Ray Optics and Optical Instruments Easy Questions and Answers | Page - 1

Ray Optics and Optical Instruments

Wavelength of light of frequency 100 Hz

1. 2 × 10⁶ m
1. 3 × 10⁶ m
1. 4 × 10⁶ m
1. 5 × 10⁶ m

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Frequency (n) = 100 Hz
v = n λ
λ = 3 × 10⁸
100

[where, velocity of light (v) = 3 × 10⁸ m/s]
λ = 3 × 10⁶ m

Correct Option: B

Frequency (n) = 100 Hz
v = n λ
λ = 3 × 10⁸
100

[where, velocity of light (v) = 3 × 10⁸ m/s]
λ = 3 × 10⁶ m

One face of a rectangular glass plate 6 cm thick is silvered. An object held 8 cm in front of the first face forms an image 12 cm behind the silvered face. The refractive index of the glass is

1. 0.4
1. 0.8
1. 1.2
1. 1.6

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Thickness of glass plate (t) = 6 cm;
Distance of the object (u) = 8 cm.
And distance of the image (v) = 12 cm.
Let x = Apparent position of the silvered surface in cm.
Since the image is formed due to reflection at the silvered face and by the property of mirror image
Distance of object from the mirror = Distance of image from the mirror
or, x + 8 = 12 + 6 – x ⇒ x = 5 cm.
Therefore, refractive index of glass = Real depth = 6 = 1.2
Apparent depth 5

Correct Option: C

Thickness of glass plate (t) = 6 cm;
Distance of the object (u) = 8 cm.
And distance of the image (v) = 12 cm.
Let x = Apparent position of the silvered surface in cm.
Since the image is formed due to reflection at the silvered face and by the property of mirror image
Distance of object from the mirror = Distance of image from the mirror
or, x + 8 = 12 + 6 – x ⇒ x = 5 cm.
Therefore, refractive index of glass = Real depth = 6 = 1.2
Apparent depth 5

Light travels through a glass plate of thickness t and refractive index µ. If c is the speed of light in vacuum, the time taken by light to travel this thickness of glass is

1. µtc
1. tc
  μ
1. t
  μc
1. μt
  c

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Total thickness = t; Refrative index = µ
Speed of light in Glass plate = c
μ

∵ v = Speed of light in vacuum
R.I. of medium

Time taken = t = μt
c c
μ

[where, t = thickness of glass plate]

Correct Option: D

Total thickness = t; Refrative index = µ
Speed of light in Glass plate = c
μ

∵ v = Speed of light in vacuum
R.I. of medium

Time taken = t = μt
c c
μ

[where, t = thickness of glass plate]

An electromagnetic radiation of frequency n, wavelength λ, travelling with velocity v in air enters in a glass slab of refractive index (µ). The frequency, wavelength and velocity of light in the glass slab will be respectively

1. n , λ and v
  μ μ
1. n , 2λ and v
  μ
1. n , λ and v
  μ μ μ
1. 2π , λ and v
  μ μ

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When electromagnetic wave enters in other medium, frequency reamains unchanged while wavelength and
velocity become 1 times.
μ

So, For e.m. wave entering from air to glass slab (µ), frequency remains n,
wavelength, λ' = λ
μ

and velocity, v' = v
μ

Correct Option: A

When electromagnetic wave enters in other medium, frequency reamains unchanged while wavelength and
velocity become 1 times.
μ

So, For e.m. wave entering from air to glass slab (µ), frequency remains n,
wavelength, λ' = λ
μ

and velocity, v' = v
μ

A point source of light is placed 4 m below the surface of water of refractive index 5 .
3

The minimum diameter of a disc, which should be placed over the source, on the surface of water to cut off all light coming out of water is

1. ∞
1. 6 m
1. 4 m
1. 3 m

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sin C = 1 = 1 = 3 ⇒ tan C = 3
μ 5 5 4
3

Now, tan C = r ; r = h tan C = 4 × 3 = 3 m
h 4

Diameter of disc = 2r = 6 m

Correct Option: B

sin C = 1 = 1 = 3 ⇒ tan C = 3
μ 5 5 4
3

Now, tan C = r ; r = h tan C = 4 × 3 = 3 m
h 4

Diameter of disc = 2r = 6 m