NCERT Exemplar: The Human Eye & the Colourful World | Science Class 10 PDF Download

Multiple Choice Questions

Q.1. A person cannot see distinctly objects kept beyond 2 m. This defect can be corrected by using a lens of power
(a) + 0.5 D
(b) – 0.5 D
(c) + 0.2 D
(d) – 0.2 D

Correct Answer is Option (b)

Q.2. A student sitting on the last bench can read the letters written on the blackboard but is not able to read the letters written in his text book. Which of the following statements is correct?
(a) The near point of his eyes has receded away
(b) The near point of his eyes has come closer to him
(c) The far point of his eyes has come closer to him
(d) The far point of his eyes has receded away

Correct Answer is Option (a)

Q.3. 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 11.1. In which of the following cases, after dispersion, the third colour from the top corresponds to the colour of the sky?
(a) (i)
(b) (ii)
(c) (iii)
(d) (iv)

Correct Answer is Option (b)

Q.4. At noon the sun appears white as
(a) light is least scattered
(b) all the colours of the white light are scattered away
(c) blue colour is scattered the most
(d) red colour is scattered the most

Correct Answer is Option (a)

Q.5. Which of the following phenomena of light are involved in the formation of a rainbow?
(a) Reflection, refraction and dispersion
(b) Refraction,  dispersion and  total internal reflection
(c) Refraction, dispersion and internal reflection
(d) Dispersion, scattering and total internal reflection

Correct Answer is Option (c)

Q.6. Twinkling of stars is due to atmospheric
(a) dispersion of light by water droplets
(b) refraction of light by different layers of varying refractive indices
(c) scattering of light by dust particles
(d) internal reflection of light by clouds

Correct Answer is Option (b)

Q.7. The clear sky appears blue because
(a) blue light gets absorbed in the atmosphere
(b) ultraviolet radiations are absorbed in the atmosphere
(c) violet and blue lights get scattered more than lights of all other colours by the atmosphere
(d) light of all other colours is scattered more than the violet and blue colour lights by the atmosphere

Correct Answer is Option (c)

Q.8. Which of the following statements is correct regarding the propagation of light of different colours of white light in air?
(a) Red light moves fastest
(b) Blue light moves faster than green light
(c) All the colours of the white light move with the same speed
(d) Yellow light moves with the mean speed as that of the red and the violet light

Correct Answer is Option (c)

Q.9. 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

Correct Answer is Option (b)

Q.10. Which of the following phenomena contributes significantly to the reddish appearance of the sun at sunrise or sunset?
(a) Dispersion of light
(b) Scattering of light
(c) Total internal reflection of light
(d) Reflection of light from the earth

Correct Answer is Option (b)

Q.11. The bluish colour of water in deep sea is due to
(a) the presence of algae and other plants found in water
(b) reflection of sky in water
(c) scattering of light
(d) absorption of light by the sea

Correct Answer is Option (c)

Q.12. 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

Correct Answer is Option (b)

Q.13. The focal length of the eye lens increases when eye muscles
(a) are relaxed and lens becomes thinner
(b) contract and lens becomes thicker
(c) are relaxed and lens becomes thicker
(d) contract and lens becomes thinner

Correct Answer is Option (a)

Q.14. Which  of the following statement is correct?
(a) A person with myopia can see distant objects clearly
(b) A person with hypermetropia can see nearby objects clearly
(c) A person with myopia can see nearby objects clearly
(d) A person with hypermetropia cannot see distant objects clearly

Correct Answer is Option (c)

Q.15. Draw ray diagrams each showing (i) myopic eye  and (ii) hypermetropic eye.

Q.16. A student sitting at the back of the classroom cannot read clearly the letters written on the blackboard. What advice will a doctor give to him? Draw ray diagram for the correction of this defect.

• If a student cannot see the blackboard distance to her she is suffering from myopia.
• Myopia is the inability of an eye in viewing long distant objects.
• The image in this case is formed before the retina.
• The short-sightedness is corrected by using a concave lens which diverges and shifts the image to the retina.
• Hence, the doctor advises a concave lens of suitable focal length.

Ray diagram for the correction of this defect

Q.17. How are we able to see the nearby as well as distant objects clearly?

This is due to the ability of the eye lens to adjust its focal length which is known as accommodation. When the ciliary muscles are relaxed, the lens becomes thin. Thus, its focal length increases. This enables us to see distant objects clearly. When we are looking at objects closer to the eye, the ciliary muscles contract. This increases the curvature of the eye lens. The eye lens becomes thicker. The focal length of the eye lens decreases. This enables us to see nearby objects clearly.

Q.18. A person needs a lens of power - 4.5 D for correction of his vision.
(a) What kind of defect in vision is he suffering from?
(b) What is the focal length of the corrective lens?
(c) What is the nature of the corrective lens?

(a) The defect is myopia (short-sightedness).
(b) Focal length
(c) The lens is a concave lens.

Q.19. 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 ? Draw the diagram.

Prism

• Prism is a transparent medium, that deflected light rays.
• In prism, the velocity is different for light rays of different frequencies (monochromatic light rays).
• For two identical prisms, the angle of deflection is the same.

Deflection of light rays in two mutually inverted prisms

• We know in the case of two identical prisms, the angle of deflection is the same. If two prisms are inverted with each other then the white light rays incident on the first prism will be deflected.
• And the deflected light rays again deflect by the second inverted prism in equal amounts.
• We know that the deflection angle of light is the same in a prism for a particular incident angle. The two opposite deflections of the same incident angle will cancel out, as shown in the figure. So the net deflection is zero.
• A narrow beam of white light incident on mutually inverted prisms will emerge out of white lights.
• In this phenomenon, white light is first dispersed into its seven constituent colors. When this dispersed light is incident on the second prison again, they recombine into white light.
  

