CBSE Class 10 Science The Human Eye and the Colourful World VBQs Set 02

Question. What is the colour of the clear sky during daytime? Give reason for it.
Answer: The colour of the clear sky is blue. This is because the fine particles in the atmosphere (molecules of air and other fine particles) have a size smaller than the wavelength of visible light. These particles are more effective in scattering light of shorter wavelengths at the blue end than light of longer wavelengths at the red end. When sunlight passes through the atmosphere, the fine particles in air scatter the blue colour more strongly than red, and the scattered blue light enters our eyes.

Question. Is the position of star as seen by us in its true position? Justify your answer.
Answer: No, the position of a star as seen by us is not its true position. Due to atmospheric refraction, the starlight, on entering the earth's atmosphere, undergoes refraction continuously before it reaches the earth. The atmospheric refraction occurs in a medium of gradually changing refractive index. Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position. The star appears slightly higher (above) than its actual position.

Question. What is the basic cause of atmospheric refraction?
Answer: The basic cause of atmospheric refraction is the variation in the optical density (or refractive index) of different layers of the earth's atmosphere. As we go higher, the air becomes rarer, and its refractive index decreases. Light rays passing through these layers of varying refractive indices undergo continuous refraction and bend towards or away from the normal.

Question. State the reason behind the following phenomenon/observations.
(a) Pathway of light visible in foggy atmosphere or a dusty room/smoke filled room.
(b) The apparent flattening of the sun’s disc at sunrise and sunset.
(c) Danger signal lights are red in colour.
Answer: (a) This is due to the Tyndall effect. The tiny water droplets in fog or dust/smoke particles scatter the light falling on them, making the path of the light beam visible.
(b) This is caused by atmospheric refraction. The layers of the atmosphere have different refractive indices, causing the light from the upper and lower edges of the sun's disc to bend by different amounts.
(c) Red light has the longest wavelength in the visible spectrum and is scattered the least by air molecules, fog, or smoke particles. Therefore, it can be seen from a long distance even in foggy conditions.

Question. Imagine you and your family members are astronauts on the space station now. You record the following observations from your spaceship and send them to a friend by e-mail.
(a) Star appears to twinkle while the planets do not similar to as observed from the earth.
(b) Sky appears black in colour.
(c) The length of the day is same as observed on the earth.
How can each observation be justified by your friend?
Answer: (a) Twinkling of stars is due to atmospheric refraction. Since there is no atmosphere in space, stars do not twinkle when observed from the space station.
(b) The sky appears black because there is no atmosphere to scatter the sunlight. Without scattering, the sky looks dark.
(c) On Earth, the day is longer by about 4 minutes due to atmospheric refraction (advanced sunrise and delayed sunset). In space, there is no atmosphere, so the sun is seen only when it actually crosses the horizon, making the duration of the day exactly equal to the time between actual sunrise and actual sunset.

INTEGRATED (MIXED) QUESTIONS

Question. (a) Which part of the eye has delicate membrane and containing large number of light sensitive cells?
(b) A person is advised to wear spectacles with convex lenses. What type of defect of vision is he suffering from?
Answer: (a) The retina is the delicate membrane that contains a large number of light-sensitive cells.
(b) The person is suffering from Hypermetropia (or long-sightedness), as convex lenses are used to correct this defect.

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: In a glass prism, the two refracting surfaces are inclined at an angle. When white light enters, different colours bend at different angles, causing dispersion. In a rectangular glass slab, the refracting surfaces are parallel. The dispersion that occurs at the first surface is reversed at the second surface, so the different colours recombine to emerge as white light parallel to the incident ray.

