CBSE Class 10 Science Human eye and colourful world Sure Shot Questions A

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THE HUMAN EYE AND THE COLOURFUL WORLD

Question. A prism ABC (with BC as base) is placed in different orientations. A narrow beam of white light is incident on the prism as shown in figure. In which of the following cases, after dispersion, the third colour from the top corresponds to the colour of the sky?
Answer : (ii) Explanation: Inverted position of prism (II) gives the same colour (blue) at third top as that of sky.

Question. A beam of white light falling on a glass prism gets split up into seven colours marked 1 to 7 as shown in the diagram. A student makes the following statements about the spectrum observed on the screen:
(a) The colours at positions marked 3 and 5 are similar to the colour of the sky and the colour of gold metal respectively. Is the above statement made by the student correct or incorrect?
Justify.
(b) Which two positions correspond closely to the colour of:
(i) Brinjal (ii) ‘danger’ or stop signal lights?
Answer : The position marked on number 3rd is YELLOW. Yellow is not the colour of the sky. Similarly, the position marked on number 5th is BLUE. Blue is not the colour of the core of the hardboiled egg. Hence, the statement made by the student of no 3rd as the colour of sky and no. 5th as the colour of the core of the hard-boiled egg is incorrect.

Question. A person with myopic eye cannot see objects beyond 1.2 metre distinctly. What should be the nature of corrective lenses to restore proper vision?
Answer : To correct, the object at infinity has to be brought to image at 1.2 m or
v = - 120 cm, u = - ∞
1/f = 1/v − 1/u
1/f = 1/−120 − 1/∞
∴ f = - 120 cm
P = 100/f = 100/−120 = -0.83 D

Question. A glass prism is able to produce a spectrum when white light passes through it but a glass slab does not produce any spectrum. Explain. Why is it so?
Answer : A glass prism is able to produce a spectrum, which is a patch of light obtained due to dispersion. This is because the opposite refracting faces of a glass prism are not parallel. Thus, the incident ray is not parallel to the emergent ray and is deviated by the angle of deviation.
A glass slab is a rectangular structure in which the opposite refracting faces are parallel to each other. The result is that, the emergent is parallel to the incident ray but is literally displaced from it.
Thus, a rectangular glass slab is unable to produce a spectrum due to the refracting faces being parallel to each other.

Question. Why does the sky appear dark instead of blue to an astronaut?
Answer : The sky appears dark instead of blue to an astronaut because there is no atmosphere containing air in the outer space to scatter sunlight. As there is no scattered light to reach our eyes in outer space, so the sky appears dark.

Question. Name the component of white light that deviates the least and the component that deviates the most while passing through a glass prism.
Answer : Least deviated component: Red Most deviated component: Blue

Question. Why there is no dispersion of light refracted through a rectangular glass slab?
Answer : After refraction at two parallel faces of a glass slab, a ray of light emerges in a direction parallel to the direction of incidence of white light. As rays of all colours emerge in same direction, i.e., the direction of the incidence of white light, there is no dispersion. However, there is a lateral displacement.

Question. When a monochromic light having only one wavelength, passes through a prism, will it show dispersion?
Answer : No, it will not show dispersion. It will only show deviation.

Question. The sun can be seen about two minutes before actual sunrise and two minutes after actual sunset. Why?
Answer : The sun can be seen about two minutes before actual sunrise and two minutes after actual sunset because atmospheric refraction.

Question. Is the position of a star as seen by us in its true position? Justify your answer.
Answer : No, light from stars undergoes atmospheric refraction which occurs in medium of gradually changing refractive index. So, we see the apparent position of the star after refraction by atmosphere.

Question. Why does the Sun appear white at noon?
Answer : The light is least scattered at noon.

