλ
where v is speed, ν is frequency and λ is wavelength.

3 Marks Q10. What happens to frequency when light enters glass?

Frequency remains unchanged because it is determined by the source. Speed and wavelength change in the medium.

4 Marks Q11. Explain the formation of spectrum by a glass prism.

White light enters the prism and is refracted. Different colours are refracted by different amounts because their refractive indices in glass are different. The colours emerge separated and form a spectrum.

4 Marks Q12. Explain why red light is deviated least.

Red has the longest wavelength among visible colours. In ordinary glass it has the lowest refractive index among the visible colours, so its speed is relatively higher and its deviation is least.

4 Marks Q13. Write four differences between refraction and dispersion.

Refraction is bending of light at a change of medium, whereas dispersion is separation of white light into colours. Refraction can occur for one colour, while dispersion separates different wavelengths.

4 Marks Q14. Explain the role of wavelength in dispersion.

Different colours have different wavelengths. Since the refractive index of glass depends on wavelength, each colour travels with a different speed and undergoes a different amount of refraction.

4 Marks Q15. Explain VIBGYOR.

VIBGYOR represents the seven commonly identified colours of the visible spectrum: Violet, Indigo, Blue, Green, Yellow, Orange and Red.

5 Marks Q16. Explain dispersion of white light through a prism with a labelled diagram.

White light enters a glass prism and undergoes refraction at both surfaces. Since the refractive index of glass varies with wavelength, different colours are deviated differently. Violet deviates most and red least, producing the VIBGYOR spectrum.

5 Marks Q17. Explain the relationship between refractive index and speed of light.

Refractive index is:
μ = c/v
For a given medium, higher refractive index means lower speed of light. Different colours have different refractive indices, so their speeds in glass differ.

5 Marks Q18. Explain Newton's contribution to the study of dispersion.

Newton used a glass prism to study the splitting of white light into colours. He demonstrated that white light contains different colours and that these colours can be recombined to produce white light.

5 Marks Q19. Explain the terms wavelength and frequency.

Wavelength is the distance between successive corresponding points of a wave. Frequency is the number of oscillations per second. Their relation with speed is v=νλ.

5 Marks Q20. Explain why different colours travel at different speeds in glass.

The refractive index of glass depends on wavelength. Since v=c/μ, different refractive indices result in different speeds for different colours.

6 Marks Q21. Describe the complete phenomenon of dispersion through a prism.

White light enters the prism and is refracted at the first surface. Its constituent colours have different wavelengths and therefore different refractive indices. They are deviated by different amounts. At the second surface they undergo further refraction and emerge separated. Violet has maximum deviation and red minimum deviation in ordinary glass. The resulting coloured band is the spectrum.

6 Marks Q22. Explain the cause of dispersion at the microscopic/physical level.

The interaction of electromagnetic waves with the material of the transparent medium depends on wavelength. Consequently the refractive index is wavelength-dependent. Since different wavelengths experience different refractive indices, they have different speeds and undergo different deviations in a prism.

6 Marks Q23. Explain the difference between wavelength, frequency and speed of light.

Wavelength is the spatial period of a wave, frequency is the number of oscillations per second, and speed is the distance travelled per second. They are related by v=νλ. Frequency remains unchanged when light enters a different medium, while speed and wavelength change.

6 Marks Q24. Explain why violet and red light behave differently in glass.

Violet has shorter wavelength and higher frequency than red. In ordinary glass its refractive index is greater. Hence violet travels more slowly and bends/deviates more, whereas red travels relatively faster and deviates less.

6 Marks Q25. Explain rainbow formation.

Sunlight enters a raindrop and is refracted and dispersed. The light undergoes internal reflection inside the drop and is refracted again as it emerges. Different colours emerge at different angles, producing the observed rainbow.

6 Marks Q26. Why does a glass slab not normally produce a spectrum like a prism?

A glass slab has parallel refracting surfaces. Although different colours may refract by different amounts at each surface, the second refraction largely cancels the angular deviation because the surfaces are parallel. The emergent ray remains parallel to the incident ray, so there is no clearly separated angular spectrum like that produced by a prism.

6 Marks Q27. Explain the importance of refractive index in dispersion.

Dispersion occurs because refractive index varies with wavelength. If the refractive index were identical for all colours, all colours would undergo the same deviation and white light would not separate into a spectrum.

6 Marks Q28. Write an experiment to demonstrate dispersion.

Take a glass prism and allow a narrow beam of white light to fall on one face in a dark room. Adjust the prism so that the emerging light falls on a white screen. A coloured band is observed on the screen. This demonstrates dispersion.

6 Marks Q29. Explain why frequency remains unchanged during refraction.

The frequency of light is fixed by the source. At the boundary of two media, the oscillations must remain continuous, so the frequency does not change. The speed and wavelength adjust according to the optical properties of the new medium.

6 Marks Q30. Write important points about the visible spectrum.

The visible spectrum is the part of electromagnetic radiation detected by the human eye. It is commonly represented by VIBGYOR. Violet has the shortest wavelength and highest frequency among these colours, while red has the longest wavelength and lowest frequency. In ordinary glass, violet deviates most and red least.