Question. (a) A person can see objects if they are placed at \( 1.5 \text{ m} \). What kind of lens would be required to read a book at a distance of \( 25 \text{ cm} \)? What kind of eye defect is it? Calculate the power of lens used to correct the defect.
(b) The limitation of an eye is that the distance from the eye lens to where the image formed is always the same. How is then it possible for the eye to form focussed images from the objects that are located at varying distances from the eye?
Answer: (a) The person is suffering from Hypermetropia. A convex lens is required.
Given: Near point \( v = -150 \text{ cm} \), Object distance \( u = -25 \text{ cm} \).
Using lens formula: \( \frac{1}{f} = \frac{1}{v} - \frac{1}{u} \)
\( \frac{1}{f} = \frac{1}{-150} - \frac{1}{-25} = \frac{-1 + 6}{150} = \frac{5}{150} = \frac{1}{30} \)
\( f = 30 \text{ cm} = 0.3 \text{ m} \).
Power \( P = \frac{1}{f(m)} = \frac{1}{0.3} = +3.33 \text{ D} \).
(b) This is possible due to the property of "accommodation". The ciliary muscles can change the curvature and thus the focal length of the eye lens. By becoming thinner or thicker, the lens adjusts its focal length so that the image is always formed on the retina.

Question. Differentiate between a glass slab and a glass prism. What happens when a narrow beam of (a) a monochromatic light, and (b) white light passes through (i) glass slab and (ii) glass prism?
Answer: A glass slab has parallel refracting surfaces, while a glass prism has refracting surfaces inclined at an angle.
(i) Through glass slab: (a) Monochromatic light: Emerges parallel to incident light with lateral displacement. (b) White light: Emerges as white light parallel to incident light (no dispersion).
(ii) Through glass prism: (a) Monochromatic light: Bends towards the base of the prism (refraction). (b) White light: Undergoes dispersion and forms a spectrum of seven colours.

Question. (a) Explain the following terms used in relation to defects in vision and correction provided by them: (i) Myopia (ii) Bifocal lenses (iii) Far-sightedness.
(b) Why is the normal eye unable to focus on an object placed within \( 10 \text{ cm} \) from the eye?
Answer: (a) (i) Myopia: A defect where a person can see nearby objects clearly but cannot see distant objects. Corrected by concave lenses.
(ii) Bifocal lenses: Lenses containing both convex and concave parts, used for people suffering from both myopia and presbyopia.
(iii) Far-sightedness (Hypermetropia): A defect where a person can see distant objects clearly but cannot see nearby objects. Corrected by convex lenses.
(b) The ciliary muscles have a limit to how much they can contract to thicken the eye lens. For objects closer than the near point (about \( 25 \text{ cm} \)), the lens cannot become thick enough to focus the image on the retina, resulting in a blurred image.

You are given that the diameter of the eyeball is about \( 2.3 \text{ cm} \) and a normal eye can adjust the focal length of its eye lens to see objects situated anywhere from \( 25 \text{ cm} \) to an infinite distance away from it.

Question. (a) What is the power of the (normal) eye lens, when ciliary muscles are fully relaxed?
Answer: When relaxed, the eye focuses on infinity. Object distance \( u = \infty \), Image distance \( v = 2.3 \text{ cm} = 0.023 \text{ m} \).
\( \frac{1}{f} = \frac{1}{v} - \frac{1}{u} = \frac{1}{0.023} - 0 \)
\( P = \frac{1}{0.023} \approx +43.48 \text{ D} \).

Question. (b) What is the power of the (normal) eye lens, when ciliary muscles are in their maximum contract position?
Answer: At maximum contraction, the eye focuses on the near point. Object distance \( u = -25 \text{ cm} = -0.25 \text{ m} \), Image distance \( v = 2.3 \text{ cm} = 0.023 \text{ m} \).
\( \frac{1}{f} = \frac{1}{0.023} - \frac{1}{-0.25} = 43.48 + 4 = 47.48 \text{ D} \).
\( P \approx +47.48 \text{ D} \).

Question. (c) The maximum variation in the power of the eye lens, when it adjust itself, from the normal relaxed position to the position where the eye can see the nearby object clearly?
Answer: Variation in power \( = P_{max} - P_{min} = 47.48 \text{ D} - 43.48 \text{ D} = 4 \text{ D} \).