Question. Draw a ray diagram showing the dispersion through a prism when a narrow beam of white light is incident on one of its refracting surfaces. Also, indicate the order of the colours of the spectrum obtained
Answer :

 Question. Why does the Sun appear reddish early in the morning?
Answer : In the early morning, the sun is situated near the horizon. Light from the Sun passes through thicker layers of air and cover larger distance before reaching our eyes. Therefore, blue light scattered the most and red light least. This is why the sun appears reddish early in the morning.

 Question. The far point of a myopic the person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?
Answer : To correct the myopia the person concerned should use concave lens (diverging lens) of suitable focal length.
For myopic eye: Far point of normal eye = u = at infinity ⇒ u = - ∞ The virtual image is formed at the far point of myopic person ⇒ v = - 80 cm
From lens formula,
1/f = 1/v − 1/u
1/f = 1/−80 − 1/∞
∴ f = - 80 cm
P = 100/f = 100/−80
= -1.25 D

 Question. How will you use two identical prisms so that a narrow beam of white light incident on one prism emerges out of the second prism as white light?
Answer : By using two identical prisms, one placed inverted with respect to other, we get a narrow beam of white light incident on one prism emerges out the second as white light.

 Question. Draw a neat diagram to show the refraction of a light ray through a glass prism, and label it.
Answer :

Question. How does refraction take place in the atmosphere? Why do stars twinkle but not the planets?
Answer : Atmosphere is made up of several layers. The layer at the top is optically rare, while the layer at the bottom is optically denser. Due to this, when light travels through different layers of the atmosphere, refraction takes place. Since light passes through denser and denser layer as it moves through atmosphere, it tends to bend towards the normal.
Stars are very far from us; compared to planet. Due to this, stars serve as point source of light. As a result, even a slightest change in their apparent position in the sky is clearly perceived by us. Hence,
stars appear to twinkle.
Planets on the other hand, are near to us. Hence, they do not serve as point source of light. Hence, minor changes in their apparent position are not perceived by us. Hence, planets do not appear to twinkle.

Question. i) Demonstrate an activity with a well labelled diagram to prove that white light is made up of seven colours.
ii) Which colour of light bends least and which one the most while passing out from the prism?
Also, state the reason for the same.
Answer : i) The phenomenon of splitting of a beam of white light into its seven constituent colours when passed through a transparent medium is known as dispersion, which was first discovered by Issac Newton in 1666. To understand this phenomenon, let us take a thick sheet of cardboard and make a small hole or narrow slit in its middle allow sunlight to fall on the slit. This gives a narrow beam of white light. Now, take a glass prism and allow this white light to fall one of its faces as shown in figure. Turn the prism slowly until the light that comes out of it appears on a nearby screen.
We see a beautiful band of seven colours on the screen called visible spectrum. The sequence of colours seen from the lower part of the screen is violet (V), indigo (I), blue (B), green (G), yellow (Y), orange (O) and red (R). The acronym for this is VIBGYOR.

ii) Red light has the maximum wavelength and violet light has the minimum wavelength, so in a transparent medium except air and vacuum, red light having the largest wavelength suffers the least deviation while violet light having the least wavelength bends the most.

Question. Read the following and answer any four questions from 16 (i) to 16 (v)
And when light rays pass through the atmosphere having air layers of different optical densities, then refraction of light takes place. The refraction of light caused by the earth's atmosphere (having air layers of varying optical densities) is called atmospheric refraction. When light from any source passed through a glass prism gets refracted and scattered in constitute colours (VIBGYOR). Blue colour has least wavelength and red colour has highest wavelength. Bending of red colour is least while of blue colour is highest.

