💡 Reflection and Refraction | Introduction to Light
Class 10 Science | CBSE + Foundation + Competitive Level
Light • Sources of Light • Ray and Beam • Rectilinear Propagation • Reflection • Refraction • Refractive Index
💡 Reflection and Refraction | Introduction to Light
1. What is Light? | प्रकाश क्या है?
Light is a form of energy that produces the sensation of vision and enables us to see objects.
हिन्दी: प्रकाश ऊर्जा का एक रूप है जो हमें वस्तुओं को देखने में सक्षम बनाता है।
Light is an electromagnetic wave. In vacuum, electromagnetic waves can travel without a material medium.
हिन्दी: प्रकाश ऊर्जा का एक रूप है जो हमें वस्तुओं को देखने में सक्षम बनाता है।
Light is an electromagnetic wave. In vacuum, electromagnetic waves can travel without a material medium.
Class 10 Focus:
In this chapter, the main ideas are propagation of light, reflection, refraction, mirrors, lenses and image formation.
2. Sources of Light | प्रकाश के स्रोत
| Type | Meaning | Examples |
|---|---|---|
| Natural Sources | Sources occurring naturally | Sun, stars, lightning |
| Artificial Sources | Man-made sources | Electric bulb, candle, LED |
Luminous objects:
Objects that emit their own light.
Examples: Sun, electric bulb, flame.
Non-luminous objects: Objects that do not produce their own light but can be seen when light falls on them.
Examples: Book, table, Moon.
Examples: Sun, electric bulb, flame.
Non-luminous objects: Objects that do not produce their own light but can be seen when light falls on them.
Examples: Book, table, Moon.
3. How Do We See an Object? | वस्तु दिखाई कैसे देती है?
Source of Light
→
Object
→
Reflected Light
→
Eye
We see a non-luminous object when light falls on it and some of the reflected light enters our eyes.
उदाहरण:
हम पुस्तक को इसलिए देख पाते हैं क्योंकि उस पर गिरा प्रकाश परावर्तित होकर हमारी आँखों तक पहुँचता है।
4. Rectilinear Propagation of Light | प्रकाश का सरल रेखीय संचरण
In a homogeneous transparent medium, light generally travels in a straight line.
This property is called:
Rectilinear Propagation of Light.
हिन्दी: समांगी पारदर्शी माध्यम में प्रकाश सामान्यतः सीधी रेखा में चलता है।
हिन्दी: समांगी पारदर्शी माध्यम में प्रकाश सामान्यतः सीधी रेखा में चलता है।
💡 Straight-Line Propagation of Light
5. Ray and Beam of Light | प्रकाश की किरण एवं किरण-पुंज
Ray:
A path showing the direction of propagation of light.
Beam: A collection of light rays travelling together.
Beam: A collection of light rays travelling together.
| Type of Beam | Description |
|---|---|
| Parallel Beam | Rays remain parallel to each other. |
| Convergent Beam | Rays move towards a common point. |
| Divergent Beam | Rays spread away from a common point. |
6. Reflection of Light | प्रकाश का परावर्तन
Reflection of light is the phenomenon in which light returns into the same medium after striking a reflecting surface.
हिन्दी: जब प्रकाश किसी परावर्तक सतह से टकराकर उसी माध्यम में वापस लौटता है, तो इसे प्रकाश का परावर्तन कहते हैं।
हिन्दी: जब प्रकाश किसी परावर्तक सतह से टकराकर उसी माध्यम में वापस लौटता है, तो इसे प्रकाश का परावर्तन कहते हैं।
Angle of Incidence = Angle of Reflection
∠i = ∠r
∠i = ∠r
7. Important Terms in Reflection | परावर्तन के महत्वपूर्ण शब्द
| Term | Meaning |
|---|---|
| Incident Ray | The ray falling on the reflecting surface. |
| Reflected Ray | The ray returning from the reflecting surface. |
| Normal | An imaginary line perpendicular to the reflecting surface at the point of incidence. |
| Point of Incidence | The point where the incident ray strikes the surface. |
| Angle of Incidence | Angle between incident ray and normal. |
| Angle of Reflection | Angle between reflected ray and normal. |
8. Animated Reflection Diagram | परावर्तन का एनिमेटेड चित्र
9. Laws of Reflection | परावर्तन के नियम
First Law:
The incident ray, reflected ray and normal at the point of incidence all lie in the same plane.
