Class 10 Science · Chapter 10

The Human Eye and the Colourful World

281 Class 10 Science questions on The Human Eye and the Colourful World from the NCERT textbook, NCERT Exemplar and CBSE board papers 2017–2026, each with its answer and a step-by-step solution.

CBSE Class 10 Science previous year papers, with answers →

Questions · page 13 of 15

1. Q241 CBSE 2018
   Explain giving reason why ​the sky appears blue to the observers from the surface of the ​Earth. What will be ​the ​colour of the sky ​for an ​Astronaut staying in an ​International Space Station orbiting the Earth ? ​Explain your answer with reason.
   Show answer

   Fine air particles scatter blue light most, so the sky looks blue; space has no air, so the astronaut sees a black sky.

   Why the sky appears blue from the Earth: the molecules of air and other very fine particles in the atmosphere are smaller than the wavelength of visible light. Such particles scatter light of shorter wavelength (blue) much more strongly than light of longer wavelength (red); red light has a wavelength about $1.8$ times that of blue light. So when sunlight passes through the atmosphere, its blue component is scattered in all directions over the sky, and this scattered blue light enters the eyes of an observer on the Earth from every part of the sky. Hence the sky appears blue. For the astronaut: the sky appears dark (black). The International Space Station orbits far above the atmosphere (about $400$ km up), where there is practically no air. With no particles to scatter sunlight, no scattered light reaches the astronaut's eyes from the sky, so the sky looks black even in daylight.

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2. Q242 CBSE 2018
   1. (a)​What is presbyopia ​? State ​its cause. How is ​it ​corrected ?
   2. (b)Why does the Sun appear reddish early in the morning ? Explain with the help ​of a ​labelled diagram.
   Show answer

   Presbyopia (loss of accommodation with age) needs a convex lens; the morning Sun looks reddish as blue light is scattered away.

   (a) Presbyopia is the defect of vision in which the power of accommodation of the eye decreases with age: the near point gradually recedes, so the person cannot see nearby objects comfortably and distinctly without corrective glasses. Cause: gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens. Correction: spectacles with a convex lens of suitable power; a person who also has myopia uses bifocal lenses, whose upper part is a concave lens (distant vision) and lower part a convex lens (near vision). (b) Early in the morning the Sun is near the horizon, so its light passes through a much thicker layer of air, a longer distance in the atmosphere, before reaching our eyes. Air molecules scatter blue light (shorter wavelength) much more than red light (longer wavelength), so most of the blue is scattered away on the way and mainly red light reaches our eyes: the Sun appears reddish. At noon the light travels a short path through the air, little blue is scattered, and the Sun appears white. The labelled diagram (the figure at step 5) shows the Sun near the horizon with its long path through the atmosphere, the Sun overhead with its short path, the observer on the Earth, and blue light scattered away along the long path.

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3. Q243 CBSE 2018
   1. (a)​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.
   2. (b)Why do stars twinkle ​? Explain.
   Show answer

   Myopia, corrected by a concave lens; stars twinkle as atmospheric refraction keeps changing their apparent position and the light reaching the eye.

   (a) The boy is suffering from myopia (near-sightedness): distant objects, such as the words on a board $3$ m away, cannot be seen distinctly, though nearby objects can. The image of a distant object is formed in front of the retina. Possible causes: (i) excessive curvature of the eye lens (the eye lens becomes too converging); (ii) elongation of the eyeball. Correction: myopia is corrected by using a concave (diverging) lens of suitable power, whose focal length equals the distance of the far point. The concave lens makes the rays from a distant object diverge as if they came from the far point of the eye, so the eye lens can form the image on the retina (ray diagram: without the lens the rays meet in front of the retina; with the concave lens they meet on the retina). (b) Stars twinkle because of atmospheric refraction of starlight. Starlight passes through layers of air of varying density (refractive index) before reaching us and is bent, so the star appears at a position slightly different from (higher than) its actual position. As the layers of air keep changing, the apparent position of the star keeps changing and the amount of starlight entering the eye flickers, so the star appears to twinkle.

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4. Q244 CBSE 2018
   (a) Write the function ​of each of the ​following parts of human eye :

   1. (i)Cornea
   2. (ii)​Iris
   3. (iii)Crystalline lens
   4. (iv)Ciliary muscles (b) Why does the sun appear reddish ​early in ​the morning ? Will this phenomenon be observed by an astronaut on the ​Moon ? Give reason to ​justify your answer.
   Show answer

   The four eye parts admit, control and focus light; the Sun looks reddish as blue is scattered away, but not on the Moon.

