Light
Chapter 16 MCQs
For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.
1
A ray of light strikes a plane mirror. The angle between the incident ray and the mirror surface is 35°.
What is the angle of reflection?
A 35°
B 55°
C 70°
D 110°
2
A ray of light is incident on a plane mirror. The angle between the incident ray and the reflected ray is 84°.
What is the angle of incidence?
A 21°
B 42°
C 84°
D 96°
3
A plane mirror is rotated through 15° while the incident ray remains fixed.
Through what angle does the reflected ray rotate?
A 7.5°
B 15°
C 30°
D 45°
4
An object is placed 0.80 m in front of a plane mirror.
How far is the image from the object?
A 0.40 m
B 0.80 m
C 1.20 m
D 1.60 m
5
A student stands 1.5 m in front of a plane mirror. She walks 0.40 m towards the mirror.
By how much does the distance between the student and her image decrease?
A 0.40 m
B 0.80 m
C 1.10 m
D 1.90 m
6
Which row correctly describes the image formed by a plane mirror?
| image type | size | distance from mirror | |
|---|---|---|---|
| A | real | same size | same as object distance |
| B | virtual | same size | same as object distance |
| C | virtual | smaller | half object distance |
| D | real | larger | twice object distance |
7
A letter F is placed in front of a plane mirror.
Which change occurs in the image?
A top and bottom are reversed only
B left and right appear reversed
C the image is upside down
D the image is smaller than the object
8
A ray of light travels from air into glass at an angle to the normal.
Which row is correct?
| speed | direction | |
|---|---|---|
| A | decreases | towards normal |
| B | decreases | away from normal |
| C | increases | towards normal |
| D | increases | away from normal |
9
A ray of light travels from glass into air at an angle to the normal and does not undergo total internal reflection.
Which row is correct?
| speed | direction | |
|---|---|---|
| A | decreases | towards normal |
| B | decreases | away from normal |
| C | increases | towards normal |
| D | increases | away from normal |
10
A ray of light passes from air into glass. The angle of incidence is 50° and the angle of refraction is 31°.
What is the refractive index of the glass?
A sin 31° / sin 50°
B sin 50° / sin 31°
C 50 / 31
D 31 / 50
For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.
11
Light travels from air into a transparent block. The angle of incidence is 40° and the angle of refraction is 25°.
Use sin 40° = 0.643 and sin 25° = 0.423.
What is the refractive index of the block?
A 0.658
B 1.09
C 1.52
D 2.37
12
A ray of light travels from air into a liquid of refractive index 1.33. The angle of incidence is 45°.
Use sin 45° = 0.707.
What is sin r, where r is the angle of refraction?
A 0.532
B 0.707
C 0.940
D 1.88
13
Light travels in air at 3.0 × 10⁸ m/s. It enters glass of refractive index 1.50.
What is the speed of light in the glass?
A 2.0 × 10⁸ m/s
B 3.0 × 10⁸ m/s
C 4.5 × 10⁸ m/s
D 6.0 × 10⁸ m/s
14
The speed of light in a transparent material is 2.25 × 10⁸ m/s.
Take speed of light in air as 3.0 × 10⁸ m/s.
What is the refractive index of the material?
A 0.75
B 1.33
C 2.25
D 5.25
15
A ray of light enters a rectangular glass block from air. It emerges from the opposite parallel face.
Which statement is correct?
A emergent ray is parallel to incident ray but laterally displaced
B emergent ray is not parallel and not displaced
C emergent ray bends towards the normal on leaving
D emergent ray travels back along its original path
16
A ray passes through a rectangular glass block. Inside the glass, the ray makes an angle of 24° with the normal at the first face.
At the second face, the ray leaves into air.
Which statement is correct?
A angle of emergence is less than 24°
B angle of emergence is equal to 24°
C angle of emergence is greater than 24°
D angle of emergence is always 90°
17
A glass block has refractive index 1.60.
What is the sine of the critical angle for light travelling from glass to air?
A 0.375
B 0.625
C 1.60
D 2.56
18
A transparent material has critical angle 42°.
Use sin 42° = 0.669.
What is the refractive index of the material?
A 0.669
B 1.00
C 1.49
D 2.98
19
Light travels from glass into air. The critical angle of the glass is 42°.
For which angle of incidence does total internal reflection occur?
