Chapter 10: The Human Eye and the Colourful World

Multiple Choice Questions

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

Answer: (b) – 0.5 D

[ The power of a lens is related to its ability to focus light. In this case, since the person cannot see distinctly objects beyond 2 meters, the person is experiencing myopia or nearsightedness.

Here,

We have, 

So, the person can correct the defect by using a concave lens with a power of -0.5 diopters. The correct answer is: (b) – 0.5 D ]

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

Answer : (a) The near point of his eyes has receded away.

[ When a person can see the letters on the blackboard (which is at a distance) but has difficulty reading the letters in the textbook (which is closer), it indicates that the near point of the eyes has receded. The near point is the closest distance at which an object can be brought into focus, and it tends to increase with age or certain eye conditions.]

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)

Answer: (b)  (ii)

[ When white light passes through a prism, it is dispersed into its component colors, forming a spectrum. The dispersion occurs because different colors of light are refracted by different amounts due to their varying wavelengths. Violet light bends the most and red light bends the least. The sequence of colors from top to bottom in the spectrum is: violet, indigo, blue, green, yellow, orange, and red.The color of the sky is typically blue.]

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

Answer: (b) all the colours of the white light are scattered away.

[ When the sun is higher in the sky, the Earth's atmosphere scatters shorter wavelengths of light (blue and violet) more effectively than longer wavelengths. However, at noon, the sunlight has a shorter distance to travel through the atmosphere, and much of the shorter wavelengths get scattered, leaving the longer wavelengths (red, orange, yellow, and green) dominant. As a result, the direct sunlight appears white.]

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

Answer: (c) Refraction, dispersion, and internal reflection.

[ Rainbows are formed when sunlight is refracted, dispersed (split into its different colors), and internally reflected within raindrops in the atmosphere. This combination of phenomena leads to the separation of sunlight into its various colors and the creation of a circular arc of colors that we perceive as a rainbow.]

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

Answer: (b) Refraction of light by different layers of varying refractive indices .

[ The twinkling of stars is caused by the refraction of light as it passes through various layers of Earth's atmosphere with different temperatures and densities. These variations in refractive index cause the light from stars to bend and shift, creating the appearance of twinkling.]

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

Answer: (c) violet and blue lights get scattered more than lights of all other colors by the atmosphere.

[ During the day, when the sun is higher in the sky, the Earth's atmosphere scatters shorter wavelengths of light, such as violet and blue, more effectively than longer wavelengths. This scattering is known as Rayleigh scattering, and it causes the sky to appear predominantly blue to our eyes.]

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

Answer: (a) Red light moves fastest

[ In air, different colors of light in white light travel at slightly different speeds due to their varying wavelengths. Red light, with the longest wavelength among the visible colors, moves faster than blue or violet light, which have shorter wavelengths.]

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

Answer: (b) is scattered the least by smoke or fog

[ Red light, with its longer wavelength, is scattered the least by smoke or fog compared to other colors. This property allows red signals to be more visible and effective for safety warnings at a distance, even in poor visibility conditions.]

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

Answer: (b) Scattering of light

[ The reddish appearance of the sun at sunrise or sunset is primarily due to the scattering of light. As sunlight passes through the Earth's atmosphere at a low angle during sunrise or sunset, the shorter wavelengths (blue and violet) are scattered out of the direct path, leaving the longer wavelengths (red and orange) to dominate the color we see. This phenomenon is known as Rayleigh scattering.]

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

Answer: (d) absorption of light by the sea

[ The bluish color of water in the deep sea is primarily due to the absorption of light. Water absorbs colors at the red end of the light spectrum more efficiently than colors at the blue end. As a result, the blue light is scattered and reflected back to our eyes, giving the water its characteristic blue color.]

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

Answer: (b) outer surface of the cornea.

[ When light rays enter the eye, the majority of the refraction occurs at the outer surface of the cornea. The cornea is the transparent front part of the eye that plays a significant role in focusing light onto the retina.]

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

Answer: (b) contract and lens becomes thicker .

[ When the eye muscles contract, they make the lens thicker and more curved, which increases its focal length. This process helps the eye focus on nearby objects, a phenomenon known as accommodation.]

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

Answer: (c) A person with myopia can see nearby objects clearly

[ Myopia, or nearsightedness, is a condition where a person can see nearby objects clearly but has difficulty seeing distant objects clearly. This is because the light focuses in front of the retina rather than directly on it.]

Short Answer Questions

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

Answer : (i) Myopic eye: Parallel rays from a distant object focus in front of the retina due to excessive curvature or elongated eyeball. Hence, distant objects appear blurred.

   

(ii) Hypermetropic eye: Rays from a nearby object focus behind the retina due to less curvature or shorter eyeball. Hence, nearby objects appear blurred.

  

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 her? Draw ray diagram for the correction of this defect.

Answer : The student is likely suffering from myopia (short-sightedness). The doctor will advise her to use a concave lens of suitable power. This lens diverges incoming rays so that they appear to come from the far point and focus on the retina. Thus, distant objects like the blackboard can be seen clearly.