   (a) (i) Cornea: light enters the eye through this transparent front surface, and most of the refraction of the light entering the eye takes place at it. (ii) Iris: controls the size of the pupil, and so the amount of light entering the eye. (iii) Crystalline lens: converges the light rays to form a real, inverted image on the retina, providing the fine adjustment of focal length needed for objects at different distances. (iv) Ciliary muscles: change the curvature, and so the focal length, of the eye lens by relaxing and contracting, so that a sharp image forms on the retina (accommodation). (b) Early in the morning the Sun is near the horizon, so its light has to travel a much larger distance through the atmosphere. Most of the blue light and other shorter wavelengths are scattered away by the particles of air, and mainly the longer wavelengths, such as red, reach our eyes; so the Sun appears reddish. No, an astronaut on the Moon will not observe this, because the Moon has no atmosphere to scatter the sunlight.

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5. Q245 CBSE 2017
   Sumit was ​sitting at the back in ​the ​classroom. He was not able to read the sentences clearly which were written on the black board. ​What type of eye defect he is ​suffering from ? Draw ray ​diagram to show this defect of vision and its correction ​using a suitable ​lens.
   Show answer

   Myopia; a concave lens corrects it by making rays from a distant object diverge as if from the far point.

   Sumit is suffering from myopia (near-sightedness). He can see nearby objects clearly but cannot see distant objects, such as the writing on the blackboard from the back of the classroom, distinctly: his far point is nearer than infinity, and the image of a distant object is formed in front of the retina (ray diagram (b)). It is corrected by a concave (diverging) lens of suitable power, whose focal length is equal in magnitude to the distance of the far point from the lens. The lens makes the parallel rays from a distant object diverge as if they came from the far point O, and the eye lens then forms the image on the retina (ray diagram (c)).

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6. Q246 CBSE 2017
   What is scattering ​of light ? ​Why ​do the colour of ​clear ​sky appear blue ​? Explain.
   Show answer

   Scattering sends light off in various directions from tiny particles; air scatters blue light most strongly, so the clear sky looks blue.

   Scattering of light: when light falls on very small particles in its path (molecules of air, fine dust, smoke, tiny water droplets), part of it is sent off in various directions. This is called scattering of light. Why the clear sky is blue: the molecules of air and other fine particles in the atmosphere are smaller than the wavelength of visible light, and such particles scatter light of shorter wavelengths (the blue end of the spectrum) more strongly than light of longer wavelengths (the red end); red light has a wavelength about $1.8$ times that of blue. So when sunlight passes through the atmosphere, its blue component is scattered much more strongly than its red. This scattered blue light reaches our eyes from all directions of the sky, and the clear sky appears blue. (If there were no atmosphere there would be no scattering, and the sky would look dark.)

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7. Q247 CBSE 2017
   Varun instead of copying from the black board use ​to copy regularly from the note book of his friend, Sudhir with whom he ​sat ​on the same desk. Sudhir told the teacher about it. The teacher asked Varun to get ​his eyes checked by a doctor and explained to whole class the reason why Varun copied from Sudhir’s notebook.

   1. (i)​What in your ​view, is wrong with Varun’s eyes and how can it be corrected ?
   2. (ii)If the doctor prescribes Varun to use lenses of power $-0.5$ D, ​write the type of these lenses.
   3. (iii)Write the values displayed by Sudhir and his teacher.
   Show answer

   Varun has myopia, corrected by a concave lens; $-0.5$ D lenses are concave; Sudhir and the teacher show care and concern.

   (i) Varun is suffering from myopia (near-sightedness): he sees nearby objects, such as the notebook on his desk, clearly, but distant objects, such as the blackboard, look blurred, because the image of a distant object is formed in front of the retina. It can be corrected by using spectacles with a concave (diverging) lens of suitable power. (ii) The power is negative, so these are concave (diverging) lenses; their focal length is $f=\frac{1}{-0.5}$ m $=-2$ m. (iii) Sudhir: concern and care for his friend, and a sense of responsibility — he reported the problem so that Varun could get help. The teacher: care and sensitivity towards the student, and a scientific attitude — instead of punishing Varun, the teacher found the real cause, sent him for an eye check-up and made the whole class aware of it.