Question. Which of the following is correct decreasing order of wavelength
a) Red>blue>orange>indigo
b) Blue>green>yellow>orange
c) Red>orange>yellow>blue
d) Blue>indigo>violet>red
Answer : C

Question. Why red colour light is used in train during fog?
a) It has low wavelength and least scattered
b) It has high wavelength and highly scattered
c) It has high wavelength and least scattered
d) It has low wavelength and highly scattered
Answer : C

Question. In the above figure, angle of prism, incident angle, angle of deviation, and angle of emergence are represented respectively as:
a) 1,3,5,4
b) 2,6,5,3
c) 5,4,1,3
d) 3,1,4,5
Answer : D

Question. Out of red, green, blue and violet, which colour deviates the most in glass prism
a) Red
b) Green
c) Blue
d) Violet
Answer : D

Question. A white light falls on a surface of a glass prism. Four students A, B, C, D recorded the sequence of colours from top to bottom of screen. Which of the student recorded the correct sequence?
a) Student A: violet, blue, green, yellow, red
b) Student B: red, blue, green, yellow, violet
c) Student C: red, yellow, green, blue, violet
d) Student D: red, green, yellow, blue, violet
Answer : C

Question. The danger signals installed at the top of tall buildings are red in colour. These can be easily seen from a distance because among all other colours, the red light
(a) is scattered the most by smoke or fog
(b) is scattered the least by smoke or fog
(c) is absorbed the most by smoke or fog
(d) moves fastest in air
Answer : B

Question. When light rays enter the eye, most of the refraction occurs at the
(a) crystalline lens
(b) outer surface of the cornea
(c) iris
(d) pupil
Answer : B

1. A man can read the number of a distant bus clearly but he finds difficulty in reading a book. From which defect of the eye is suffering from?

2. What type of spectacles should be worn by a person having the defects of myopia as well as hypermetropia? How does it help?

3. The sun near the horizon appears flattened at the sun set and sun rise. Explain why.

4. Explain why and when the sun is overhead at noon it appears white

5. A boy uses spectacles of focal length -50 cm. Name the defect of vision he is suffering from. Compute the power of this lens.

6. Give the meaning of the term ‚ VIBGYOR‛ with which phenomenon is it connected?

7. Explain the following terms connected with the eye. (i) Ciliary muscles (ii) Accommodation.

8. What is meant by spectrum of white light?

9. What will be colour of the sky in the absence of atmosphere?

10. Why are the traffic light signals (or danger signals) of red colour?

11. Why does the sky appear dark and black to an astronaut instead of blue?

12. Explain why, when the sun is overhead at noon, it appears white?

13. What is Atmospheric Refraction?

14. A person with myopic eye cannot see objects beyond 1.2metre distinctly. What should be the nature of corrective lenses to restore  roper vision?

15. The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?

16. The far point of myopic person is 80 cm in front of the eye. What is the nature an power of the lens required to enable him to see very distant objects distinctly?

17. The far point of a myopic person is 150 cm in front the eye. Calculate the focal length and power of a lens required to enable him to see distant objects clearly.

18. How is the eye lens held in its position?

19. What is meant by near point?

20. What is meant by least distance of distinct vision?

21. Which part of the eye controls the amount of the light entering the eye?

22. Which liquid fills the space behind the cornea?

49. Your school laboratory has one large window. To find the focal length of a concave mirror using one of the walls as the screen, the experiment may be perfomed.

(a) Near the wall opposite to the window.

(b) On the same wall as the window

(c) On the wall adjacent to the window

(d) Only on the table as per the laboratory arrangement

 

50. A students obtains a blurr image of a object on a screen by using a concave mirror. In order to obtain a sharp image on the screen, he will have to shift the mirror :–

(a) towards the screen

(b) away from the screen

(c) either towards or away from the screen depending upon the position of the object

(d) to a position very far away from the screen

 

51. The focal length of the concave mirror in an experimental setup shown below, is

(a) 10.2cm

(b) 11.0cm

(c) 11.4cm

(d) 12.2cm

 

52. Four students, Ameeta, Zahira, Ravi and David performed the experiment for determination of focal length of a concave mirror separately. They measured the distance between the screen and the mirror as shown in the following diagram.

Which one of these students is likely to get the correct value of focal length of the concave mirror? (a) Ameeta (b) Zahira (c) Ravi (d) David

 

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