17. How are we able to see nearby and also the distant objects clearly?

Answer: We are able to see nearby and distant objects clearly due to the power of accommodation of the eye. The ciliary muscles change the curvature and focal length of the eye lens, so that images of objects at different distances are formed clearly on the retina.

18. A person needs a lens of power – 4.5 D for correction of her vision.

(a) What kind of defect in vision is she suffering from?

(b) What is the focal length of the corrective lens?

(c) What is the nature of the corrective lens?

Answer : (a) The person is suffering from myopia (nearsightedness).

(b) Here ,  

We have,  

 

(c) The nature of the corrective lens is a concave lens, as it is used to correct myopia and has a negative focal length.

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.

Answer: The first prism disperses white light into its constituent colours. The second prism is placed in an inverted position, so it recombines these colours back into white light. Thus, the beam emerging from the second prism becomes white again, cancelling the effect of dispersion.

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.

Answer: When a narrow beam of white light passes through a prism, it gets refracted and splits into its constituent colours due to different wavelengths. Each colour bends by a different amount. Red deviates the least, while violet deviates the most. The order of colours obtained is VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red).

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

Answer: No, the position of a star seen by us is not its true position. Due to atmospheric refraction, light from the star bends while passing through different layers of Earth’s atmosphere. This makes the star appear slightly higher than its actual position in the sky.

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

Answer: We see a rainbow only after rainfall because tiny water droplets remain suspended in the air. These droplets act like small prisms and split sunlight into different colours by refraction, dispersion and internal reflection, forming a rainbow in the sky.

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

Answer: The colour of the clear sky appears blue due to scattering of sunlight by tiny particles in the atmosphere. Shorter wavelengths like blue are scattered more than longer wavelengths like red. So, blue light reaches our eyes from all directions, making the sky look blue.

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

Answer: During sunrise and sunset, the Sun appears reddish because sunlight travels a longer distance through the atmosphere. Shorter wavelengths like blue scatter more, while red light reaches our eyes.

At noon, the Sun appears white because sunlight travels a shorter distance through the atmosphere, so very little scattering occurs and almost all colours reach our eyes.

Long Answer Questions

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

Answer : The structure of human eye :

The human eye is a sense organ that helps us to see objects. Its main parts are:

(a) Cornea – Transparent front part that allows light to enter the eye.

(b) Iris – Coloured part of the eye that controls the size of the pupil.

(c) Pupil – Opening through which light enters.

(d) Eye lens – Convex lens that focuses light on the retina.

(e) Ciliary muscles – Change the shape of the lens for focusing.

(f) Retina – Light-sensitive screen where image is formed.

(g) Optic nerve – Carries messages from retina to the brain.

Functioning of human eye : Light rays from an object enter the eye through the cornea and pupil. The eye lens focuses the light on the retina to form a real and inverted image. The retina converts it into electrical signals which are sent to the brain through the optic nerve. The brain interprets these signals, and we are able to see the object clearly.

We are able to see nearby as well as distant objects due to the power of accommodation of the eye. The ciliary muscles adjust the curvature and focal length of the eye lens so that clear images of objects at different distances are formed on the retina.

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?

Answer: A person is myopic (short-sighted) when they can see nearby objects clearly but not distant ones. This happens when the image forms in front of the retina. It is corrected using a concave lens, which diverges light rays so the image forms on the retina.

  

A person is hypermetropic (long-sighted) when they can see distant objects clearly but not nearby ones. The image forms behind the retina. It is corrected using a convex lens, which converges light rays to focus on the retina.

   

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

Answer : We consider an ABC triangular glass prism. Here, PE is the incident ray, EF is the refracted ray, and FS is the emergent ray. A ray of light enters from air into glass at the first surface AB, and the light ray bends towards the normal upon refraction. At the second surface AC, the light ray exits from glass into air, and hence it bends away from the normal. The angle of incidence and the angle of refraction at each refracting surface of the prism differ due to the change in the medium. This bending of light is different from the bending that occurs in a glass slab, as the peculiar shape of the prism causes the emergent ray to deviate at an angle from the direction of the incident ray. This angle is called the angle of deviation, denoted by ∠D.

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

Answer : The sun appears reddish at sunrise and sunset because its light passes through more of Earth's atmosphere, scattering shorter blue wavelengths and allowing longer red wavelengths to dominate. At noon, the sun is overhead and its light travels through less atmosphere, so less scattering occurs, and the sun appears white or yellow, not red.

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

Answer: Dispersion is the splitting of white light into its constituent colours when it passes through a glass prism. When white light enters the prism, different colours refract by different amounts due to their different wavelengths. Violet bends the most, while red bends the least. Thus, a spectrum of colours (VIBGYOR) is formed on the other side of the prism.

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

Answer: Refraction in the atmosphere takes place because different layers of air have different densities due to changes in temperature and pressure. When light passes through these layers, it bends continuously.

Stars twinkle because they are very far away and appear as point sources of light. Atmospheric refraction changes their apparent position and brightness continuously. Planets do not twinkle because they appear larger and their light is not affected much by atmospheric refraction.