CLASS X - PHYSICS
Chapter 9: Light Reflection and Refraction
Time: 2 Hours | Total Marks: 80
Instructions:
- All questions are compulsory.
- Section A: 1 mark each (20 questions)
- Section B: 2 marks each (6 questions)
- Section C: 3 marks each (6 questions)
- Section D: 5 marks each (2 questions)
- Show your working for numerical problems.
SECTION A - 1 Mark Questions (20 × 1 = 20 marks)
1M
1.
What is the SI unit of focal length?
1M
2.
A mirror that curves inward is called a _____ mirror.
1M
3.
The relationship f = R/2 is true for which optical element?
1M
4.
What is the value of refractive index for vacuum?
1M
5.
A virtual image can be projected on a screen. True or False?
1M
6.
The power of a lens is measured in _____ (SI unit).
1M
7.
Which law of reflection states that angle of incidence equals angle of reflection?
1M
8.
Refractive index of diamond is approximately _____ .
1M
9.
A convex lens always forms which type of image?
a) Real and inverted
b) Virtual and erect
c) Real or virtual depending on object position
d) Always inverted and reduced
1M
10.
The phenomenon of bending of light when it passes from one medium to another is called _____.
1M
11.
What is the magnification of a plane mirror?
1M
12.
The centre of curvature is represented by the letter _____.
1M
13.
Which lens is used to correct nearsightedness?
a) Convex lens
b) Concave lens
c) Plane lens
d) Bifocal lens
1M
14.
Snell's law is represented as: n₁ sin(i) = n₂ sin(r). What does 'i' represent?
1M
15.
The speed of light in a medium with refractive index 1.5 is _____ m/s.
1M
16.
A concave mirror produces a virtual image when the object is placed _____.
1M
17.
The relationship between focal length and radius of curvature is: f = R/_____
1M
18.
Which optical element has zero power?
a) Convex lens
b) Plane lens
c) Concave lens
d) Concave mirror
1M
19.
When light travels from denser to rarer medium, it bends _____ the normal.
1M
20.
The magnification formula for mirrors is m = _____.
SECTION B - 2 Mark Questions (6 × 2 = 12 marks)
2M
21.
State the laws of reflection of light.
2M
22.
Differentiate between real and virtual images formed by mirrors.
2M
23.
A concave mirror has a focal length of 15 cm. Calculate its radius of curvature.
2M
24.
A lens has a power of +2 diopters. Find its focal length in centimeters.
2M
25.
Draw a ray diagram showing image formation by a convex lens when the object is placed at 2F.
2M
26.
Explain why light bends when it travels from one medium to another.
SECTION C - 3 Mark Questions (6 × 3 = 18 marks)
3M
27.
A concave mirror with focal length 10 cm forms an image at 20 cm from the mirror. Calculate the object distance and magnification. State the nature of the image.
3M
28.
Light travels from air (n = 1) to glass (n = 1.5) at an angle of incidence of 30°. Calculate the angle of refraction using Snell's law.
3M
29.
An object is placed 30 cm in front of a convex lens of focal length 10 cm. Calculate the image distance, magnification, and describe the nature of the image.
3M
30.
Compare the properties of concave and convex mirrors. Create a comparison table.
3M
31.
A lens has a power of -4 diopters. Find its focal length and state which type of lens it is. What is its magnifying property?
3M
32.
An eyeglass lens has focal length -25 cm. Calculate its power in diopters. What is the name of the defect it corrects?
SECTION D - 5 Mark Questions (2 × 5 = 10 marks)
5M
33.
Explain the formation of images by a concave mirror when the object is placed at different positions. Draw ray diagrams for each case: (a) beyond 2F, (b) at 2F, (c) between F and 2F, (d) between P and F, (e) at F.
5M
34.
Explain refraction of light with the help of Snell's law. Derive the formula and explain what happens when: (a) light goes from rarer to denser medium, (b) light goes from denser to rarer medium. Give real-life examples of refraction.
Total: 80 Marks
CLASS X - PHYSICS
Answer Key: Chapter 9 - Light Reflection and Refraction
Total Marks: 80
SECTION A - Answer Key (20 marks)
1. Meter (m)
Focal length is the distance from the mirror/lens to the focal point, measured in meters.
2. Concave
A concave mirror curves inward and is also called a converging mirror.
3. Mirror (Concave or Convex)
The relationship f = R/2 applies to all curved mirrors, where f is focal length and R is radius of curvature.
4. 1
Refractive index n = c/v. For vacuum, v = c (speed of light in vacuum), so n = 1.
