CBSE Class 10 Science HOTs Human Eye and Colourful World Set 04

The Human Eye

Question. Define the term power of accommodation. Write the modification in the curvature of the eye lens which enables us to see the nearby objects clearly?
Answer: The ability of the eye lens to adjust its focal length is called power of accommodation. To see nearby objects clearly, the ciliary muscles contract. This makes the eye lens thicker (more convex) and its curvature increases. Consequently, the focal length of the eye lens decreases, enabling us to focus nearby objects onto the retina.

Question. Trace the sequence of events which occur when a bright light is focused on your eyes.
Answer: When bright light falls on the eye, the sequence of events is as follows:

• The light rays pass through the cornea and enter the eye.
• The intensity of light is sensed by the iris.
• The iris automatically contracts the pupil, thereby reducing the amount of light entering the eye to prevent damage to the retina.
• This is a reflex action controlled by the nervous system.

Question. Write about power of accommodation of human eye. Explain why the image distance in the eye does not change when we change the distance of an object from the eye?
Answer: Power of accommodation is the ability of the eye lens to change its focal length so that objects at various distances can be clearly focused on the retina. The image distance in the eye (the distance between the eye lens and the retina) is fixed. When the object distance changes, the ciliary muscles modify the curvature of the crystalline lens. By changing the lens's curvature, its focal length is adjusted such that the image always forms exactly on the retina, maintaining a constant image distance.

Question. (a) State the role of ciliary muscles present in our eye.
(b) Identify the defect of vision in each of the following cases and suggest its corrective measure :
(i) The eye lens has become milky and cloudy.
(ii) The eye lens has excessive curvature.
(iii) The eye lens has large focal length (longer than normal).
(iv) Ciliary muscles have weakened.
Answer: (a) Ciliary muscles help in changing the curvature and hence the focal length of the eye lens. This allows the eye to focus on both near and distant objects.
(b) (i) Cataract; Corrective measure: Surgical replacement of the opaque lens with an artificial intraocular lens.
(ii) Myopia (Short-sightedness); Corrective measure: Use of a concave (diverging) lens of appropriate power.
(iii) Hypermetropia (Long-sightedness); Corrective measure: Use of a convex (converging) lens of appropriate power.
(iv) Presbyopia; Corrective measure: Use of bifocal lenses or a convex lens for reading.

Question. Write the function of each of the following parts of human eye:
(i) Cornea (ii) Iris
(iii) Crystalline lens (iv) Ciliary muscles
Answer: (i) Cornea: It is the outermost transparent layer that provides most of the refraction for light entering the eye.
(ii) Iris: It is a dark muscular diaphragm that controls the size of the pupil.
(iii) Crystalline lens: It provides the finer adjustment of focal length required to focus objects at different distances on the retina.
(iv) Ciliary muscles: They adjust the curvature of the eye lens to facilitate accommodation.

Question. State the function of each of the following parts of the human eye :
(i) Cornea (ii) Iris (iii) Pupil (iv) Retina
Answer: (i) Cornea: Admits light into the eye and performs initial refraction.
(ii) Iris: Regulates the amount of light entering the eye by adjusting pupil size.
(iii) Pupil: Acts as a variable aperture through which light enters the inner eye.
(iv) Retina: Acts as a screen where the image is formed; it contains light-sensitive cells (rods and cones) that convert light into electrical signals.

Question. (a) List the parts of the human eye that control the amount of light entering into it. Explain how they perform this function?
(b) Write the function of retina in human eye.
Answer: (a) The iris and the pupil control the amount of light. The iris is a muscular diaphragm that regulates the size of the pupil. In bright light, the iris expands, making the pupil smaller to let less light in. In dim light, the iris contracts, making the pupil larger to allow more light in.
(b) The retina is a delicate membrane having an enormous number of light-sensitive cells. When illuminated, these cells get activated and generate electrical signals. These signals are sent to the brain via the optic nerves, where the brain interprets them to perceive the object.

Defects of Vision and their Correction

Question. Person suffering from cataract has
(a) elongated eyeball
(b) excessive curvature of eye lens
(c) weakened ciliary muscles
(d) opaque eye lens.
Answer: (d) opaque eye lens.

Question. A student uses spectacles of focal length – 2.5 m.
(a) Name the defect of vision he is suffering from.
(b) Which lens is used for the correction of this defect ?
(c) List two main causes of developing this defect.
(d) Compute the power of this lens.
Answer: (a) Myopia (Short-sightedness).
(b) Concave lens (diverging lens).
(c) (i) Excessive curvature of the eye lens. (ii) Elongation of the eyeball.
(d) Power \( P = \frac{1}{f(\text{in m})} \)
\( \implies \) \( P = \frac{1}{-2.5} = -0.4 \text{ D} \).