Question. (a) What is meant by the term ‘power of accommodation’? Name the component of eye that is responsible for the power of accommodation.
(b) A student sitting at the back bench in a class has difficulty in reading. What could be his defect of vision? Draw ray diagrams to illustrate the image formation of the blackboard when he is seated at the (i) back seat (ii) front seat. State two possible causes of this defect. Explain the method of correcting this defect with the help of a ray diagram.
Answer: (a) Power of accommodation is the ability of the eye lens to adjust its focal length to see objects at different distances. The ciliary muscles are responsible for this.
(b) The student has Myopia (Short-sightedness).
Causes: (1) Excessive curvature of the eye lens. (2) Elongation of the eyeball.
Correction: This defect is corrected by using a concave lens of suitable focal length which brings the image back onto the retina.

Question. (a) (i) What is meant by scattering of light?
(ii) State the factors on which the colour of scattered light perceived by us depends.
(b) Which of the two is scattered more easily-light of short wavelength or light of longer wavelength? Give reason.
(c) How is the eye defect of old person differing from near-sightedness and far-sightedness?
Answer: (a) (i) Scattering of light is the phenomenon where light rays get deviated from their straight path upon striking obstacles like dust, air molecules, or water droplets.
(ii) The colour of scattered light depends on the size of the scattering particles. Very fine particles scatter mainly blue light (shorter wavelength), while larger particles scatter light of longer wavelengths (like red).
(b) Light of short wavelength (blue/violet) is scattered more easily. This is because the amount of scattering is inversely proportional to the fourth power of the wavelength (Rayleigh scattering).
(c) The eye defect of old people is often Presbyopia, which is due to the gradual weakening of ciliary muscles and diminishing flexibility of the eye lens. It is different from myopia (eyeball elongation) and hypermetropia (eyeball shortening) as it is caused by aging and often requires bifocal lenses.

ASSERTION AND REASON QUESTIONS

Question. Assertion: The near-point of a hypermetropic eye is more than \( 25 \text{ cm} \) away.
Reason: Hypermetropia is corrected using spectacles containing concave lenses.
(a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.
(b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) The statement of the Assertion is false but the Reason is true.
Answer: (c) Assertion is true but the Reason is false.

Question. Assertion: Myopia is the defect of vision in which a person cannot see the distant objects clearly.
Reason: This due to eye-ball being too short.
(a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.
(b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) The statement of the Assertion is false but the Reason is true.
Answer: (c) Assertion is true but the Reason is false.

Question. Assertion: Pupil is black in colour.
Reason: Pupil is black in colour as no light is reflected back.
(a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.
(b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) The statement of the Assertion is false but the Reason is true.
Answer: (a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.

Question. Assertion: The rainbow is a man made spectrum of sunlight in the sky.
Reason: The rainbow is formed in the sky when the sun is shining and presence of water droplets at the same time.
(a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.
(b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) The statement of the Assertion is false but the Reason is true.
Answer: (d) The statement of the Assertion is false but the Reason is true.

Question. Assertion: Sky appears blue in the day time.
Reason: White light is composed of seven colours.
(a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.
(b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) The statement of the Assertion is false but the Reason is true.
Answer: (b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.

Question. Assertion: When objects are observed through a turbulent stream of hot air, they appear to be flickered.
Reason: Hotter air has a refractive index less than that of the cooler air.
(a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.
(b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) The statement of the Assertion is false but the Reason is true.
Answer: (a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.

Question. Assertion: A person suffering from myopia cannot see the distant object clearly.
Reason: Converging lens is used for the correction of myopic eye as it can form real as well as virtual images of the objects placed in front of it.
(a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.
(b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) The statement of the Assertion is false but the Reason is true.
Answer: (c) Assertion is true but the Reason is false.