5. False
Virtual images cannot be projected on a screen because they are formed by diverging light rays. Only real images can be projected.
6. Diopter (D)
Power P = 1/f (in meters). If f is in cm, divide by 100. Unit: Diopter (D) = m⁻¹
7. First law of reflection (or Second law of reflection)
The first law states: angle of incidence = angle of reflection. The second law states: rays are in the same plane.
8. 2.42
Diamond has the highest refractive index among transparent materials, causing it to sparkle brilliantly.
9. c) Real or virtual depending on object position
Convex lenses form real images when object is beyond F, and virtual images when object is between P and F.
10. Refraction
Refraction is the bending of light at the boundary when it changes from one medium to another.
11. +1 or 1
Plane mirror always produces images with magnification = 1, meaning same size as object.
12. C
C represents the centre of curvature. The radius of curvature R is the distance from P to C.
13. b) Concave lens
Concave (diverging) lenses spread out light rays, correcting myopia (nearsightedness).
14. Angle of incidence
In Snell's law: n₁sin(i) = n₂sin(r), 'i' is angle of incidence and 'r' is angle of refraction.
15. 2 × 10^8 (or 200,000,000)
v = c/n = (3 × 10^8)/1.5 = 2 × 10^8 m/s
16. Between the pole and the focal point (or at F or between P and F)
When object is between P and F of concave mirror, virtual, erect, and magnified image is formed.
17. 2
The relationship is f = R/2. For a spherical mirror, focal length is half the radius of curvature.
18. b) Plane lens
A plane (flat) lens has zero power because P = 1/f and f = infinity for plane surface.
19. Away from
When light goes from denser to rarer medium (higher to lower n), it bends away from the normal.
20. -v/u (or h'/h)
Magnification m = h'/h = -v/u where h' is image height, h is object height, v is image distance, u is object distance.
SECTION B - Answer Key (12 marks)
21. Laws of Reflection:
First Law: The incident ray, reflected ray, and normal all lie in the same plane.
Second Law: The angle of incidence is equal to the angle of reflection (∠i = ∠r).
Both angles are measured from the normal to the reflecting surface.
Second Law: The angle of incidence is equal to the angle of reflection (∠i = ∠r).
Both angles are measured from the normal to the reflecting surface.
22. Real vs Virtual Images:
Real Images: Formed by actual convergence of light rays; can be projected on screen; inverted; formed by concave mirrors when object is beyond F.
Virtual Images: Formed by divergence of light rays (appear to come from behind); cannot be projected; erect; formed by convex mirrors and plane mirrors; formed by concave mirrors when object is between P and F.
Virtual Images: Formed by divergence of light rays (appear to come from behind); cannot be projected; erect; formed by convex mirrors and plane mirrors; formed by concave mirrors when object is between P and F.
23. Radius of Curvature Calculation:
Given: f = 15 cm
Using: f = R/2
Therefore: R = 2f = 2 × 15 = 30 cm
Answer: Radius of curvature = 30 cm
Using: f = R/2
Therefore: R = 2f = 2 × 15 = 30 cm
Answer: Radius of curvature = 30 cm
24. Focal Length from Power:
Given: Power P = +2 diopters
Using: P = 1/f (where f is in meters)
Therefore: f = 1/P = 1/2 = 0.5 m = 50 cm
Answer: Focal length = 50 cm (or 0.5 m)
Using: P = 1/f (where f is in meters)
Therefore: f = 1/P = 1/2 = 0.5 m = 50 cm
Answer: Focal length = 50 cm (or 0.5 m)
25. Ray Diagram - Convex Lens with Object at 2F:
When object is at 2F of a convex lens:
- Three rays: (1) Parallel to axis → through F (2) Through optical center → straight (3) Through F → parallel to axis
- Image formed: At 2F on opposite side
- Image properties: Real, inverted, same size as object (m = -1)
- Three rays: (1) Parallel to axis → through F (2) Through optical center → straight (3) Through F → parallel to axis
- Image formed: At 2F on opposite side
- Image properties: Real, inverted, same size as object (m = -1)
26. Why Light Bends (Refraction):
When light travels from one medium to another:
- Speed of light changes (c in vacuum, v in medium where v = c/n)
- Change in speed causes change in direction (wavelength changes, frequency remains constant)
- Denser medium → light slows down → bends toward normal
- Rarer medium → light speeds up → bends away from normal
The amount of bending depends on the refractive indices of the two media.
