Human Eye And Colourful World Test

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Human Eye And Colourful World Test - 3

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Weekly Quiz Competition

Question 1
1 / -0

An equilateral prism is made of a material of refractive index . The angle of minimum deviation for the prism is

A
90°

B
60°

C
45°

D
30°

Solution

Equilateral prism has all angles equal to 60° each.
Let a prism ABC be taken.

For minimum angle of deviation, the refractive index of material of prism is given by:

n =

Since prism is an equilateral one, its angle is 60°.

Given, n =

sin
(∵ )

= 120° - 60°

= 60°

Hence, minimum angle of deviation is equal to angle of prism.
Question 2
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A student can distinctly see the object up to a distance 15 cm. He wants to see the blackboard at a distance of 3 cm. The focal length and power of lens used will be

A
-4.8 cm and -3.3 D

B
-5.8 cm and -4.3 D

C
-7.5 cm and 6.3 D

D
-15.7 cm and -6.33 D

Solution

The student should use a lens which forms image at distance f = 15 cm of the object placed at 3 m.u = -3 m = -300 cm

v = -15 cm

Using lens formula, we have

Power of the lens is
Question 3
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Directions: In the following question, two statements are given. One is Assertion and the other is Reason. Examine the statements carefully and mark the correct answer accordingly.

Assertion: Blue colour of sky appears due to scattering of blue colour.
Reason: Blue colour has the shortest wavelength in the visible spectrum.

A
Both Assertion and Reason are true and Reason is the correct explanation of Assertion.

B
Both Assertion and Reason are true but Reason is not the correct explanation of Assertion.

C
Assertion is true but Reason is false.

D
Assertion is false but Reason is true.

Solution

From Rayleigh criteria of scattering, we have
Scattering ∝
Since, the wavelength of blue colour is least as compared to the other colours, it gets scattered the most by the gases in the atmosphere and the sky appears blue.
Hence, both Assertion and Reason are true and Reason is the correct explanation of Assertion.
Question 4
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A doctor prescribes spectacles to a patient with a combination of a convex lens of focal length 40 cm and concave lens of focal length 25 cm. The power of spectacles will be

A
– 6.5 D

B
1.5 D

C
– 1.5 D

D
– 8.5D

Solution

Power of combination is addition of reciprocal of focal lengths of individual lenses.
The power of a thin lens is equal to the reciprocal of its focal length (f) measured in metre.
P =
When two lenses are combined, focal length of combination is given by:
; where f₁ = 40 cm, f₂ = –25 cm

Hence, power – ^{= -1.5 D}
Note: Since power is negative, the combination behaves like a diverging lens.
Question 5
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What is the increasing order of the extent of angle changes undergone by the colours of the spectrum during splitting of white light?

A
Red < Yellow < Blue

B
Yellow < Blue < Red

C
Red < Blue < Yellow

D
Blue < Yellow < Red

Solution

The refractive index of the medium depends upon the wavelength of the light. The greater is the wavelength, the lesser is the refractive index, hence lesser deviation. The wavelength of red colour is maximum, followed by yellow, and blue colour has the least wavelength.
Hence, red colour will form the least angle, followed by yellow, and the angle made by the blue with the direction of the incident ray will be maximum.
Question 6
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_______ is the dark muscular diaphragm that controls the size of the pupil.

A
Aqueous humour

B
Ciliary muscle

C
Cornea

D
Iris

Solution

Iris controls the size of the pupil.
Iris is a pigmented muscular curtain near the front of the eye, between the cornea and the lens. The iris is located in front of the lens and ciliary body and behind the cornea. The iris consists of two sheets of smooth muscle with contrary actions: dilation (expansion) and contraction (constriction). These muscles control the size of the pupil and thus determine how much light reaches the sensory tissue of the retina.
Question 7
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An eye specialist prescribes spectacles having a combination of convex lens of focal length 50 cm in contact with a concave lens of focal length 25 cm. The power of this combination of lenses (in dioptre) is

A
+2.5 D

B
-2.0 D

C
-2.5 D

D
+2.0 D

Solution

For the combination of two lenses, effective focal length is
Here f₁ = 50 cm, f₂ = - 25 cm
Question 8
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Which of the following defects can be rectified by using cylindrical lenses?

A
Myopia

B
Presbyopia

C
Astigmatism

D
Hypermetropia

Solution

Astigmatism, in which cornea has an abnormal curvature, can be rectified by cylindrical lenses.
Question 9
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Which of the following phenomena is used in formation of rainbow

A
dispersion

B
total internal reflection

C
Both of these

D
None of these

Solution

Water droplets in the atmosphere behave as tiny prisms. When white light enters the water droplets, it splits into its constituent colours due to dispersion whereas total internal reflection takes place at the rear surface of the water droplet. Hence, we can conclude that the formation of rainbow is due to atmospheric refraction.
Question 10
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The change in focal length of an eye lens is caused by the action of

A
pupil

B
retina

C
ciliary muscles

D
iris

Solution

The change in focal length of an eye lens is caused by the action of ciliary muscles. The cilliary muscle changes the curvature of lens present in eye hence it is responsible for change in focal length of eye lens.
Question 11
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White colour of the clouds is due to