A 0°
B 30°
C 42°
D 50°
20
Which conditions are both needed for total internal reflection?
A light travels from less dense to more dense medium, and incidence angle is less than critical angle
B light travels from more optically dense to less optically dense medium, and incidence angle is greater than critical angle
C light travels from less dense to more dense medium, and incidence angle is greater than critical angle
D light travels from more dense to less dense medium, and incidence angle is less than critical angle
21
A ray in glass strikes the glass-air boundary at the critical angle.
Which statement is correct?
A the ray emerges along the boundary
B the ray reflects back with no refracted ray
C the ray passes into air along the normal
D the ray stops at the boundary
22
The critical angle of glass is 42°. A ray in glass strikes the glass-air boundary at an angle of incidence of 35°.
What happens?
A total internal reflection occurs
B the ray refracts into air away from the normal
C the ray refracts into air towards the normal
D the ray emerges along the boundary
23
A ray of light travels inside an optical fibre. It repeatedly reflects from the inner surface.
What condition must be met at each reflection?
A angle of incidence is less than critical angle
B angle of incidence is equal to zero
C angle of incidence is greater than critical angle
D angle of reflection is greater than angle of incidence
24
Why is total internal reflection useful in optical fibres?
A it allows light to escape gradually through the sides
B it keeps light trapped inside the core with very little loss
C it changes infrared radiation into sound
D it makes the speed of light zero
25
An optical fibre has a core of refractive index 1.48 and cladding of refractive index 1.40.
Why is cladding needed?
A to make the outside refractive index slightly lower and allow total internal reflection at the core boundary
B to make the fibre opaque
C to make the critical angle impossible to reach
D to absorb all the light that reaches the boundary
For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.
26
A ray travels from glass into air. The refractive index of glass is 1.50.
Use sin 41.8° = 0.667 and sin 48.2° = 0.746.
Which is closest to the critical angle?
A 33.3°
B 41.8°
C 48.2°
D 60.0°
27
A ray of light enters a semicircular glass block through the curved surface along a radius.
Why is there no refraction at the curved surface?
A the ray travels along the normal at the point of entry
B curved surfaces cannot refract light
C the glass has refractive index 1
D the ray is totally internally reflected immediately
28
A ray inside a semicircular glass block reaches the flat surface. The critical angle of the glass is 42°.
Which row is correct?
| angle of incidence at flat surface | result |
|---|---|
| A | 30° |
| B | 42° |
| C | 50° |
| D | 60° |
29
A coin at the bottom of a pool appears shallower than it really is because:
A light from the coin bends away from the normal as it leaves water
B light from the coin bends towards the normal as it leaves water
C the coin emits less light in water
D water absorbs all reflected light from the coin
30
A pool appears to have depth 1.2 m. The refractive index of water is 1.33.
What is the approximate real depth?
A 0.90 m
B 1.2 m
C 1.6 m
D 2.5 m
31
A glass block has real thickness 6.0 cm. It appears to have thickness 4.0 cm when viewed from above.
What is the refractive index of the glass?
A 0.67
B 1.5
C 2.0
D 10
32
A converging lens is used to focus parallel rays of light.
Where do rays parallel to the principal axis meet after passing through the lens?
A at the centre of the lens
B at the principal focus
C at twice the focal length always
D at the object position
33
A ray passes through the optical centre of a thin converging lens.
What happens to the ray?
A it passes through undeviated
B it reflects back along its path
C it bends through the principal focus
D it becomes parallel to the principal axis
34
A ray parallel to the principal axis strikes a converging lens.
After passing through the lens, it:
A passes through the principal focus
B passes through the optical centre undeviated
C becomes parallel to the axis on the other side
D reflects back from the lens surface
35
A ray passing through the principal focus before reaching a converging lens emerges:
A through the optical centre
B parallel to the principal axis
C reflected back along the same path
D at 90° to the principal axis
36
A converging lens has focal length 10 cm. An object is placed 30 cm from the lens.
What type of image is formed?
A real, inverted and diminished
B real, inverted and magnified
C virtual, upright and magnified
D virtual, inverted and diminished
37
A converging lens has focal length 10 cm. An object is placed 15 cm from the lens.
Which description of the image is correct?
A real, inverted and magnified
B real, inverted and diminished
C virtual, upright and magnified
D no image is formed
38
A converging lens has focal length 10 cm. An object is placed 5.0 cm from the lens.