- Speed of light changes (c in vacuum, v in medium where v = c/n)
- Change in speed causes change in direction (wavelength changes, frequency remains constant)
- Denser medium → light slows down → bends toward normal
- Rarer medium → light speeds up → bends away from normal
The amount of bending depends on the refractive indices of the two media.
SECTION C - Answer Key (18 marks)
27. Concave Mirror Calculation:
Given: f = 10 cm, v = 20 cm (on the same side as object for real image)
Using Mirror Formula: 1/f = 1/v + 1/u
1/10 = 1/20 + 1/u
1/u = 1/10 - 1/20 = (2-1)/20 = 1/20
u = 20 cm
Object distance: 20 cm
Magnification: m = -v/u = -20/20 = -1
Nature of Image: Real, inverted, same size as object (at 2F of mirror)
Using Mirror Formula: 1/f = 1/v + 1/u
1/10 = 1/20 + 1/u
1/u = 1/10 - 1/20 = (2-1)/20 = 1/20
u = 20 cm
Object distance: 20 cm
Magnification: m = -v/u = -20/20 = -1
Nature of Image: Real, inverted, same size as object (at 2F of mirror)
28. Snell's Law Refraction Calculation:
Given: n₁ = 1 (air), n₂ = 1.5 (glass), θ₁ = 30°
Using Snell's Law: n₁sin(θ₁) = n₂sin(θ₂)
1 × sin(30°) = 1.5 × sin(θ₂)
1 × 0.5 = 1.5 × sin(θ₂)
sin(θ₂) = 0.5/1.5 = 1/3 = 0.333
θ₂ = arcsin(0.333) ≈ 19.5° (or approximately 19° 28')
Angle of refraction ≈ 19.5°
Note: Light bends toward the normal when entering denser medium.
Using Snell's Law: n₁sin(θ₁) = n₂sin(θ₂)
1 × sin(30°) = 1.5 × sin(θ₂)
1 × 0.5 = 1.5 × sin(θ₂)
sin(θ₂) = 0.5/1.5 = 1/3 = 0.333
θ₂ = arcsin(0.333) ≈ 19.5° (or approximately 19° 28')
Angle of refraction ≈ 19.5°
Note: Light bends toward the normal when entering denser medium.
29. Convex Lens Calculation:
Given: u = 30 cm, f = 10 cm
Using Lens Formula: 1/f = 1/v - 1/u
1/10 = 1/v - 1/(-30) [u is negative for convex lens convention]
1/10 = 1/v + 1/30
1/v = 1/10 - 1/30 = (3-1)/30 = 2/30 = 1/15
v = 15 cm
Image distance: 15 cm (on opposite side of object)
Magnification: m = v/u = 15/30 = 0.5 (or -1/2)
Nature of Image: Real, inverted, diminished (half the size of object)
Using Lens Formula: 1/f = 1/v - 1/u
1/10 = 1/v - 1/(-30) [u is negative for convex lens convention]
1/10 = 1/v + 1/30
1/v = 1/10 - 1/30 = (3-1)/30 = 2/30 = 1/15
v = 15 cm
Image distance: 15 cm (on opposite side of object)
Magnification: m = v/u = 15/30 = 0.5 (or -1/2)
Nature of Image: Real, inverted, diminished (half the size of object)
30. Concave vs Convex Mirror Comparison:
Concave Mirror (Converging):
- Curves inward
- Forms real, inverted images (when object beyond F)
- Also forms virtual, erect images (when object between P and F)
- f and R are positive
- Used in telescopes, headlamps, shaving mirrors
Convex Mirror (Diverging):
- Curves outward
- Always forms virtual, erect, diminished images
- f and R are negative
- Used in rear-view mirrors, security mirrors
- Larger field of view
- Curves inward
- Forms real, inverted images (when object beyond F)
- Also forms virtual, erect images (when object between P and F)
- f and R are positive
- Used in telescopes, headlamps, shaving mirrors
Convex Mirror (Diverging):
- Curves outward
- Always forms virtual, erect, diminished images
- f and R are negative
- Used in rear-view mirrors, security mirrors
- Larger field of view
31. Lens Power and Type:
Given: Power P = -4 diopters
Focal length: f = 1/P = 1/(-4) = -0.25 m = -25 cm
Type of Lens: Concave (diverging) lens because f is negative
Magnifying Property: Concave lenses always produce virtual, erect, diminished images. Magnification is always less than 1 and the image is always smaller than the object. Used to correct myopia (nearsightedness).
Focal length: f = 1/P = 1/(-4) = -0.25 m = -25 cm
Type of Lens: Concave (diverging) lens because f is negative
Magnifying Property: Concave lenses always produce virtual, erect, diminished images. Magnification is always less than 1 and the image is always smaller than the object. Used to correct myopia (nearsightedness).