Second Law: The angle of incidence is equal to the angle of reflection.
Second Law: The angle of incidence is equal to the angle of reflection.
∠i = ∠r
10. Regular and Diffuse Reflection | नियमित एवं विसरित परावर्तन
| Regular Reflection | Diffuse Reflection |
|---|---|
| Occurs on smooth surfaces. | Occurs on rough surfaces. |
| Reflected rays remain orderly. | Reflected rays scatter in different directions. |
| Can form a clear image. | Usually does not form a clear image. |
| Example: Plane mirror | Example: Rough wall |
11. Plane Mirror | समतल दर्पण
A plane mirror has a flat reflecting surface.
The image formed by a plane mirror is generally:
- Virtual
- Erect
- Same size as object
- Laterally inverted
- At the same distance behind the mirror as the object is in front
Object distance = Image distance
|u| = |v|
|u| = |v|
12. Refraction of Light | प्रकाश का अपवर्तन
Refraction is the change in direction of light when it passes obliquely from one transparent medium to another due to a change in its speed.
हिन्दी: जब प्रकाश तिरछे रूप से एक पारदर्शी माध्यम से दूसरे पारदर्शी माध्यम में जाता है और उसकी चाल बदलने के कारण उसकी दिशा बदलती है, तो इसे अपवर्तन कहते हैं।
हिन्दी: जब प्रकाश तिरछे रूप से एक पारदर्शी माध्यम से दूसरे पारदर्शी माध्यम में जाता है और उसकी चाल बदलने के कारण उसकी दिशा बदलती है, तो इसे अपवर्तन कहते हैं।
13. Why Does Refraction Occur?
The speed of light is different in different transparent media.
When light enters another medium at an angle, its speed changes and its path may bend.
Speed of Light in Vacuum
c ≈ 3 × 10⁸ m/s
c ≈ 3 × 10⁸ m/s
14. Refraction Through a Glass Slab | काँच की पट्टी से अपवर्तन
🔦 Refraction Through Glass Slab
When a ray passes through a rectangular glass slab:
- It bends towards the normal on entering glass from air.
- It bends away from the normal while emerging from glass into air.
- The emergent ray is parallel to the incident ray.
- There is a lateral displacement between the incident and emergent ray.
15. Rarer and Denser Medium | विरल एवं सघन माध्यम
For a given pair of media:
Optically denser medium:
Light travels more slowly in it.
Optically rarer medium:
Light travels faster in it.
| Movement | Bending |
|---|---|
| Rarer → Denser | Towards the normal |
| Denser → Rarer | Away from the normal |
Important:
“Optically denser” does not necessarily mean physically heavier or having greater mass density.
16. Refractive Index | अपवर्तनांक
The refractive index of a medium indicates how much the speed of light is reduced in that medium compared with vacuum.
n = c / v
n = Refractive Index
c = Speed of light in vacuum
v = Speed of light in the medium
n = Refractive Index
c = Speed of light in vacuum
v = Speed of light in the medium
17. Snell's Law | स्नेल का नियम
For light passing between two given transparent media:
n₁ sin i = n₂ sin r
where:
- n₁ = refractive index of first medium
- n₂ = refractive index of second medium
- i = angle of incidence
- r = angle of refraction
18. Important Difference: Reflection vs Refraction
| Reflection | Refraction |
|---|---|
| Light returns into the same medium. | Light enters another medium. |
| Occurs at a reflecting surface. | Occurs at the boundary between transparent media. |
| Direction changes by reflection. | Direction and speed may change. |
| ∠i = ∠r | n₁ sin i = n₂ sin r |
19. Optical Phenomena in Daily Life
Refraction helps explain several familiar observations:
- A pencil partly immersed in water appears bent.