A
reflection of seven colours of light

B
refraction of seven colours of light

C
scattering of seven colours of light

D
absorption of seven colours of light

Solution

Large dust particles and the water droplets present in the clouds scatter all the wavelengths almost equally. Hence, clouds appear white.
Therefore, option (3) is correct.
Question 12
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White light is incident on the interface of the glass and air as shown in the figure. If the green light is just totally internally reflected, the emerging ray in the air contains

A
yellow, orange, red

B
violet, indigo, blue

C
all the colours, except green

D
all the colours

Solution

The critical angle of incidence is given by sini_c=
Refractive index is dependent upon the wavelength. The greater the wavelength, the lesser will be the refractive index which means greater the wavelength, lesser will be the refractive index and hence, greater will be the critical angle of incidence. The critical angle for violet, indigo and blue will be less than the critical angle of incidence of the green colour, hence these colours will undergo total internal reflection whereas the critical angle for orange, yellow and red colours will be more as their wavelengths are more than the green colour and these colours will undergo normal refraction.
Question 13
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The far point of myopic person is 40 cm. To see distant objects clearly, the focal length and the power of the lens used should be

A
-40 cm, -2.5 D

B
-25 cm, -4.0 D

C
40 cm, 2.5 D

D
-40 cm, +2.5 D

Solution

Here, v = -40 cm, u =

f = -40 cm

= -2.5 D
Question 14
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For a prism, the angle of minimum deviation is directly proportional to the

A
wavelength of light used

B
refractive index of the prism material

C
size of the prism

D
frequency of light used

Solution

Angle of minimum deviation is directly proportional to refractive index. Refractive index for red is least while for violet is maximum. Refractive index is inversely proportional to the wavelength of the light. As red has a higher wavelength than violet light, it has a smaller refractive index than violet.
Question 15
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The Sun is visible a little before the actual sunrise because of

A
atmospheric reflection

B
atmospheric dispersion

C
atmospheric diffraction

D
atmospheric refraction

Solution

This observation is because of atmospheric refraction. When the Sun is slightly below the horizon, the light coming from it travels from less dense to more dense air, and is refracted downwards. Thus, the Sun can be seen two minutes before actual sunrise and remains visible for two minutes after actual sunset.
Question 16
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For a defective eye, the power of lens required is –2 D. What is the distance beyond which he is not able to see the objects clearly?

A
50 cm

B
55 cm

C
60 cm

D
100 cm

Solution

Power, P = – 2 D
Or f = m
This is the case of myopic eye, where the person is suffering from short sightedness and the far point is less than infinity. We need far point after which vision is not possible and where the image of an object kept at infinity has to be formed.
u = – (infinity)
v = ?
f = – 0.5 cm
(Lens formula)
Or 1/v + 0 = - 1/0.5 or v = - 0.5 m = - 50 cm
The distance beyond which he is not able to see the objects clearly is 50 cm.
Question 17
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The blind spot refers to the area where __________.

A
cone cells are absent

B
rod cells are absent

C
Both (1) and (2)

D
None of these

Solution

The blind spot refers to the area, where both rod cells and cone cells are absent. Because there are no cells to detect light on the optic disc, the corresponding part of the field of vision is invisible. Hence, option (3) is correct.
Question 18
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Which of the following options represents the angle of deviation for a ray of light passing through a prism?

A
A₁

B
A₂

C
A₃

D
A₄

Solution

A light ray PQ is incident on the refracting surface DE of the prism at an incident angle i.
After refraction of the ray at Q, it bends towards the normal N₁ and travels along QR.
The refracted ray QR again suffers refraction at R and bends away from the normal N₂ and travels along RS.
The ray RS is called emergent ray. The angle made by the emergent ray with the normal is called angle of emergence .
When the emergent ray is produced backwards, it meets the incident ray produced at point G.
The angle between the emergent ray and the incident ray is called angle of deviation ().
Angle of deviation is the angle through which incident ray is turned by the prism while passing through it.

Hence, option (3) is correct.
Question 19
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One cannot see through fog because

A
fog absorbs light

B
light is scattered by the droplets in the fog

C
light suffers total reflection in the droplets in the fog

D
the refractive index of fog is infinity

Solution

One cannot see through fog because light is scattered by the droplets in the fog.
Scattering of light is the phenomenon in which when light passes through a medium like fog, then a part of the light is absorbed by particles of the medium preceded by its subsequent radiation in a particular direction.
Question 20
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Which of the following statements is incorrect?

A
Human eye is a refracting system containing a diverging lens.

B
The retina of the human eye contains millions of light sensitive cells, called rods and cones, which convert the light into electrical messages.

C
Every image that is focused on the retina is upside down.

D
We need both eyes to judge the relative positions of objects accurately.

Solution

Refraction is the phenomenon which makes image formation possible by the eyes. Our eyes comprise of convex lens, which is a converging lens. When parallel rays of light pass through a convex lens, the refracted rays converge at one point called the principal focus.