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8. Q248 CBSE 2017
   Name the type ​of ​defect of vision ​a person is suffering from, ​if he ​uses convex lenses in his spectacles for the correction of ​his ​vision. If the power of the lenses is $+0.5$ D, ​find the focal length of the lenses.
   Show answer

   Hypermetropia (long-sightedness); the lens is convex, with $f=+2$ m.

   The person has hypermetropia (long-sightedness): nearby objects cannot be seen clearly because the near point of the eye is farther than $25$ cm, and a convex (converging) lens corrects it. (Presbyopia, in which the near point recedes with age, is also corrected with convex lenses for near vision.) Focal length: $f=\frac{1}{P}=\frac{1}{+0.5\text{D}}=+2$ m $=+200$ cm, a convex lens of focal length $2$ m.

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9. Q249 CBSE 2017
   What ​is scattering ​of light ? ​Why ​is the colour of the ​clear sky blue ​? ​Explain.
   Show answer

   Scattering is the spreading of light by very small particles; the sky is blue because air molecules scatter blue light most.

   Scattering of light: when light falls on very small particles in its path, such as molecules of air, fine dust or tiny water droplets, the particles send the light out in many different directions. This spreading of light by the particles is called scattering of light. The clear sky is blue because the molecules of air and other fine particles in the atmosphere are smaller than the wavelength of visible light, and such particles scatter light of shorter wavelengths (the blue end of the spectrum) much more strongly than light of longer wavelengths (the red end); red light has a wavelength about $1.8$ times that of blue. So when sunlight passes through the atmosphere, its blue component is scattered strongly in all directions, and this scattered blue light reaches our eyes from every part of the sky. Hence the clear sky appears blue.

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10. Q250 CBSE 2017
    Name the phenomenon ​responsible for the early sunrise and delayed sunset. ​Explain with the help ​of a diagram the reason why the sun ​is ​visible to us about 2 minutes before the actual sun-rise and about 2 minutes after ​the ​actual sunset.
    Show answer

    Atmospheric refraction bends sunlight, so the Sun is seen about 2 minutes before actual sunrise and about 2 minutes after actual sunset.

    The phenomenon is atmospheric refraction (the refraction of sunlight by the Earth’s atmosphere). Reason: the air near the Earth’s surface is denser than the air higher up, so the refractive index of the atmosphere increases gradually towards the ground. When the Sun is still slightly below the horizon, its light enters the atmosphere and passes from rarer into denser layers, bending towards the normal at every layer, so its path curves towards the Earth and reaches the observer. The observer sees the Sun along the direction from which the light finally enters the eye, and that direction is above the horizon. So the Sun appears above the horizon about 2 minutes before the actual sunrise (its actual crossing of the horizon). In the same way, at sunset its light is still bent towards us after it has gone below the horizon, so it stays visible for about 2 minutes after the actual sunset. Diagram: the observer on the Earth, the horizon, the actual Sun below the horizon, the ray bending in the layers of air, and the apparent Sun above the horizon on the line of sight.

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11. Q251 CBSE 2017
    State ​the ​main cause of atmospheric ​refraction of ​light. ​With the ​help of a ​diagram, explain the phenomenon of twinkling of stars.
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    Atmospheric refraction arises from air of changing density and temperature; starlight keeps bending, so the star's apparent position and brightness fluctuate, causing twinkling.

    Main cause of atmospheric refraction: the air is made up of layers of different temperature and density, and hence of different refractive index (hot air is less dense and has a slightly lower refractive index than cooler air; the air is generally denser near the Earth), and these conditions keep changing. Light passing through the atmosphere is therefore refracted continuously. Twinkling of stars: starlight entering the atmosphere passes into denser and denser layers and bends towards the normal each time, so it reaches the eye along a steeper path, and the star is seen at an apparent position slightly higher than its actual position (see the diagram). Since the physical conditions of the atmosphere keep changing, the path of the starlight keeps changing slightly: the apparent position of the star fluctuates, and the amount of starlight entering the eye flickers — the star appears brighter at one moment and fainter at the next. Stars are so far away that they act as point sources, so this flickering is seen as twinkling.

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12. Q252 CBSE 2017
    What is ​myopia ? ​List ​its two causes. ​Draw a ray diagram to explain ​its correction using an appropriate ​lens.
    Show answer

    Myopia is near-sightedness: the image of distant objects forms before the retina; a concave lens of suitable power corrects it.