- A coin in water may appear raised.
- The bottom of a swimming pool appears shallower.
- Stars appear to twinkle due to atmospheric refraction.
- The Sun can be visible slightly before actual sunrise and after actual sunset because of atmospheric refraction.
20. Apparent Depth | आभासी गहराई
When an object under water is viewed from air, it may appear closer to the surface than its actual position.
This is called apparent depth.
Refractive Index
≈
Real Depth / Apparent Depth
This relation is commonly used for an object viewed normally from air for a simple apparent-depth situation.
21. Important Light Terms | महत्वपूर्ण प्रकाश शब्दावली
| Term | Meaning |
|---|---|
| Transparent | Allows most light to pass through. |
| Translucent | Allows some light to pass through. |
| Opaque | Does not allow light to pass through. |
| Normal | Line perpendicular to a surface at the point of incidence. |
| Medium | Material through which light travels. |
| Optical Density | Related to the speed of light in a medium. |
22. Important Formula Sheet 📘
Speed of Light in Vacuum
c ≈ 3 × 10⁸ m/s
c ≈ 3 × 10⁸ m/s
Law of Reflection
∠i = ∠r
∠i = ∠r
Refractive Index
n = c / v
n = c / v
Snell's Law
n₁ sin i = n₂ sin r
n₁ sin i = n₂ sin r
Apparent Depth Relation
n ≈ Real Depth / Apparent Depth
n ≈ Real Depth / Apparent Depth
23. 30 MCQs | Multiple Choice Questions
Q1. Light is a form of:
A. Energy
B. Matter only
C. Sound
D. Pressure
Answer: A
Light is a form of electromagnetic energy.
Q2. Which is a luminous object?
A. Moon
B. Sun
C. Book
D. Chair
Answer: B
The Sun emits its own light.
Q3. In a homogeneous transparent medium, light generally travels:
A. In a circle
B. In a zig-zag path
C. In a straight line
D. Randomly
Answer: C
This property is called rectilinear propagation.
Q4. A collection of light rays travelling together is called:
A. Beam
B. Pixel
C. Spectrum
D. Shadow
Answer: A
A beam consists of multiple light rays travelling together.
Q5. Reflection occurs when light:
A. Is absorbed completely
B. Returns into the same medium after striking a surface
C. Stops permanently
D. Changes into sound
Answer: B
Reflection is the return of light into the same medium.
Q6. The angle of incidence is measured between:
A. Incident ray and surface
B. Reflected ray and surface
C. Incident ray and normal
D. Normal and surface
Answer: C
Angle of incidence is measured between the incident ray and normal.
Q7. According to the law of reflection:
A. i > r
B. i < r
C. i = r
D. i + r = 180°
Answer: C
The angle of incidence equals the angle of reflection.
Q8. A normal is drawn:
A. Parallel to the surface
B. Perpendicular to the surface
C. At 45° to the surface always
D. Randomly
Answer: B
Normal is perpendicular to the reflecting/refracting surface.
Q9. Regular reflection occurs mainly from:
A. Smooth surfaces
B. Very rough surfaces
C. Clouds only
D. Dust particles
Answer: A
Smooth surfaces produce orderly reflection.
Q10. A plane mirror forms an image that is:
A. Real and inverted
B. Virtual and erect
C. Always enlarged
D. Always diminished
Answer: B
The image in a plane mirror is virtual and erect.
Q11. The image formed by a plane mirror is:
A. Laterally inverted
B. Always upside down
C. Always real
D. Always magnified
Answer: A
Plane mirror produces lateral inversion.
Q12. Refraction occurs mainly because light:
A. Changes colour only
B. Changes speed when entering another medium
C. Stops moving
D. Becomes sound
Answer: B
A change in speed causes bending when the light enters obliquely.