Which description of the image is correct?
A real, inverted and diminished
B real, inverted and same size
C virtual, upright and magnified
D real, upright and magnified
39
A converging lens forms a real image of an object placed beyond 2F.
Where is the image formed?
A between F and 2F on the other side
B at F on the same side
C beyond 2F on the other side
D between lens and F on the same side
40
A converging lens forms an image of an object placed between F and 2F.
Which row is correct?
| image position | image nature |
|---|---|
| A | beyond 2F |
| B | between F and 2F |
| C | same side as object |
| D | at F |
For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.
41
A converging lens forms an image of height 6.0 cm from an object of height 2.0 cm.
What is the magnification?
A 0.33
B 3.0
C 4.0
D 8.0
42
A lens forms an image of height 1.5 cm. The object height is 6.0 cm.
What is the magnification?
A 0.25
B 1.5
C 4.0
D 9.0
43
A converging lens forms an image 24 cm from the lens. The object is 8.0 cm from the lens.
What is the magnification?
A 0.33
B 3.0
C 16
D 32
44
A converging lens is used as a magnifying glass.
Where must the object be placed?
A beyond 2F
B at 2F
C between F and 2F
D between the lens and F
45
Which lens is used to correct short-sightedness?
A converging lens because the eye focuses behind the retina
B diverging lens because the eye focuses in front of the retina
C converging lens because the eye focuses in front of the retina
D diverging lens because the eye focuses behind the retina
46
Which lens is used to correct long-sightedness?
A diverging lens because near objects focus behind the retina
B converging lens because near objects focus behind the retina
C diverging lens because distant objects focus in front of the retina
D converging lens because distant objects focus in front of the retina
47
White light passes through a glass prism and forms a spectrum.
Why does this happen?
A all colours travel at the same speed in glass
B different colours are refracted by different amounts
C the prism absorbs red light only
D white light contains only one wavelength
48
White light is dispersed by a prism.
Which colour is usually deviated the most?
A red
B yellow
C green
D violet
49
Which colour of visible light has the longest wavelength?
A red
B green
C blue
D violet
50
A red ray and a violet ray enter a glass prism from air at the same angle.
Which statement is correct?
A red is refracted more because it has longer wavelength
B violet is refracted more because it has a higher refractive index in glass
C red and violet are refracted equally because both are visible light
D violet is not refracted because it is electromagnetic radiation
For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.
Chapter 16 Answer Key
| Q | Ans | Q | Ans | Q | Ans | Q | Ans | Q | Ans |
|---|---|---|---|---|---|---|---|---|---|
| 1 | B | 11 | C | 21 | A | 31 | B | 41 | B |
| 2 | B | 12 | A | 22 | B | 32 | B | 42 | A |
| 3 | C | 13 | A | 23 | C | 33 | A | 43 | B |
| 4 | D | 14 | B | 24 | B | 34 | A | 44 | D |
| 5 | B | 15 | A | 25 | A | 35 | B | 45 | B |
| 6 | B | 16 | C | 26 | B | 36 | A | 46 | B |
| 7 | B | 17 | B | 27 | A | 37 | A | 47 | B |
| 8 | A | 18 | C | 28 | B | 38 | C | 48 | D |
| 9 | D | 19 | D | 29 | A | 39 | A | 49 | A |
| 10 | B | 20 | B | 30 | C | 40 | A | 50 | B |
For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.
Detailed Explanations
1. B
-
Angles of incidence and reflection are measured from the normal, not the mirror surface.
-
Angle between incident ray and mirror surface = 35°.
-
Angle of incidence = 90° − 35° = 55°.
-
Angle of reflection = angle of incidence = 55°.
2. B
-
For a plane mirror:
-
angle of incidence = angle of reflection
-
-
Angle between incident ray and reflected ray = 2i.
-
2i = 84°.
-
i = 42°.
3. C
-
When a plane mirror rotates by θ, the reflected ray rotates by 2θ.
-
Mirror rotation = 15°.
-
Reflected ray rotation = 2 × 15° = 30°.
4. D
-
In a plane mirror:
-
image distance behind mirror = object distance in front of mirror
-
-
Object is 0.80 m in front.
-
Image is 0.80 m behind.
-
Distance from object to image = 0.80 + 0.80 = 1.60 m.