Question. Assertion: The angle through which a ray of light bends on passing through a prism is called the angle of deviation.
Reason: The peculiar shape of a prism makes the emergent ray bend at angle to the direction of the incident ray.
(a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.
(b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) The statement of the Assertion is false but the Reason is true.
Answer: (a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.

CASE-BASED QUESTIONS

Two children went to the park with their grandfather. On reaching the park, the children joined others to play, while their grandfather after taking three rounds of the park, sat on the chair, took out newspaper from his bag and began to read with the help of his spectacle. After sometime, he realised that it was too long to see the children, he looked around, but though he has worn spectacle, he couldn’t see anything. He then realised that he had forgotten his other spectacle, which he used to see the far away places, were left at home. He began calling them by their names, but due to the large distance, his voice was not reaching the children. Another man sitting beside him realised the problem and helped him to reach the children.

Question. (a) Name the defect of vision the grandfather is suffering from.
Answer: The grandfather is suffering from Presbyopia (as he needs different spectacles for near and far vision).

Question. (b) Which type of lens is used to correct this eye defect?
Answer: Bifocal lenses (consisting of both concave and convex lenses) are used to correct this defect.

Question. (c) State the cause of eye defect by which grandfather is suffering.
Answer: The cause of Presbyopia is the gradual weakening of the ciliary muscles and the diminishing flexibility of the crystalline eye lens due to old age.

Question. Or (c) If a grandfather wears lens of power \( -6\text{D} \) for the distant vision and for correcting his near vision, he needs a lens of \( +2\text{D} \). Determine the focal length of the lenses in both the cases.
Answer: For distant vision: \( P = -6\text{D} \)
\( f = \frac{1}{P} = \frac{1}{-6} \text{ m} = -16.67 \text{ cm} \).
For near vision: \( P = +2\text{D} \)
\( f = \frac{1}{P} = \frac{1}{2} \text{ m} = +50 \text{ cm} \).

To understand the formation of rainbow one should actually study the refraction, internal reflection, dispersion and total deviation of white light by the spherical water drop. A rainbow is produced when sunlight falls and gets diverted to the eyes of the observer due to a large number of water droplets in the sky on a rainy day. In addition to primary Rainbow there is a secondary rainbow. It occurs in the same manner as the primary rainbow but due to two internal reflections. In nature we can observe only primary and secondary rainbows. Higher order rainbows are never seen since they are weaker than (i) the background sky brightness (ii) the light reflected from the outside surface of the drops and (iii) the light transmitted through the process with no internal reflections.

Question. (a) What is refraction?
(b) List two essential conditions for observing a rainbow.
(c) What is the shape of a rainbow?
Or
(c) Why the red appears on the top of rainbow?
Answer: (a) Refraction is the bending of light as it passes from one transparent medium to another of different optical density.
(b) (i) The sun should be shining in one part of the sky while it is raining in the opposite part. (ii) An observer should stand with their back towards the sun.
(c) A rainbow is typically an arc of a circle. (Or) Red appears on top because it is the least deviated colour of the spectrum when light undergoes dispersion and internal reflection in a water droplet.

Pranav has too much enthusiasm to celebrate the end of winter season and welcomes the season of fruitfulness with family members and relatives. In the evening all family members gather up and move in a circle around the bonfire. At particular moment, Pranav observe that the apparent position of object, as seen through the hot air or beyond and above the fire changes continuously. After sometimes, he find that this phenomena can also be used to explain several observation around us.

Question. (a) Name the phenomenon which could explain the wavering of objects above a fire.
(b) Define the phenomena observed by Pranav.
(c) Name two observations which can be explained by the above phenomena.
Or
(c) How does the refractive index of earth’s atmosphere vary with height?
Answer: (a) Atmospheric refraction.
(b) The wavering of objects is due to the continuous change in the physical conditions of the air above the fire. The air just above the fire becomes hotter and thus lighter (less dense) than the cooler air above it. The refractive index of hot air is less than that of cool air. Since the physical conditions of the refracting medium (air) are not stationary, the apparent position of the object fluctuates.
(c) (i) Twinkling of stars. (ii) Advanced sunrise and delayed sunset. (Or) The refractive index of the earth's atmosphere generally decreases with increasing height from the surface.