Q13. When light travels from a rarer to a denser medium, it generally bends:
A. Away from the normal
B. Towards the normal
C. Along the surface always
D. Backwards
Answer: B
Light bends towards the normal when entering an optically denser medium obliquely.
Q14. When light travels from a denser to a rarer medium, it generally bends:
A. Towards the normal
B. Away from the normal
C. Into the surface
D. It never bends
Answer: B
It bends away from the normal.
Q15. Refractive index is given by:
A. n = v/c
B. n = c/v
C. n = c + v
D. n = c − v
Answer: B
n = c/v for a medium relative to vacuum.
Q16. The speed of light in vacuum is approximately:
A. 3 × 10⁸ m/s
B. 3 × 10⁶ m/s
C. 3 × 10⁴ m/s
D. 3 × 10² m/s
Answer: A
c ≈ 3 × 10⁸ m/s.
Q17. Snell's law is:
A. n₁ sin i = n₂ sin r
B. n₁ cos i = n₂ cos r
C. i + r = 180°
D. n = c + v
Answer: A
Snell's law relates refractive indices and angles.
Q18. In a rectangular glass slab, the emergent ray is generally:
A. Perpendicular to incident ray
B. Parallel to incident ray
C. Opposite to incident ray
D. Always curved
Answer: B
For parallel faces of a rectangular slab, the emergent ray is parallel to the incident ray.
Q19. The sideways shift of a ray through a glass slab is called:
A. Lateral displacement
B. Magnification
C. Dispersion
D. Diffraction
Answer: A
The perpendicular separation between incident and emergent rays is lateral displacement.
Q20. A pencil partly immersed in water appears bent because of:
A. Reflection only
B. Refraction
C. Absorption
D. Scattering only
Answer: B
Refraction at the water-air interface changes the apparent position of the submerged part.
Q21. The Moon is:
A. Luminous
B. Non-luminous
C. A source of its own visible light
D. An artificial source
Answer: B
The Moon is visible mainly because it reflects sunlight.
Q22. Which is an example of diffuse reflection?
A. Reflection from a rough wall
B. Reflection from a plane mirror
C. Reflection from a polished metal surface
D. Reflection from a smooth glass mirror
Answer: A
A rough surface scatters reflected rays in different directions.
Q23. Which statement is correct about an optically denser medium?
A. Light travels faster in it than in vacuum
B. Light travels more slowly in it than in an optically rarer medium
C. It must have greater mass density
D. It cannot transmit light
Answer: B
Optical density is related to the speed of light, not simply mass density.
Q24. If refractive index of a medium is 1.5, then speed of light in it is:
A. c/1.5
B. 1.5c
C. c + 1.5
D. 1.5/c
Answer: A
From n = c/v, v = c/n = c/1.5.
Q25. A convergent beam consists of rays that:
A. Move towards a common point
B. Move away from one another
C. Are always parallel
D. Never intersect
Answer: A
Convergent rays approach a common point.
Q26. A divergent beam consists of rays that:
A. Spread apart
B. Meet at one point
C. Remain parallel
D. Stop at the source
Answer: A
Divergent rays spread away from one another.
Q27. Which phenomenon explains the apparent shallowness of a swimming pool?
A. Refraction
B. Reflection only
C. Rectilinear propagation only
D. Absorption
Answer: A
Refraction changes the apparent position of the bottom.
Q28. The angle of reflection is measured between:
A. Reflected ray and normal
B. Reflected ray and mirror surface
C. Incident ray and mirror surface
D. Surface and normal only
Answer: A
Angle of reflection is measured between reflected ray and normal.
Q29. If the angle of incidence is 40°, the angle of reflection is:
A. 20°
B. 40°
C. 50°
D. 80°
Answer: B
According to the law of reflection, i = r.
Q30. Which statement is correct?
A. Reflection and refraction are exactly the same phenomenon.
B. Reflection occurs only in vacuum.
C. Refraction involves a change in speed when light enters another medium.
D. Refraction never changes direction.
Answer: C
Refraction is associated with a change in speed and, for oblique incidence, a change in direction.