5. B
-
Initial distance from mirror = 1.5 m.
-
Initial distance between student and image = 3.0 m.
-
New distance from mirror = 1.5 − 0.40 = 1.10 m.
-
New distance between student and image = 2.20 m.
-
Decrease = 3.0 − 2.20 = 0.80 m.
-
Trap: image also moves by 0.40 m.
6. B
-
Image in a plane mirror is:
-
virtual
-
upright
-
same size as object
-
same distance behind mirror as object is in front
-
7. B
-
A plane mirror produces lateral inversion.
-
Left and right appear reversed.
-
The image is not upside down.
-
The image is the same size.
8. A
-
Light enters glass from air.
-
Glass is optically denser than air.
-
Light slows down and bends towards the normal.
9. D
-
Light travels from glass to air.
-
Air is optically less dense.
-
Light speeds up and bends away from the normal.
10. B
-
Refractive index:
-
n = sin i / sin r
-
-
i = 50°.
-
r = 31°.
-
n = sin 50° / sin 31°.
11. C
-
n = sin i / sin r
-
n = sin 40° / sin 25°
-
n = 0.643 / 0.423
-
n = 1.52
12. A
-
n = sin i / sin r
-
1.33 = 0.707 / sin r
-
sin r = 0.707 / 1.33
-
sin r = 0.532
13. A
-
Refractive index:
-
n = speed of light in air / speed in material
-
-
speed in glass = 3.0 × 10⁸ / 1.50
-
speed = 2.0 × 10⁸ m/s
14. B
-
n = speed in air / speed in material
-
n = 3.0 × 10⁸ / 2.25 × 10⁸
-
n = 1.33
15. A
-
In a rectangular glass block:
-
light bends towards the normal entering
-
bends away from the normal leaving
-
-
The emergent ray is parallel to the incident ray but laterally displaced.
16. C
-
At the second face, light goes from glass to air.
-
It bends away from the normal.
-
So the angle of emergence is greater than 24°.
17. B
-
For light travelling from glass to air:
-
sin c = 1 / n
-
-
sin c = 1 / 1.60
-
sin c = 0.625
18. C
-
n = 1 / sin c
-
n = 1 / 0.669
-
n = 1.49
19. D
-
Total internal reflection occurs when:
-
light travels from glass to air
-
angle of incidence is greater than critical angle
-
-
Critical angle = 42°.
-
50° is greater than 42°.
-
Answer = 50°.
20. B
-
Total internal reflection requires:
-
light travelling from more optically dense to less optically dense medium
-
angle of incidence greater than the critical angle
-
For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.
21. A
-
At the critical angle, the refracted ray travels along the boundary.
-
Angle of refraction = 90°.
-
If incidence angle becomes greater than this, total internal reflection occurs.
22. B
-
Critical angle = 42°.
-
Incidence angle = 35°.
-
Since 35° is less than the critical angle, total internal reflection does not occur.
-
The ray refracts into air.
-
Going from glass to air, it bends away from the normal.
23. C
-
In an optical fibre, light is kept inside by total internal reflection.
-
At each internal reflection:
-
angle of incidence must be greater than critical angle.
-
24. B
-
Total internal reflection keeps light trapped inside the fibre core.
-
This allows light signals to travel long distances with little loss.
-
Optical fibres are used in communication and medical endoscopes.
25. A
-
The fibre core has higher refractive index than the cladding.
-
This allows light to go from more optically dense core to less optically dense cladding.
-
If incidence angle is large enough, total internal reflection occurs.
-
Cladding also protects the core and reduces signal loss.
26. B
-
Critical angle:
-
sin c = 1 / n
-
-
sin c = 1 / 1.50 = 0.667
-
Given sin 41.8° = 0.667.
-
Critical angle = 41.8°.
27. A
-
A ray entering through the curved surface along a radius strikes the surface normally.
-
If the ray is along the normal, angle of incidence = 0°.
-
So no refraction occurs at the curved surface.
28. B
-
Critical angle = 42°.
-
At exactly the critical angle, the ray emerges along the surface.
-
At angles greater than 42°, total internal reflection occurs.
-
So 42° gives ray along surface.
29. A
-
Light from the coin travels from water to air.
-
It bends away from the normal.
-
The eye traces rays backwards in straight lines.
-
This makes the coin appear shallower than it really is.