Q.20. 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.

Q.21. Is the position of a star as seen by us its true position? Justify your answer.

No. light from stars undergoes atmospheric refraction which occurs in medium of gradually changing refractive index.

Q.22. Why do we see a rainbow in the sky only after rainfall?

Rainbow is formed by the dispersion of sunlight by tiny water droplets present in the atmosphere. A rainbow is always formed in a direction opposite to that of the Sun. The water droplets act like small prisms. They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop.

Q.23. Why is the colour of the clear sky blue?

A clear cloudless day-time sky is blue because molecules in the air scatter blue light from the sun more than they scatter red light. Blue colour gets scattered the maximum since the particles present in the atmosphere have size which is comparable to the wavelength of blue light.

Q.24. What is the difference in colours of the Sun observed during sunrise/sunset and noon? Give explanation for each.

Sun appears red at sunrise and sunset: At sunrise and sunset, sun is closer to the horizon. The sunlight passes through denser layer of air and covers larger distance before reaching our eyes. Most of the blue light gets scattered. And red color reaches us which make the sun appears red at sunrise. At noon, the sun is overhead. The sunlight passes through layers of air and covers shorter distance before reaching  our eye. So, almost all colours of light are scattered equally. Hence the sun appears white.

Long Answer Type Questions

Q.25. Explain the structure and functioning of Human eye. How are we able to see nearby as well as distant objects?

The human eye has the following main parts:

• Cornea: Human eye is spherical in shape. It has a tough white coat that protects the interior of the eye. The front portion of this coat is transparent and is called the cornea.
• Iris: This is a dark muscular structure behind the cornea. The unique colour of a person's eye is because of the colour of the iris.
• Pupil: The small opening in the iris is called the pupil. Iris controls the size of the pupil and thus controls the amount of light entering the eye. Light enters the eye through the pupil.
• Lens: Lens is thicker in the middle and is made of transparent material. The lens focuses the light on the back of the eye; called the retina.
• Retina: The back of the eye is called the retina. It works like a screen; on which an image is formed.
• Rods and cones function as photoreceptors in the retina of the eye, turning visible light into neuronal signals, which are sent to the brain. This process is called transduction. These nerve cells are connected to the optic nerve.
• Formation of Image in Eye: Light rays enter the eye through the pupil and pass through the lens. The lens focuses light rays on the retina. The real, inverted and smaller image is formed on the retina. The optic nerve carries the message to the brain. The brain interprets the message and we get the sense of vision.
  

Q.26. When do we consider a person to be myopic or hypermetropic?
Explain using diagrams how the  defects associated with myopic and hypermetropic eye can be corrected?

Myopia:

• Myopia is a condition when a person is not able to view an object at distance clearly. This condition is also known as short-sightedness.
• The condition occurs when the image of an object viewed by a person is formed in front of the retina rather than on it.
• This condition can be corrected using a concave lens to diverge the light rays and make them fall on the retina And hence adjust the defect.
  

Hypermetropia:

• Hypermetropia is a condition opposite to that of myopia. In this case, a person is able to view an object at distance clearly while an object that is closer is not visible clearly. This is also known as longsightedness.
• The condition occurs when the image of the object viewed is formed beyond the retina rather than on it.
• This condition is corrected using a convex lens which converges the light rays and aid in making it fall on the retina thereby adjusting the defect.

Q.27. Explain the refraction of light through a triangular glass prism using a labelled ray diagram. Hence define the angle of deviation.

Step 1: Refraction

• Refraction is the phenomenon in which when light travels from one medium to another, it changes its speed and hence its path of propagation.
• The amount of refraction can be estimated using the refractive index of the given medium.
• If c is the speed of light in vacuum and v is the speed of light in a given medium, then the refractive index of this medium is μ = c/v.

Step 2: Refraction through a Prism:

The given figure shows the passage of light through a triangular prism ABC .

The angle of incidence and refraction at the first face AB are i and r₁, while the angle of incidence (from glass to air) at the second face AC is r₂ and the angle of refraction or emergence .

Step 3: Angle of Deviation
The angle between the emergent ray and the direction of the incident ray is called the angle of deviation, δ.
The formula for angle of deviation is given as,
δ = i + e - A
where,  is the angle of the prism as shown in the figure.

Q.28. How can we explain the reddish appearance of sun at sunrise or sunset? Why does it not appear red at noon?

Sun appears red at sunrise and sunset : At sunrise and sunset, sun is closer to the horizon. The sunlight passes through denser layer of air and covers larger distance before reaching our eyes. Most of the blue light gets scattered. And red color reaches us which make the sun appears red at sunrise. During noon sunlight has to travel less distance to reach us. Because most of the colors get scattered we see sun  white in color during noon.

Q.29. Explain the phenomenon of dispersion of white light through a glass prism, using suitable ray diagram.

Dispersion: The splitting of white light into its constituent colours is called dispersion. The colour sequence is given by the acronym V I B G Y O R - Violet, Indigo, Blue, Green, Yellow, Orange, and Red. This colour pattern is called Dispersion which occurs because refraction or bending differs with the colour. The speed of light of different colours in a medium like glass, water etc. is different. Varying speed for different colours leads to different refractive indices for different colours. It has been observed that the refractive index of glass for violet colour is more than that for red colour. The speed of light, however, is the same for all colours in free space or vacuum.

Q.30. How does refraction take place in the atmosphere? Why do  stars twinkle but not the planets?

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.