10. The Human Eye and the Colourful World

Class 10 Chapter 10 The Human Eye and the Colourful World

Chapter 11: 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

(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. To correct myopia, a concave lens is needed.

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

(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?

Fig.11.1

(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

(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

(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

(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

(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

(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

(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

(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

(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

(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

(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

(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) The diagram of myopic eye :

(ii) The diagram of hypermetropic eye:

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 doctor will likely advise the student that she may have myopia (nearsightedness) and suggest getting a prescription for corrective lenses, such as concave lenses, to improve distant vision.

The ray diagram for the correction of myopia defect :

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

Answer: We can see nearby and distant objects clearly through the eye's ability to adjust the shape of the lens, known as accommodation. The ciliary muscles change the lens's curvature, making it thicker for near objects and thinner for distant ones.

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?

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 : To make the white light emerge unchanged after passing through two identical prisms, place the prisms in an inverted arrangement relative to each other. The first prism disperses the white light into its constituent colors, and the second prism, placed upside down, recombines these colors back into white light, allowing the beam to emerge as white light again.

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 : The ray diagram of the dispersion through a prism when a narrow beam of white light is incident on one of its refracting surfaces :

The order of the colours of the spectrum are : Violet , Indigo , Blue , Green , Yellow , Orange and Red .(VIBGYOR) .

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

Answer : No, the position of a star as seen by us is not its true position. The Earth's atmosphere refracts (bends) the light from the star, causing the light to reach our eyes from a slightly different direction than its actual position. This atmospheric refraction makes stars appear slightly shifted from their true locations.

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

Answer : A rainbow is seen in the sky after rainfall because rain droplets act as tiny prisms. When sunlight enters these droplets, it undergoes refraction, dispersion, and internal reflection, splitting the light into its constituent colors. The dispersed light exits the droplets at different angles, forming a circular spectrum of colors visible as a rainbow. Rain is essential as it provides the water droplets needed for this light phenomenon.

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

Answer : The color of the clear sky is blue due to Rayleigh scattering. Shorter wavelengths of light, such as blue and violet, are scattered more efficiently by the molecules and particles in the Earth's atmosphere than longer wavelengths like red and yellow. Since our eyes are more sensitive to blue light and the atmosphere absorbs some violet light, the sky predominantly appears blue.

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

Answer : The color of the Sun differs at sunrise/sunset and noon due to the scattering of light in the Earth's atmosphere:

Sunrise/Sunset:

(i) Color: The Sun appears red or orange.

(ii) At sunrise and sunset, sunlight passes through a thicker layer of the Earth's atmosphere compared to noon. The longer path length causes more scattering of shorter wavelengths (blue and violet), leaving the longer wavelengths (red, orange) to dominate the Sun's appearance.

Noon:

(i) Color: The Sun appears yellowish-white.

(ii) At noon, the Sun is higher in the sky, and its light travels through a shorter path in the atmosphere. This reduces the scattering effect, allowing more of the shorter wavelengths (blue and green) to reach the observer, giving the Sun a yellowish-white color.

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 human eye is a highly complex and sensitive organ that allows us to perceive the world around us. It functions like a camera, capturing light and converting it into electrical signals that the brain interprets as visual images.

Structure and Function of the Human Eye are :

Cornea: The cornea is the transparent, curved front surface of the eye that allows light to enter. It provides the majority of the eye's optical power by refracting (bending) light rays as they enter the eye. This initial refraction is crucial for focusing light onto the retina.

Crystalline Lens: Located just behind the iris, the crystalline lens fine-tunes the focus of the light rays refracted by the cornea. By adjusting its shape, thanks to the action of the ciliary muscles, the lens changes its focal length to focus on objects at varying distances, ensuring a clear image is formed on the retina.

Iris: The iris is the colored, muscular diaphragm situated behind the cornea. It controls the size of the pupil (the opening in the center of the iris) by contracting or dilating. This regulates the amount of light entering the eye, similar to the aperture of a camera.

Pupil: The pupil is the dark, circular opening in the center of the iris. It controls the amount of light that enters the eye by adjusting its size in response to the brightness of the environment. In bright light, the pupil constricts to reduce light entry, while in dim light, it dilates to allow more light in.

Ciliary Muscles: These muscles are attached to the crystalline lens and are responsible for adjusting its shape. When the ciliary muscles contract, they cause the lens to become more curved, increasing its refractive power to focus on nearby objects. When they relax, the lens flattens, allowing the eye to focus on distant objects.

Retina: The retina is the light-sensitive layer at the back of the eye, functioning as the "screen" where images are formed. It contains millions of photoreceptor cells (rods and cones) that detect light and color. These cells convert light into electrical signals that are sent to the brain for image processing.

Optic Nerve: The optic nerve carries the electrical signals generated by the retina's photoreceptor cells to the brain. The brain then interprets these signals to create the images we see, allowing us to perceive the world around us.

Aqueous Humour: This is the clear, watery fluid found in the space between the cornea and the lens (anterior chamber of the eye). It helps maintain intraocular pressure, provides nutrients to the eye tissues, and removes waste products.

Vitreous Humour: The vitreous humour is the clear, jelly-like substance filling the large cavity between the lens and the retina (posterior chamber of the eye). It helps maintain the eye's shape and provides a pathway for light to reach the retina.

We can see nearby and distant objects through a process called accommodation. The eye's lens changes shape - thicker for nearby objects and thinner for distant ones by - the action of ciliary muscles. This adjustment allows light to focus correctly on the retina, ensuring clear vision at varying distances.

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 : (i) A person is considered myopic (nearsighted) when they can see nearby objects clearly but have difficulty seeing distant objects. This occurs because the light focuses in front of the retina due to the eyeball being too long or the cornea being too curved.

A concave lens of suitable power will bring the image back onto the retina, thus correcting the defect .

(ii) A person is considered hypermetropic (farsighted) when they can see distant objects clearly but have difficulty seeing nearby objects. This occurs because the light focuses behind the retina due to the eyeball being too short or the lens being less convex.

A convex lens of suitable power will bring the image back onto the retina, thus correcting the defect .

Both conditions can be corrected with glasses, contact lenses, or refractive surgery.

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 of white light through a glass prism occurs when the light enters the prism and is refracted. Different colors, which make up white light, have different wavelengths and bend by varying amounts. This causes the light to spread out into a spectrum of colors: red, orange, yellow, green, blue, indigo, and violet, from least to most bent.

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

Answer : Refraction occurs when light passes through layers of air with varying densities in the atmosphere. As light moves through these layers, it bends due to changes in the air's refractive index. This bending causes phenomena such as the apparent shifting of stars' positions and the distortion of the sun's shape near the horizon.

Stars twinkle due to their distant light traveling through many turbulent layers of the atmosphere, which causes rapid variations in their apparent brightness and position. Planets, being closer to Earth, appear as disks rather than points of light, so their light is less affected by atmospheric turbulence, resulting in a steadier appearance.