24. 30 Subjective Questions with Answers
2 Marks
Q1. What is light?
Light is a form of electromagnetic energy that enables us to see objects.
2 Marks
Q2. What is rectilinear propagation of light?
The property by which light travels in a straight line in a homogeneous transparent medium is called rectilinear propagation.
2 Marks
Q3. Define reflection of light.
Reflection is the phenomenon in which light returns into the same medium after striking a reflecting surface.
2 Marks
Q4. State the second law of reflection.
The angle of incidence is equal to the angle of reflection:
∠i = ∠r.
∠i = ∠r.
2 Marks
Q5. Define refraction of light.
Refraction is the change in direction of light when it passes obliquely from one transparent medium to another due to a change in its speed.
3 Marks
Q6. Differentiate between luminous and non-luminous objects.
Luminous objects emit their own light, such as the Sun and a bulb. Non-luminous objects do not produce their own visible light and are seen by reflected light, such as a book or chair.
3 Marks
Q7. What is a ray and what is a beam?
A ray represents the direction/path of propagation of light. A beam is a collection of light rays travelling together.
3 Marks
Q8. State the two laws of reflection.
1. Incident ray, reflected ray and normal lie in the same plane.
2. Angle of incidence equals angle of reflection.
2. Angle of incidence equals angle of reflection.
3 Marks
Q9. Differentiate between regular and diffuse reflection.
Regular reflection occurs on smooth surfaces and reflected rays remain orderly. Diffuse reflection occurs on rough surfaces and reflected rays scatter in different directions.
3 Marks
Q10. Why does refraction occur?
Refraction occurs because the speed of light changes when it passes from one transparent medium to another. If the ray enters obliquely, this speed change causes bending.
4 Marks
Q11. Explain the terms incident ray, reflected ray, normal and point of incidence.
The incident ray falls on the reflecting surface. The reflected ray returns from the surface. The normal is perpendicular to the surface at the point of incidence. The point of incidence is where the incident ray strikes the surface.
4 Marks
Q12. Write four characteristics of the image formed by a plane mirror.
The image is virtual, erect, same in size as the object and laterally inverted. It is formed at the same distance behind the mirror as the object is in front.
4 Marks
Q13. Explain optical density.
Optical density describes how strongly a medium reduces the speed of light compared with another medium. Light travels more slowly in an optically denser medium. It is not the same as ordinary mass density.
4 Marks
Q14. Explain what happens when light travels from air to glass.
Glass is optically denser than air. When light enters glass obliquely, its speed decreases and the ray bends towards the normal.
4 Marks
Q15. Explain what happens when light travels from glass to air.
Air is optically rarer than glass. When light emerges obliquely from glass into air, its speed increases and the ray bends away from the normal.
5 Marks
Q16. Explain refraction through a rectangular glass slab.
When a ray enters a rectangular glass slab from air, it bends towards the normal. Inside the slab it travels in a straight line. At the second surface it emerges into air and bends away from the normal. Because the two surfaces are parallel, the emergent ray is parallel to the incident ray but displaced sideways. This sideways shift is called lateral displacement.
5 Marks
Q17. Explain the terms refractive index and speed of light.
Refractive index is the ratio of the speed of light in vacuum to its speed in the medium:
n = c/v
A larger refractive index generally means light travels more slowly in that medium compared with a medium having a lower refractive index.
n = c/v
A larger refractive index generally means light travels more slowly in that medium compared with a medium having a lower refractive index.
5 Marks
Q18. Explain the difference between reflection and refraction.
Reflection occurs when light returns into the same medium after striking a surface. Refraction occurs when light enters another transparent medium and its speed changes, often causing a change in direction. Reflection follows ∠i = ∠r, while refraction follows Snell's law.
5 Marks
Q19. Explain why a pencil appears bent when partly immersed in water.