16. Assertion–Reason Questions

Q1.
Assertion: Violet light is deviated more than red light by an ordinary glass prism.
Reason: The refractive index of ordinary glass is generally greater for violet light than for red light.

Answer: Both Assertion and Reason are true, and Reason correctly explains the Assertion.

Q2.
Assertion: Dispersion occurs when white light passes through a prism.
Reason: Different colours have different wavelengths and refractive indices in glass.

Answer: Both are true, and the Reason correctly explains the Assertion.

Q3.
Assertion: Frequency of light changes when it enters glass from air.
Reason: Speed of light changes in glass.

Answer: Assertion is false, Reason is true.

Q4.
Assertion: Red light has the longest wavelength among the visible colours.
Reason: Red light has the minimum frequency among the visible colours.

Answer: Both are true, and the Reason is related through v=νλ for a fixed speed in vacuum.

Q5.
Assertion: A prism can separate white light into colours.
Reason: The refractive index of glass is the same for every colour.

Answer: Assertion is true, but Reason is false.

17. HOTS Questions | उच्च स्तरीय चिंतन

HOTS 1: If the refractive index of glass were exactly the same for all colours, what would happen to dispersion?

There would be no angular separation of colours due to dispersion because all colours would undergo the same deviation.

HOTS 2: Why does a prism produce a spectrum while a parallel-sided glass slab does not normally produce a clearly separated spectrum?

A prism has non-parallel refracting surfaces, producing net angular deviation that separates colours. In a slab, the parallel surfaces largely cancel the angular deviation.

HOTS 3: If two colours have the same frequency, would they necessarily have the same wavelength in the same medium?

For a given medium, wavelength is related to speed and frequency. If their optical properties are different, the speeds may differ. Thus, frequency alone does not guarantee equal wavelength in different optical conditions.

HOTS 4: Why is the spectrum obtained from a prism not arranged randomly?

The colours are arranged according to their wavelength/frequency and the wavelength-dependent refractive index of the medium. Hence a definite order VIBGYOR is obtained.

HOTS 5: Why is a rainbow a natural example of dispersion?

Sunlight is white light. Water droplets refract and disperse its different wavelengths, producing separated colours. Internal reflection inside the droplets also contributes to the observed rainbow.

18. Diagram Labelling Practice

Students should practise labelling:

• Incident white light
• Glass prism
• Refracted rays
• Emergent rays
• Spectrum
• Violet
• Red

19. CBSE Golden Points | परीक्षा के लिए महत्वपूर्ण बिंदु

✔ Dispersion = splitting of white light into colours.

✔ Spectrum = band of colours produced after dispersion.

✔ VIBGYOR = Violet, Indigo, Blue, Green, Yellow, Orange, Red.

✔ Violet has the shortest wavelength among visible colours.

✔ Red has the longest wavelength among visible colours.

✔ Violet has maximum deviation in ordinary glass.

✔ Red has minimum deviation in ordinary glass.

✔ Refractive index depends on wavelength.

✔ Frequency remains unchanged during refraction.

✔ Speed and wavelength change when light enters a different medium.

✔ μ = c/v.

✔ v = νλ.

✔ Rainbow involves refraction, dispersion and internal reflection.

20. Memory Tricks | याद रखने की ट्रिक

🌈 VIBGYOR:
Violet → Indigo → Blue → Green → Yellow → Orange → Red

🔺 Prism Trick:
Violet = Maximum Deviation
Red = Minimum Deviation

📏 Wavelength Trick:
Violet → Short wavelength
Red → Long wavelength

⚡ Formula Trick:
v = νλ
Speed = Frequency × Wavelength

🔬 Refractive Index:
μ = c/v

21. One-Minute Revision | एक मिनट में पूरा अध्याय

White Light → mixture of visible colours

Prism → causes different colours to deviate differently

Dispersion → splitting of white light

Spectrum → coloured band

VIBGYOR → visible spectrum order

Violet → shortest wavelength → maximum deviation

Red → longest wavelength → minimum deviation

Frequency → unchanged during refraction

Speed → changes in medium

Wavelength → changes in medium

Rainbow → refraction + dispersion + internal reflection

22. Final Summary | अंतिम सारांश

Dispersion of white light is an important optical phenomenon in which white light separates into its constituent colours because different wavelengths experience different refractive indices in a transparent medium. A glass prism produces a visible spectrum commonly represented as VIBGYOR. In ordinary glass, violet light is deviated the most and red light the least.

श्वेत प्रकाश का विक्षेपण वह घटना है जिसमें श्वेत प्रकाश प्रिज्म से गुजरते समय अपने विभिन्न रंगों में विभाजित हो जाता है। इसका मुख्य कारण यह है कि अलग-अलग रंगों के लिए काँच का अपवर्तनांक अलग होता है। सामान्य काँच में बैंगनी प्रकाश का विचलन सबसे अधिक तथा लाल प्रकाश का विचलन सबसे कम होता है।

🌈 White Light → Prism → Dispersion → VIBGYOR Spectrum