Question. (a) A person is suffering from both myopia and hypermetropia.
(i) What kind of lenses can correct this defect?
(ii) How are these lenses prepared?
(b) A person needs a lens of power +3 D for correcting his near vision and –3 D for correcting his distant vision. Calculate the focal lengths of the lenses required to correct these defects.
Answer: (a) (i) Bifocal lenses can correct this defect (Presbyopia). (ii) These lenses consist of both concave and convex lenses. Usually, the upper portion is a concave lens for distant vision and the lower portion is a convex lens for near vision.
(b) For near vision correction: \( P_1 = +3 \text{ D} \)
\( \implies \) \( f_1 = \frac{1}{P_1} = \frac{1}{3} \text{ m} \approx +0.33 \text{ m} \text{ or } 33.3 \text{ cm} \).
For distant vision correction: \( P_2 = -3 \text{ D} \)
\( \implies \) \( f_2 = \frac{1}{P_2} = \frac{1}{-3} \text{ m} \approx -0.33 \text{ m} \text{ or } -33.3 \text{ cm} \).

Question. A student is unable to see clearly the words written on the black board placed at a distance of approximately 3 m from him. Name the defect of vision the boy is suffering from. State the possible causes of this defect and explain the method of correcting it.
Answer: The student is suffering from Myopia.

• Causes: Excessive curvature of the eye lens or elongation of the eyeball.
• Correction: This defect is corrected by using a concave lens of suitable focal length. The concave lens diverges the incoming parallel rays just enough so that the eye lens can focus them exactly on the retina.

Question. Write the importance of ciliary muscles in the human eye. Name the defect of vision that arises due do gradual weakening of the ciliary muscles in old age. What type of lenses are required by the persons suffering from this defect to see the objects clearly?
Answer: Importance: Ciliary muscles change the focal length of the eye lens by modifying its curvature, enabling accommodation. The defect arising from their weakening is Presbyopia. Bifocal lenses are required to see both near and distant objects clearly.

Question. Akshay, sitting in the last row in his class, could not see clearly the words written on the blackboard. When the teacher noticed it, he announced if any student sitting in the front row could volunteer to exchange his seat with Akshay. Salman immediately agreed to exchange his seat with Akshay. He could now see the words written on the blackboard clearly. The teacher thought it fit to send the message to Akshay’s parents advising them to get his eyesight checked.
(a) Which defect of vision is Akshay suffering from? Which type of lens is used to correct this defect?
(b) State the values displayed by the teacher and Salman.
(c) In your opinion, in what way can Akshay express his gratitude towards the teacher and Salman?
Answer: (a) Akshay is suffering from Myopia. A concave lens is used for correction.
(b) Teacher: Concern, vigilance, and helpfulness. Salman: Empathy, kindness, and a cooperative spirit.
(c) Akshay can express his gratitude by saying "Thank you" and showing improved academic performance and helpfulness in return.

Dispersion of White Light by a Glass Prism

Question. Differentiate between a glass slab and a glass prism. What happens when a narrow beam of (i) a monochromatic light and (ii) white light passes through (a) glass slab and (b) glass prism?
Answer: In a glass slab, the opposite faces are parallel, whereas in a prism, the refracting faces are inclined at an angle.

• (a) Glass slab: (i) Monochromatic light undergoes lateral displacement but emerges parallel to its original path. (ii) White light also emerges as white light with lateral displacement (no dispersion).
• (b) Glass prism: (i) Monochromatic light is deviated from its path. (ii) White light is dispersed into its seven constituent colors.

Atmospheric Refraction

Question. Which one of the following is the correct reason for twinkling of stars ?
(a) Atmospheric reflection of starlight.
(b) Atmospheric refraction of starlight.
(c) Scattering of starlight.
(d) Dispersion of starlight.
Answer: (b) Atmospheric refraction of starlight.

Question. Why do stars appear to twinkle ? Explain.
Answer: Starlight undergoes continuous refraction as it passes through the layers of the Earth's atmosphere, whose optical density varies constantly. Since the atmosphere is dynamic, the apparent position of the star fluctuates slightly. This, coupled with the change in the amount of light reaching our eye, causes the star to appear to twinkle.

Question. Explain why the planets do not twinkle.
Answer: Planets are much closer to Earth than stars and appear as extended sources (a collection of many point sources) rather than single points. Although light from each point fluctuates due to atmospheric refraction, the average intensity from the whole planet remains constant. The variations in light from different points cancel each other out, so planets do not twinkle.