    Myopia (near-sightedness) is the defect of vision in which a person can see nearby objects clearly but cannot see distant objects distinctly. Its far point is nearer than infinity, and the image of a distant object is formed in front of the retina. Two causes: (i) excessive curvature of the eye lens, so that it converges light too strongly; (ii) elongation of the eyeball, so that the retina lies too far behind the eye lens. Correction: a concave (diverging) lens of suitable power, whose focal length is equal in magnitude to the distance of the far point. It spreads the parallel rays from a distant object so that they appear to come from the far point F; the eye can focus on F, so the image is now formed on the retina (see the ray diagram).

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13. Q253 CBSE 2017
    What ​is hypermetropia ​? List ​its two causes. ​Draw a ray diagram ​to explain the correction ​of this defect using ​an appropriate lens.
    Show answer

    Hypermetropia, in which nearby objects are blurred, is caused by a long focal length or small eyeball and corrected by a convex lens.

    Hypermetropia (far-sightedness) is the defect of vision in which a person can see distant objects clearly but cannot see nearby objects distinctly. The near point of the eye lies farther away than the normal near point (25 cm): light from a nearby object would be focussed at a point behind the retina. Two causes: (i) the focal length of the eye lens is too long; (ii) the eyeball has become too small. Correction: a convex (converging) lens of suitable power. As the ray diagram shows, rays from an object at the normal near point N (25 cm) are refracted by the convex lens so that they appear to come from N′, the near point of the defective eye; the eye can focus N′, so the image is formed on the retina.

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14. Q254 CBSE 2017
    Describe ​an ​activity ​that demonstrates the ​scattering ​of ​light.
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    A beam through sodium thiosulphate and acid solution shows blue light scattered at the sides and red transmitted light.

    Activity: Place a strong source of white light at the focus of a convex lens so that a parallel beam passes through a transparent glass tank of clear water, then through a circular hole in a cardboard; a second convex lens forms a sharp image of the hole on a screen. Dissolve about 200 g of sodium thiosulphate in about 2 L of water in the tank and add about 1 to 2 mL of concentrated sulphuric acid. Observation: in 2 to 3 minutes fine particles of sulphur begin to form. Blue light is then seen from the three sides of the tank, while the image of the hole on the screen (the transmitted light, seen through the fourth side) appears at first orange-red and then bright crimson red. Explanation: the very fine colloidal sulphur particles scatter the short wavelengths (blue) much more than the long ones (red). The scattered blue light is seen from the sides, and the transmitted beam, having lost much of its blue, looks red. This demonstrates the scattering of light, which also explains the blue colour of the sky and the reddish Sun at sunrise and sunset.

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15. Q255 CBSE 2017
    Curvature ​of eye lens of human eye can be modified by ​ciliary muscles to some ​extent so that its focal length is changed as per requirement. How ​will the curvature and focal length of eye lens change when

    1. (a)a ​distant object ​is to be seen and
    2. (b)​a nearby object is to be seen clearly ? Write the reason why a normal eye is not ​able to see clearly, the objects placed closer than 25 cm, without any strain on the eye.
    Show answer

    The eye lens thins for distant objects and thickens for near ones, but cannot thicken enough for objects closer than 25 cm.

    (a) To see a distant object, the ciliary muscles relax and the eye lens becomes thin: its curvature decreases and its focal length increases (it is then at its longest), so that parallel rays from the distant object are focused on the retina. (b) To see a nearby object clearly, the ciliary muscles contract and the eye lens becomes thicker: its curvature increases and its focal length decreases, so that the image of the near object still forms on the retina. Reason: the focal length of the eye lens cannot be decreased below a certain minimum limit, because the lens can be made only so curved. For a normal eye this limit corresponds to an object about 25 cm away (the near point, or least distance of distinct vision). An object closer than 25 cm would need a still shorter focal length; its image would fall behind the retina, so it looks blurred and the eye is strained.

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16. Q256 CBSE 2017
    Due to ​gradual weakening of ciliary muscles and diminishing flexibility of ​the eye lens a certain defect of vision arises. Write the name of this ​defect. Name the type of lens required ​by ​such ​persons to improve the ​vision. Explain the structure and function ​of such a lens.
    Show answer

    Presbyopia; corrected by bifocal lenses, with a concave upper part for distant vision and a convex lower part for near vision.

    The defect is presbyopia: with age the power of accommodation of the eye decreases, the near point recedes, and it becomes difficult to see nearby objects comfortably and distinctly. Such persons need a convex lens for near vision, and since with age many of them also find distant objects blurred, they commonly use bifocal lenses. Structure: a common bifocal lens consists of both a concave and a convex lens; the upper portion is a concave lens and the lower portion is a convex lens. Function: the upper concave portion facilitates distant vision, and the lower convex portion facilitates near vision, such as reading.