The foundation of modern optics lays in 1672 when Sir Isaac Newton publishes his paper on the bending of light through prism. His experiments in bending of light through prisms led, eventually, to the revolutionary discovery of distinct coloured rays in white light and, distinguishable when refracted through a prism. In his experiment, he set up a prism near his window, and projected a beautiful spectrum 22 feet onto the far wall. Further, to prove that the prism was not colouring the light, he refracted the light back together by placing the second identical prism in an inverted position with respect to the first prism.

Question. (a) State the phenomenon of light used by the prism to form spectrum.
(b) How does the angle of minimum deviation of glass prism vary if the incident violet light is replaced by red light?
(c) The refractive index of the material of the prism is \( \sqrt{2} \) and angle of prism is \( 30^{\circ} \). One of the two refracting surfaces of the prism is made a mirror inwards with a silver coating. At what angle of incidence, a beam of monochromatic light entering the prism from the other face will retrace its path (after reflection from the silvered surface)?
Or
(c) What conclusion would you like to draw about angle of incident and angle of emergence in the case of refraction of light through a triangular glass prism?
Answer: (a) Dispersion of light.
(b) The angle of minimum deviation decreases because the refractive index of glass is smaller for red light than for violet light.
(c) To retrace the path, light must strike the silvered surface normally (\( r_2 = 0 \)). From \( A = r_1 + r_2 \), \( 30^{\circ} = r_1 + 0 \implies r_1 = 30^{\circ} \). Using Snell's law: \( n = \frac{\sin i}{\sin r_1} \implies \sqrt{2} = \frac{\sin i}{\sin 30^{\circ}} \implies \sin i = \sqrt{2} \times \frac{1}{2} = \frac{1}{\sqrt{2}} \implies i = 45^{\circ} \). (Or) In a triangular prism, the angle of incidence is generally not equal to the angle of emergence unless the prism is in the position of minimum deviation.

A student sitting at the back bench in a class has difficulty in reading. He observed that he has no difficulty in reading if he seated at front seat of the class. Doctor prescribed him a suitable lens of negative power and explain him that this lens shifts the image back onto the retina instead of in front of it. He is now able to read the blackboard while sitting at the back bench in the class.

Question. (a) Name the defect of vision in the student’s eye.
(b) If the doctor prescribes the lenses of power \( -0.5\text{D} \) write the type of these lenses.
(c) Why the student is unable to see distinctly written on the blackboard from the back bench of the class?
Or
(c) Write the function of retina in human eye.
Answer: (a) Myopia (near-sightedness).
(b) Concave lens (diverging lens).
(c) In a myopic eye, the image of a distant object (blackboard) is formed in front of the retina rather than on the retina itself. (Or) The retina acts as a screen where the image is formed. It contains light-sensitive cells (rods and cones) that generate electrical signals upon illumination, which are sent to the brain via the optic nerve.

Glasses are among the most common forms of eye wear to correct sight, adding or subtracting power to manage near-sightedness (myopia), far sightedness (hypermetropia), misshapen corneas (astigmatism i.e., imperfection in the curvature of eye’s cornea or lens), or farsightedness associated with age (presbyopia). eyeglasses are used to correct presbyopia. Three prescriptions for three persons having different eye diseases are shown below:

For person A
Distance: Right Eye (OD) Sph \( -1.5 \), V/A \( 6/6 \); Left Eye (OS) Sph \( -2.25 \), V/A \( 6/6 \)

For person B
Near: Right Eye (OD) Sph \( +1.00 \), V/A \( 6/6 \); Left Eye (OS) Sph \( +0.25 \), V/A \( 6/6 \)

