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