Light from the underwater part of the pencil travels from water to air. It is refracted away from the normal at the water-air boundary. The eye traces the refracted rays backward in straight lines, making the submerged part appear shifted from its actual position. Therefore, the pencil appears bent.
5 Marks
Q20. Explain apparent depth.
An object under water viewed from air appears closer to the surface than its actual depth. This apparent position occurs due to refraction at the water-air boundary. For simple normal viewing:
Refractive Index ≈ Real Depth / Apparent Depth.
Refractive Index ≈ Real Depth / Apparent Depth.
6 Marks
Q21. Explain reflection of light with a labelled diagram.
Reflection is the return of light into the same medium after striking a reflecting surface. The incident ray falls on the surface at the point of incidence. A normal is drawn perpendicular to the surface. The reflected ray travels back into the original medium. The angle between incident ray and normal is angle of incidence, while that between reflected ray and normal is angle of reflection. According to the laws of reflection, all three rays/normal lie in one plane and ∠i = ∠r.
6 Marks
Q22. Explain refraction of light with a suitable example.
Refraction is the change in direction of light when it passes obliquely between transparent media because its speed changes. For example, when a ray travels from air to glass, its speed decreases and it bends towards the normal. When it travels from glass to air, its speed increases and it bends away from the normal. A partly immersed pencil appearing bent is a common example.
6 Marks
Q23. Explain the laws of reflection.
The first law states that the incident ray, reflected ray and normal at the point of incidence lie in the same plane. The second law states that the angle of incidence is equal to the angle of reflection. Thus:
∠i = ∠r.
These laws apply to reflection from reflecting surfaces.
∠i = ∠r.
These laws apply to reflection from reflecting surfaces.
6 Marks
Q24. Explain the formation of an image by a plane mirror.
A plane mirror reflects rays from an object. The reflected rays appear to come from behind the mirror when extended backward. Therefore, the image is virtual and cannot be obtained on a screen. It is erect, same in size as the object, laterally inverted and located at the same distance behind the mirror as the object is in front.
6 Marks
Q25. Explain regular and diffuse reflection with examples.
Regular reflection occurs when parallel incident rays fall on a smooth surface and the reflected rays remain orderly. A plane mirror is an example. Diffuse reflection occurs on a rough surface. Although each point still obeys the law of reflection, the normals differ from point to point, causing reflected rays to scatter in many directions. A rough wall is an example.
6 Marks
Q26. Derive the relation n = c/v.
The refractive index of a medium with respect to vacuum is defined as the ratio of the speed of light in vacuum to the speed of light in the medium.
Therefore:
n = c/v
where c is the speed of light in vacuum and v is the speed of light in the medium.
Therefore:
n = c/v
where c is the speed of light in vacuum and v is the speed of light in the medium.
6 Marks
Q27. State and explain Snell's law.
For light passing from one transparent medium to another:
n₁ sin i = n₂ sin r
where n₁ and n₂ are refractive indices of the two media, i is the angle of incidence and r is the angle of refraction. The law quantitatively relates the bending of light to the optical properties of the media.
n₁ sin i = n₂ sin r
where n₁ and n₂ are refractive indices of the two media, i is the angle of incidence and r is the angle of refraction. The law quantitatively relates the bending of light to the optical properties of the media.
6 Marks
Q28. Explain the factors responsible for bending of light at a boundary.
The main factor is the change in speed of light when it enters another medium. The direction changes when the ray strikes the boundary obliquely. The amount of bending depends on the optical properties/refractive indices of the two media and the angle of incidence.
6 Marks
Q29. Explain why the bottom of a swimming pool appears raised.
Light from the bottom travels from water, an optically denser medium, into air, an optically rarer medium. The rays bend away from the normal. When the eye traces these rays backward, they appear to originate from a point above the actual bottom. Therefore, the bottom appears raised and the pool appears shallower.
6 Marks
Q30. Explain the major concepts of introduction to light.