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17. Q257 CBSE 2017
    With the help of ciliary ​muscles the human eye can change its curvature and thus ​alter the focal ​length of its lens. State the changes ​that ​occur in the curvature and focal length of the eye lens while viewing

    1. (a)​a distance object, ​
    2. (b)nearby objects. Explain, why a normal eye is not able to see distinctly ​the objects placed closer than 25 cm, without putting any strain on the eye.
    Show answer

    The lens thins for distant objects and thickens for near ones, but its focal length cannot shorten enough for objects closer than 25 cm.

    (a) Distant object: the ciliary muscles relax, the eye lens becomes thin, its curvature decreases and its focal length increases (it is longest). (b) Nearby objects: the ciliary muscles contract, the eye lens becomes thicker, its curvature increases and its focal length decreases. Why not nearer than 25 cm: the focal length of the eye lens cannot be decreased below a certain minimum, because the ciliary muscles cannot make the lens more curved beyond a limit. For a normal eye this shortest focal length focuses an object at about 25 cm (the near point, or least distance of distinct vision) on the retina. An object placed nearer would need a still shorter focal length, which the lens cannot provide, so its image would form behind the retina and look blurred, and the eye strains in trying to focus it.

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18. Q258 CBSE 2017
    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 ?
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    Accommodation is the eye lens adjusting its focal length; the image distance stays fixed because the lens-retina distance cannot change.

    Power of accommodation: the eye lens is flexible, and the ciliary muscles can change its curvature and so its focal length. The ability of the eye lens to adjust its focal length so that objects at different distances are focused sharply on the retina is called the power of accommodation. For a distant object the ciliary muscles relax, the lens becomes thin and its focal length increases; for a nearby object they contract, the lens becomes thicker and its focal length decreases. A normal eye can focus objects from about 25 cm (the near point) to infinity (the far point). Why the image distance does not change: for an object to be seen clearly its image must be formed on the retina, and the distance between the eye lens and the retina is fixed by the size of the eyeball. So the image distance cannot change; when the object distance changes, the eye keeps the image on the retina by changing the focal length of its lens (accommodation), not by changing the image distance.

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19. Q259 CBSE 2017
    With the help of a ​diagram, explain ​how atmospheric refraction ​is responsible ​for the advance sunrise and delayed ​sunset. How much time difference does it cause on the ​duration of ​day on the Earth ?
    Show answer

    Atmospheric refraction bends sunlight towards the Earth, so the Sun is seen about 2 minutes before actual sunrise and after actual sunset.

    Air near the Earth is denser than the air above it, and its refractive index increases gradually towards the ground. At sunrise, while the Sun is still slightly below the horizon, its light passes into denser and denser layers of air and bends continuously towards the normal, that is towards the Earth, so it reaches the observer along a curved path. The eye sees the Sun along the last direction of this light, extended backwards, which points above the horizon: the Sun appears above the horizon before it has actually risen (advance sunrise). In the same way, at sunset light from the Sun keeps reaching the observer for a short time after the Sun has actually gone below the horizon (delayed sunset). The diagram shows the actual and apparent positions of the Sun. Time difference: about 2 minutes. The Sun is seen about 2 minutes before the actual sunrise and stays visible for about 2 minutes after the actual sunset, so the day as a whole is about 4 minutes longer.

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20. Q260 CBSE 2017
    What is scattering of light ? Use ​this phenomenon to explain why ​the clear ​sky ​appears blue ​and the sun appears ​reddish at sunrise.
    Show answer

    Fine particles scatter blue light much more than red, so the clear sky looks blue and the Sun looks reddish at sunrise.

    Scattering of light: when light falls on very small particles in its path (molecules of air, dust, smoke, water droplets), part of it is sent out in different directions; this is called scattering of light. Very fine particles scatter mainly the shorter wavelengths, at the blue end of the spectrum. Blue sky: the molecules of air and other fine particles in the atmosphere are smaller than the wavelength of visible light, so they scatter blue light much more strongly than red light (red has a wavelength about 1.8 times that of blue). The scattered blue light reaches our eyes from all parts of the sky, so the clear sky appears blue. Reddish Sun at sunrise: near the horizon, sunlight passes through much thicker layers of air, a far longer path, before reaching us. On the way most of the blue and other shorter wavelengths are scattered out of the beam, and the light that reaches us is mainly of longer wavelengths, so the Sun appears reddish.

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