Question. Explain in brief the reason for each of the following:
(a) Advanced sun-rise
(b) Delayed sun-set
(c) Twinkling of stars
Answer: (a) & (b) Advanced sunrise and delayed sunset occur because starlight/sunlight from the Sun below the horizon gets refracted by the Earth's atmosphere towards the eye, making the Sun appear above the horizon.
(c) Twinkling of stars is due to the fluctuating atmospheric refraction of starlight passing through layers of air with varying optical densities.

Atmospheric Refraction

Question. Which one of the following is the correct reason for twinkling of stars ?
(a) Atmospheric reflection of starlight.
(b) Atmospheric refraction of starlight.
(c) Scattering of starlight.
(d) Dispersion of starlight.
Answer: (b) Atmospheric refraction of starlight.

Question. Why do stars appear to twinkle ? Explain.
Answer: Stars are very distant and appear as point-sized sources of light. As the light from a star enters the Earth's atmosphere, it undergoes refraction continuously due to the varying optical density of the air layers. Since the physical conditions of the Earth's atmosphere are not stationary, the path of light rays from the star varies slightly, causing the apparent position of the star to fluctuate. This fluctuation, combined with the varying amount of starlight entering the eye, makes the star appear to twinkle.

Question. Explain why the planets do not twinkle.
Answer: Planets are much closer to the Earth than stars and are therefore seen as extended sources (a collection of many point-sized sources of light). Although light from each individual point source undergoes atmospheric refraction and fluctuates, the total variation in the amount of light entering our eye from all the individual point-sized sources will average out to zero. This nullifies the twinkling effect, making planets appear with steady brightness.

Question. Explain in brief the reason for each of the following:
(a) Advanced sun-rise
(b) Delayed sun-set
(c) Twinkling of stars
Answer: (a) and (b) Advanced sunrise and delayed sunset are caused by atmospheric refraction. When the Sun is slightly below the horizon, light rays coming from it travel from rarer to denser layers of the atmosphere and bend towards the normal. As a result, the Sun appears to be above the horizon about 2 minutes before actual sunrise and 2 minutes after actual sunset.
(c) Twinkling of stars is due to the atmospheric refraction of starlight. Since the atmosphere is dynamic and its refractive index changes constantly, the starlight reaching our eyes fluctuates in intensity and position, causing the twinkling effect.

Question. What is atmospheric refraction ? List two natural phenomena based on atmospheric refraction.
Answer: The refraction of light caused by the Earth's atmosphere due to the varying optical densities of different air layers is called atmospheric refraction. Two natural phenomena based on this are: (i) Twinkling of stars and (ii) Advanced sunrise and delayed sunset.

Scattering of Light

Question. Which of the following statements is not true for scattering of light?
(a) Colour of the scattered light depends on the size of particles of the atmosphere.
(b) Red light is least scattered in the atmosphere.
(c) Scattering of light takes place as various colours of white light travel with different speed in air.
(d) The fine particles in the atmospheric air scatter the blue light more strongly than red. So the scattered blue light enters our eyes.
Answer: (c) Scattering of light takes place as various colours of white light travel with different speed in air.

Light seems to travel along straight line paths in a transparent medium. But when light enters obliquely from one transparent medium to another, some changes are observed. This is because different mediums have different optical densities. The extent of the change in the direction of light that takes place when it enters obliquely in a given pair of media is expressed in terms of a 'constant'. Light travels the fastest in vacuum. Light gets refracted through a transparent prism. Several phenomena are observed due to the reflection, refraction, dispersion and scattering of light by various mediums.

Question. Rainbow is a natural spectrum. It is produced because of :
(a) dispersion of sunlight by tiny water droplets.
(b) refraction of sunlight by dust particles.
(c) reflection of sunlight by plane shining surfaces.
(d) scattering of sunlight by tiny water droplets.
Answer: (a) dispersion of sunlight by tiny water droplets.

Question. Blue colour of clear sky is due to :
(a) Refraction of light
(b) Reflection of light
(c) Absorption of light
(d) Scattering of light
Answer: (d) Scattering of light

Question. The apparent flattening of the Sun's disc at sunrise and sunset is due to:
(a) Dispersion of light
(b) Scattering of light
(c) Atmospheric refraction of light
(d) Tyndall effect
Answer: (c) Atmospheric refraction of light

Question. Consider the following statements:
I. Very fine particles scatter mainly blue light.
II. Advance sunrise and delayed sunset are due to atmospheric refraction.
III. Violet light bends the least while red light bends the most when a beam of white light passes through a glass prism.
The correct statement(s) is/are :
(a) I only
(b) III only
(c) I and II
(d) II and III
Answer: (c) I and II

Question. The sky appears dark to passengers flying at very high altitudes mainly because
(a) Scattering of light is not enough at such heights.
(b) There is no atmosphere at great heights.
(c) The size of molecules is smaller than the wavelength of visible light.
(d) The light gets scattered towards the earth.
Answer: (a) Scattering of light is not enough at such heights.