For person C
Distance: Right Eye (OD) Sph \( -0.5 \), Cyl \( +1.25 \), Axis \( 50^{\circ} \), V/A \( 6/6 \); Left Eye (OS) Sph \( -1.00 \), Axis \( 75^{\circ} \), V/A \( 6/6 \)
Near: Right Eye (OD) Sph \( +2.50 \); Left Eye (OS) Sph \( +2.50 \)

Question. (a) What does the number in the column of spherical lens represents?
(b) After analysing the table, state the persons who have suffered from myopia and hypermetropia.
(c) Is the defect in vision a disease? If not, why?
Or
(c) State the nature of corrective lens and its focal length for the right eye of person B as prescribed by the doctor.
Answer: (a) It represents the power of the spherical lens in Dioptres (D).
(b) Person A suffers from Myopia. Person B suffers from Hypermetropia. Person C suffers from a combination of defects including Presbyopia and Astigmatism.
(c) No, a defect in vision is usually not a disease but a refractive error caused by an mismatch between the eye's focal length and its physical length. (Or) For person B (Right Eye): Sph \( +1.00\text{D} \). Nature: Convex lens. Focal length \( f = \frac{1}{P} = \frac{1}{+1.00} = 1\text{ m} = 100\text{ cm} \).

Question. In an experiment, Pooja used a equilateral triangular glass prism and projected a narrow beam of white light source from one side of the surface of the prism. She placed a screen on the other side and saw many colours appearing as patches on the screen. But when she used a red light source, she could only see a red patch on the screen. Similarly she used a blue and green light source and could only see one colour patch on both occasions.
(a) State the phenomenon that she was trying to demonstrate.
(b) Give reason: Why she could not see any other colour when the red light was used?
(c) She also could relate to another natural phenomenon that we observe on a rainy humid day as sun comes out. What could be that phenomenon?
Or
(c) State the reason that Pooja can give to her friends to explain the phenomenon observed by her in the experiment.
Answer: (a) Dispersion of white light.
(b) Red light is monochromatic (single color), so it cannot be further split into other colors.
(c) Rainbow. (Or) The glass prism splits white light into its constituent seven colours because different colours of light bend through different angles with respect to the incident beam, as they travel at different speeds through the glass.

Question. 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 medium.
(a) Which phenomena is/are responsible for the formation of rainbow?
(b) Why the clear sky appears blue to us?
Or
(c) Consider the following statements.
I. Violet light bends the least while red light bends the most when a beam of white light passes through a glass prism.
II. The path of the beam of light passing through a true solution is not visible while visible through a colloidal solution.
III. The refractive index of hotter air is slightly less than that of the cooler air.
Choose the correct statement(s) from above.
Answer: (a) Refraction, dispersion, and total internal reflection.
(b) The clear sky appears blue because molecules in the air scatter blue light (shorter wavelength) more strongly than red light. (Or) Statements II and III are correct. (Statement I is incorrect because red bends the least and violet the most).

QUESTIONS

Question. What is meant by power of accommodation of the eye?
Answer: Power of accommodation: The ability of eye lens to adjust its focal length to form a sharp image of the object at varying distances on the retina is called power of accommodation.

Question. A person with a myopic eye cannot see objects beyond \( 1.2\text{ m} \) distinctly. What should be the type of the corrective lens used to restore proper vision?
Answer: A person with myopic eye defect should use concave lens of focal length \( 1.2\text{ m} \) to restore his proper vision.

Question. What is the far point and near point of the human eye with normal vision?
Answer: For normal vision, the near point is about \( 25\text{ cm} \) and far point is infinity. Thus, a normal eye can see objects clearly that are between \( 25\text{ cm} \) and infinity.

Question. A student has difficulty reading the blackboard while sitting in the last row. What could be the defect the child is suffering from? How can it be corrected?
Answer: Child is suffering from myopia or short sightedness. The defect is corrected by using a concave lens of suitable power placed in front of eye defective.