Light is electromagnetic energy that enables vision. In a homogeneous transparent medium it travels in straight lines. Reflection is the return of light into the same medium, while refraction is the change in direction caused by a change in speed when light enters another medium. Reflection obeys ∠i = ∠r. Refraction is described by Snell's law and refractive index n = c/v. These concepts form the foundation for the study of mirrors and lenses.
25. Assertion–Reason Questions
Q1.
Assertion: The angle of incidence is equal to the angle of reflection.
Reason: This is one of the laws of reflection.
Reason: This is one of the laws of reflection.
Answer: Both Assertion and Reason are true, and Reason correctly explains Assertion.
Q2.
Assertion: A ray bends towards the normal when it travels from air to glass obliquely.
Reason: Glass is optically denser than air.
Reason: Glass is optically denser than air.
Answer: Both Assertion and Reason are true, and Reason correctly explains Assertion.
Q3.
Assertion: A plane mirror forms a real image.
Reason: Reflected rays actually meet behind the mirror.
Reason: Reflected rays actually meet behind the mirror.
Answer: Both Assertion and Reason are false.
Q4.
Assertion: A pencil partly immersed in water appears bent.
Reason: Light is refracted at the water-air boundary.
Reason: Light is refracted at the water-air boundary.
Answer: Both Assertion and Reason are true, and Reason correctly explains Assertion.
Q5.
Assertion: Optical density and mass density are exactly the same concept.
Reason: Optical density is related to the speed of light in a medium.
Reason: Optical density is related to the speed of light in a medium.
Answer: Assertion is false, but Reason is true.
26. HOTS / Competency-Based Questions
HOTS 1.
A ray strikes a plane mirror at an angle of incidence of 35°. What is the angle between the incident and reflected rays?
Angle of reflection = 35°.
The angle between incident and reflected rays = i + r = 35° + 35° = 70°.
The angle between incident and reflected rays = i + r = 35° + 35° = 70°.
HOTS 2.
A medium has refractive index 1.5. If the speed of light in vacuum is 3 × 10⁸ m/s, calculate the speed of light in the medium.
n = c/v
Therefore, v = c/n
v = (3 × 10⁸)/1.5
v = 2 × 10⁸ m/s.
Therefore, v = c/n
v = (3 × 10⁸)/1.5
v = 2 × 10⁸ m/s.
HOTS 3.
Why does a coin placed at the bottom of a water-filled container become visible when water is added, even though it was not visible before?
Refraction at the water-air interface changes the direction of light rays from the coin. The refracted rays can enter the observer's eyes, making the coin appear to be at a shallower position.
HOTS 4.
Why does a rectangular glass slab not change the overall direction of an emergent ray, although the ray bends twice?
The two opposite faces of the slab are parallel. The ray bends towards the normal at the first surface and away from the normal at the second surface by corresponding amounts. Hence the emergent ray becomes parallel to the incident ray, though laterally displaced.
HOTS 5.
A student measures an angle of reflection from the mirror surface instead of the normal. Will the value equal the angle of incidence?
No. The law of reflection compares both angles with the normal, not with the mirror surface. If the angle with the surface is θ, the corresponding angle with the normal is 90° − θ.
27. Important Numericals | महत्वपूर्ण संख्यात्मक प्रश्न
Numerical 1.
The angle of incidence is 50°. Find the angle of reflection.
According to law of reflection:
∠i = ∠r
Therefore:
∠r = 50°
Numerical 2.
The refractive index of glass is 1.5. Calculate the speed of light in glass if c = 3 × 10⁸ m/s.
n = c/v
Therefore:
v = c/n
v = 3 × 10⁸ / 1.5
v = 2 × 10⁸ m/s
Numerical 3.
A medium has speed of light 2.0 × 10⁸ m/s. Find its refractive index.
n = c/v
n = (3 × 10⁸)/(2 × 10⁸)
n = 1.5
Numerical 4.
The real depth of a liquid is 12 cm and its refractive index is 1.5. Find the apparent depth.
n = Real Depth / Apparent Depth
Therefore:
Apparent Depth = 12/1.5
= 8 cm
Numerical 5.