30. C
-
Refractive index approximately:
-
n = real depth / apparent depth
-
-
real depth = n × apparent depth
-
real depth = 1.33 × 1.2
-
real depth = 1.6 m
31. B
-
n = real thickness / apparent thickness
-
n = 6.0 / 4.0
-
n = 1.5
32. B
-
Parallel rays entering a converging lens parallel to the principal axis meet at the principal focus.
-
The focal length is the distance from lens centre to this focus.
33. A
-
A ray passing through the optical centre of a thin lens passes through undeviated.
-
This is one of the standard lens construction rays.
34. A
-
A ray parallel to the principal axis passes through the principal focus after refraction by a converging lens.
35. B
-
A ray passing through the principal focus before reaching a converging lens emerges parallel to the principal axis.
-
This is the reverse of the parallel-ray rule.
36. A
-
Focal length = 10 cm.
-
2F = 20 cm.
-
Object distance = 30 cm, so object is beyond 2F.
-
Image is:
-
real
-
inverted
-
diminished
-
formed between F and 2F
-
37. A
-
Focal length = 10 cm.
-
Object distance = 15 cm, so object is between F and 2F.
-
Image is:
-
real
-
inverted
-
magnified
-
formed beyond 2F
-
38. C
-
Focal length = 10 cm.
-
Object distance = 5.0 cm, so object is between lens and F.
-
Image is:
-
virtual
-
upright
-
magnified
-
-
This is how a magnifying glass works.
39. A
-
For a converging lens:
-
object beyond 2F gives image between F and 2F
-
-
The image is real, inverted and diminished.
40. A
-
Object between F and 2F gives:
-
image beyond 2F
-
real
-
inverted
-
magnified
-
For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.
41. B
-
Magnification = image height / object height
-
M = 6.0 / 2.0
-
M = 3.0
42. A
-
Magnification = image height / object height
-
M = 1.5 / 6.0
-
M = 0.25
-
Since M < 1, image is diminished.
43. B
-
Magnification = image distance / object distance
-
M = 24 / 8.0
-
M = 3.0
44. D
-
A converging lens works as a magnifying glass when the object is placed between the lens and the principal focus.
-
The image formed is virtual, upright and magnified.
45. B
-
Short-sightedness:
-
distant objects focus in front of the retina
-
-
Correction:
-
diverging lens spreads rays before they enter the eye
-
image then focuses on the retina
-
46. B
-
Long-sightedness:
-
near objects focus behind the retina
-
-
Correction:
-
converging lens brings rays together before they enter the eye
-
image then focuses on the retina
-
47. B
-
White light contains different colours.
-
Different colours travel at different speeds in glass.
-
They are refracted by different amounts.
-
This separates white light into a spectrum.
48. D
-
Violet light is deviated the most by a glass prism.
-
Red light is deviated the least.
-
Order of deviation: red least, violet most.
49. A
-
Red light has the longest wavelength in the visible spectrum.
-
Violet has the shortest wavelength.
50. B
-
Violet light is refracted more than red light in glass.
-
This is because glass has a higher refractive index for violet light than for red light.
-
Therefore violet deviates more in a prism.
For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.
Common Traps From This Chapter
| Trap | Correct Rule |
|---|---|
| Reflection angles | measured from the normal |
| Angle with mirror surface | subtract from 90° |
| Plane mirror image | virtual, upright, same size |
| Object-image distance | twice object-mirror distance |
| Mirror rotates θ | reflected ray rotates 2θ |
| Air to glass | slows, bends towards normal |
| Glass to air | speeds up, bends away from normal |
| Refractive index | n = sin i / sin r |
| Speed in medium | v = c / n |
| Rectangular block | emergent ray parallel but displaced |
| Critical angle | sin c = 1/n |
| At critical angle | ray emerges along boundary |
| Total internal reflection | denser to less dense and i > c |
| Optical fibre | uses total internal reflection |
| Apparent depth | appears less than real depth |
| Converging lens: object beyond 2F | real, inverted, diminished |
| Object between F and 2F | real, inverted, magnified |
| Object between lens and F | virtual, upright, magnified |
| Magnification | image height / object height |
| Short-sightedness | diverging lens |
| Long-sightedness | converging lens |
| Prism dispersion | different colours refracted differently |
| Red light | longest visible wavelength, least deviated |
| Violet light | shortest visible wavelength, most deviated |