Question. The colour of clear sky from the earth appears blue but from the space it appears black. Why?
Answer: On Earth, sunlight is scattered by the atmosphere. Molecules and fine particles in the air are smaller than the wavelength of visible light and scatter the shorter (blue) wavelengths more strongly. In space, there is no atmosphere to scatter light, so light travels in straight paths and the sky appears black.

Question. Give reasons :
(a) Red colour is selected for danger signals.
(b) The sky appears dark in space.
(c) The time difference between actual sunset and apparent sunset is about 2 minutes.
Answer: (a) Red light has the longest wavelength in the visible spectrum. It is scattered the least by fog, smoke, or dust particles, making it visible from long distances.
(b) Space is a vacuum with no atmosphere. Since there are no particles to scatter sunlight, the sky appears dark.
(c) Due to atmospheric refraction, starlight from the Sun below the horizon bends towards the Earth, making the Sun visible for about 2 minutes after it has actually crossed below the horizon.

Question. Why is Tyndall effect shown by colloidal particles? State four instance of observing the Tyndall effect.
Answer: Tyndall effect is the scattering of a beam of light by colloidal particles. It occurs because the size of these particles is comparable to the wavelength of visible light, allowing them to redirect the light rays.
Four instances: (i) Sunlight entering a dusty room through a small hole. (ii) Sunlight passing through the canopy of a dense forest. (iii) Headlights of a car in a foggy night. (iv) Blue colour of the sky.

Question. (a) State the relation between colour of scattered light and size of the scattering particle.
(b) The apparent position of an object, when seen through the hot air, fluctuates or waves. State the basic cause of this observation.
Answer: (a) Very fine particles scatter mainly blue light (shorter wavelengths). Larger particles scatter longer wavelengths. Very large particles scatter all wavelengths equally, making the scattered light appear white.
(b) This is caused by atmospheric refraction. Hot air is less dense and has a lower refractive index than cooler air. As the hot air above a source (like fire) rises and fluctuates, the refractive index of the medium through which light passes changes constantly, causing the object's apparent position to wave.
 

Refraction of Light Through a Prism

Question. If a beam of red light and a beam of violet light are incident at the same angle on the inclined surface of a prism from air medium and produce angles of refraction r and v respectively, which of the following is correct?
(a) r = v
(b) r > v
(c) r = 1/v
(d) r < v
Answer: (b) r > v

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: This can be done by arranging two identical prisms together in an inverted position with respect to each other. The first prism disperses white light into its seven constituent colours, and the second inverted prism recombines these colours into a single beam of white light.

Case Based Questions

Read the passage given below and answer the following questions :
Atmospheric refraction is the phenomenon of bending of light on passing through earth’s atmosphere. As we move above the surface of earth, density of air goes on decreasing. Local conditions like temperature etc., also affect the optical density of earth’s atmosphere. On account of atmospheric refraction, stars seen appear higher than they actual are; advanced sunrise; delayed sunset, oval appearance of the sun at sunrise and sunset; stars twinkle, planets do not.

Question. What is the effect of atmospheric refraction on the apparent length of the day?
Answer: Due to atmospheric refraction, the apparent length of the day increases by about 4 minutes (2 minutes for advanced sunrise and 2 minutes for delayed sunset).

Question. Why is the apparent position of the star appears raised?
Answer: Starlight enters the earth's atmosphere and undergoes continuous refraction from rarer to denser layers. As a result, the light bends towards the normal, and the star appears at a position slightly higher than its actual position when viewed from the earth.

Question. Why does sun appear oval shaped or flattened?
Answer: The sun appears oval-shaped or flattened at sunrise and sunset because the light rays from the upper and lower edges of the sun's disc are refracted by different amounts due to the varying density of air layers near the horizon.

Question. What are the reason for twinkling of stars and non-twinkling of planets?
Answer: Twinkling of stars is caused by the fluctuating atmospheric refraction of starlight as it passes through the dynamic atmosphere. Planets do not twinkle because they are much closer to Earth and act as extended sources of light; the average variations in light from different points on a planet cancel each other out.