A ray is incident on a plane mirror at 30°. Find the angle between the incident and reflected rays.
i = 30°
r = 30°
Angle between incident and reflected rays:
i + r = 30° + 30°
= 60°
28. Diagram Labelling Practice
📐 Reflection Diagram – Label the Parts
Label these:
- Incident Ray
- Reflected Ray
- Normal
- Point of Incidence
- Angle of Incidence (i)
- Angle of Reflection (r)
- Reflecting Surface
29. CBSE Golden Points ⭐
- Light is a form of electromagnetic energy.
- Light generally travels in a straight line in a homogeneous transparent medium.
- Reflection means return of light into the same medium.
- Angle of incidence is measured from the normal.
- Angle of reflection is also measured from the normal.
- ∠i = ∠r.
- Incident ray, reflected ray and normal lie in the same plane.
- Regular reflection occurs on smooth surfaces.
- Diffuse reflection occurs on rough surfaces.
- Plane mirror forms a virtual and erect image.
- Plane mirror produces lateral inversion.
- Refraction occurs because the speed of light changes between media.
- Rarer → denser: bends towards normal.
- Denser → rarer: bends away from normal.
- n = c/v.
- Snell's law: n₁ sin i = n₂ sin r.
- Optical density should not be confused with mass density.
- A rectangular glass slab produces lateral displacement.
- Emergent ray from a rectangular glass slab is parallel to the incident ray.
- Apparent depth is less than real depth when an underwater object is viewed from air.
30. Memory Tricks 🧠
REFLECTION = SAME MEDIUM
Light hits surface → Returns to same medium → Reflection → i = r
Light hits surface → Returns to same medium → Reflection → i = r
REFRACTION = SPEED CHANGE
New Medium → Speed Changes → Direction may Change → Refraction
New Medium → Speed Changes → Direction may Change → Refraction
RARER → DENSER
TOWARDS NORMAL
DENSER → RARER
AWAY FROM NORMAL
TOWARDS NORMAL
DENSER → RARER
AWAY FROM NORMAL
REFRACTIVE INDEX TRICK:
n = c/v
Higher n → Lower speed of light
n = c/v
Higher n → Lower speed of light
31. One-Line Revision
Light → Straight-Line Propagation
Reflection → Same Medium → ∠i = ∠r
Refraction → Speed Change → Bending
n = c/v
n₁ sin i = n₂ sin r
Reflection → Same Medium → ∠i = ∠r
Refraction → Speed Change → Bending
n = c/v
n₁ sin i = n₂ sin r
32. Final Summary | अंतिम सारांश
Light is electromagnetic energy that enables us to see objects.
In a homogeneous transparent medium, light generally travels in a straight line. When light strikes a reflecting surface and returns into the same medium, the phenomenon is called reflection. Reflection obeys two laws, the most important being:
∠i = ∠r.
When light passes obliquely from one transparent medium to another, its speed changes and its direction may change. This phenomenon is called refraction.
The refractive index is:
n = c/v
and refraction between two media is described by:
n₁ sin i = n₂ sin r.
These fundamental ideas form the basis for understanding spherical mirrors, lenses, image formation and optical instruments.
FINAL EXAM FORMULA:
💡 Light ↓ 📏 Straight-Line Propagation ↓ 🪞 Reflection ↓ ∠i = ∠r ↓ 🔦 Refraction ↓ Speed Changes ↓ n = c/v ↓ n₁ sin i = n₂ sin r
💡 Light ↓ 📏 Straight-Line Propagation ↓ 🪞 Reflection ↓ ∠i = ∠r ↓ 🔦 Refraction ↓ Speed Changes ↓ n = c/v ↓ n₁ sin i = n₂ sin r
💡 Reflection and Refraction | Introduction to Light
Class 10 Science | CBSE + Foundation + Competitive Level
© Educational Study Material
Class 10 Science | CBSE + Foundation + Competitive Level
© Educational Study Material