32. Eyeglass Lens Calculation:
Given: f = -25 cm = -0.25 m
Power: P = 1/f = 1/(-0.25) = -4 diopters
Defect Corrected: Myopia (Nearsightedness)
- People with myopia can see near objects clearly but far objects appear blurry
- The concave lens diverges light rays to compensate for the eye's excessive converging power
- This allows the image to fall directly on the retina
Power: P = 1/f = 1/(-0.25) = -4 diopters
Defect Corrected: Myopia (Nearsightedness)
- People with myopia can see near objects clearly but far objects appear blurry
- The concave lens diverges light rays to compensate for the eye's excessive converging power
- This allows the image to fall directly on the retina
SECTION D - Answer Key (10 marks)
33. Image Formation by Concave Mirror:
(a) Object beyond 2F:
- Image: Real, inverted, diminished (between F and 2F)
- Used in telescopes
(b) Object at 2F:
- Image: Real, inverted, same size, at 2F
(c) Object between F and 2F:
- Image: Real, inverted, magnified (beyond 2F)
(d) Object between P and F:
- Image: Virtual, erect, magnified, behind mirror
- Used in shaving/makeup mirrors
(e) Object at F:
- Image: At infinity (highly magnified real image)
- Used in searchlights
Ray Diagrams: Each should show three principal rays: (1) Parallel to axis through F, (2) Through optical center along axis, (3) Through F parallel to axis after reflection.
- Image: Real, inverted, diminished (between F and 2F)
- Used in telescopes
(b) Object at 2F:
- Image: Real, inverted, same size, at 2F
(c) Object between F and 2F:
- Image: Real, inverted, magnified (beyond 2F)
(d) Object between P and F:
- Image: Virtual, erect, magnified, behind mirror
- Used in shaving/makeup mirrors
(e) Object at F:
- Image: At infinity (highly magnified real image)
- Used in searchlights
Ray Diagrams: Each should show three principal rays: (1) Parallel to axis through F, (2) Through optical center along axis, (3) Through F parallel to axis after reflection.
34. Refraction and Snell's Law:
Definition: Refraction is the bending of light when it passes from one medium to another.
Snell's Law: n₁sin(θ₁) = n₂sin(θ₂)
Where n₁, n₂ are refractive indices and θ₁, θ₂ are angles from normal.
Derivation Concept: When light enters a denser medium, its speed decreases (v = c/n), causing the wavelength to decrease while frequency remains constant. This change in speed causes the direction to change.
(a) Rarer to Denser Medium:
- Light slows down (v₂ < v₁)
- Bends toward normal
- Angle of refraction < angle of incidence
- Example: Light entering water from air
(b) Denser to Rarer Medium:
- Light speeds up (v₂ > v₁)
- Bends away from normal
- Angle of refraction > angle of incidence
- Example: Light leaving water to air (can have total internal reflection)
Real-Life Examples:
1. Pencil appears bent in water (refraction at water surface)
2. Objects underwater appear closer than actual distance
3. Rainbow formation (refraction + dispersion in water droplets)
4. Diamonds sparkle (high refractive index, total internal reflection)
5. Mirage on hot roads (refraction in hot air layers)
Snell's Law: n₁sin(θ₁) = n₂sin(θ₂)
Where n₁, n₂ are refractive indices and θ₁, θ₂ are angles from normal.
Derivation Concept: When light enters a denser medium, its speed decreases (v = c/n), causing the wavelength to decrease while frequency remains constant. This change in speed causes the direction to change.
(a) Rarer to Denser Medium:
- Light slows down (v₂ < v₁)
- Bends toward normal
- Angle of refraction < angle of incidence
- Example: Light entering water from air
(b) Denser to Rarer Medium:
- Light speeds up (v₂ > v₁)
- Bends away from normal
- Angle of refraction > angle of incidence
- Example: Light leaving water to air (can have total internal reflection)
Real-Life Examples:
1. Pencil appears bent in water (refraction at water surface)
2. Objects underwater appear closer than actual distance
3. Rainbow formation (refraction + dispersion in water droplets)
4. Diamonds sparkle (high refractive index, total internal reflection)
5. Mirage on hot roads (refraction in hot air layers)
Total Marks: 80
(Section A: 20 + Section B: 12 + Section C: 18 + Section D: 10 = 80)
(Section A: 20 + Section B: 12 + Section C: 18 + Section D: 10 = 80)
