CBSE Class 10 Science HOTs Human Eye and Colourful World Set 06

This page has 20 higher order thinking (HOTs) questions with answers for CBSE Class 10 Science Chapter 10, The Human Eye and the Colourful World, taken from question banks and board-style practice. The questions ask you to reason about accommodation, the far point and power of corrective lenses, twinkling and the colour of the sky, atmospheric refraction, dispersion by prisms and the rainbow, and each answer is explained step by step.

Very Short Answer HOTS Questions

Question 1. What happens to the image distance in the normal human eye when we decrease the distance of an object, say from 10 m to 1 m? Justify your answer.
Answer:
The image distance remains the same.
The image is always formed on the retina, which is at a fixed distance from the eye lens. When the object comes closer, the ciliary muscles make the lens thicker, so its focal length decreases and the image stays on the retina.
In simple words: The eye changes the focal length of its lens, not the distance of the retina.

Teacher's Note:
1. The retina is at a fixed distance from the lens.
2. Focal length changes; image distance does not.

Question 2. No rainbow could be observed from the surface of the Moon by the astronauts. Give reason.
Answer:
A rainbow needs water droplets in the air to disperse sunlight.
The Moon has no atmosphere and therefore no water droplets, so no dispersion of sunlight can take place and no rainbow is formed.
In simple words: No air and no water drops means no rainbow.

Teacher's Note:
1. Link the rainbow to droplets in the atmosphere.
2. Mention both the missing atmosphere and the missing droplets.

Question 3. A star appears on the horizon. What is the true position of the star? Explain.
Answer:
The true position of the star is below the horizon.
Starlight travels from the rarer upper air to the denser lower air layers and bends towards the normal each time. So it seems to come from a slightly higher position than the real one, and the star is seen on the horizon even when it is actually just below it.
In simple words: Bent starlight makes the star look a little higher than it really is.

Teacher's Note:
1. The bending is called atmospheric refraction.
2. Apparent position is higher than true position.

Question 4. Mala can see the objects lying between 25 cm and 100 cm from her eye. Her vision can be corrected by using a lens of power −0.1 D. Is the statement true or false?
Answer:
Her far point is 100 cm, so the corrective lens must have f = −100 cm = −1 m.
P = 1/f = 1/(−1) = −1 D.
The given power −0.1 D is wrong, so the statement is false.
In simple words: The correct lens power is −1 D, so the statement is false.

Teacher's Note:
1. The far point equals the magnitude of the focal length.
2. Check units: use metres.

Question 5. If Ram, a myopic person, uses spectacles of power −0.5 D, what will be the distance of the far point of his eye?
Answer:
f = 1/P = 1/(−0.5) = −2 m.
The lens forms a virtual image of a far object at the far point, so the far point is at 2 m from the eye.
In simple words: Without spectacles he can see clearly only up to 2 m.

Teacher's Note:
1. Far point = focal length magnitude.
2. Negative power means a concave lens.

Short Answer HOTS Questions

Question 6. Explain why the planets do not twinkle.
Answer:
Planets are much closer to the Earth than stars and are seen as extended sources of light.
A planet can be taken as a collection of a large number of point-sized sources. The variation in the light from some points is cancelled by the opposite variation from others, so the total amount of light entering our eye stays almost constant. Hence planets do not twinkle.
In simple words: A planet is a patch of light, so the flickering averages out.

Teacher's Note:
1. Contrast with stars, which are point sources.
2. The cause of twinkling is atmospheric refraction.

Question 7. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
Answer:
The ciliary muscles can make the eye lens thicker only up to a limit, so the focal length of the eye lens cannot be decreased below a certain minimum.
For an object closer than 25 cm, the rays are not converged enough to form a sharp image on the retina, so the object looks blurred and the eye feels strain.
In simple words: The lens cannot become thicker than its limit, so very close objects stay blurred.

Teacher's Note:
1. Mention the minimum focal length.
2. The 25 cm limit is the near point.

Question 8. Why does the sky at noon appear white?
Answer:
At noon the Sun is directly overhead, so its light travels the shortest path through the atmosphere.
Very little scattering of any one colour takes place, so the light that reaches us is nearly the same as sunlight and appears white.
In simple words: A short path through air means little scattering and white light.

Teacher's Note:
1. Compare with the long path at sunrise and sunset.
2. Scattering depends on the path length.

Question 9. Ram can read a book clearly only if he holds it at an arm's length from him. Name the defect of vision if he is an old man.
Answer:
He is suffering from presbyopia.
With age the ciliary muscles weaken and the eye lens becomes less flexible, so the near point moves farther than 25 cm and he cannot see nearby objects clearly.
In simple words: Holding a book far away is the sign of presbyopia in old age.

Teacher's Note:
1. Link the symptoms with age.
2. Reading glasses or bifocal lenses correct it.

Question 10. In a certain murder investigation, it was important to discover whether the victim was long-sighted or short-sighted. How could a detective decide by examining his spectacles?
Answer:
If the spectacle lenses are concave (thinner at the centre), the victim was short-sighted (myopic).
If the spectacle lenses are convex (thicker at the centre), the victim was long-sighted (hypermetropic).
In simple words: Concave lenses mean short-sighted; convex lenses mean long-sighted.

Teacher's Note:
1. Use the type of lens to name the defect.
2. A quick way to check: a concave lens makes things look smaller.

Question 11. By how much time would the day have been shorter if the Earth had no atmosphere?
Answer:
Because of atmospheric refraction, the Sun is seen about 2 minutes before the actual sunrise and about 2 minutes after the actual sunset.
Without the atmosphere these gains would not occur, so the day would be shorter by about 4 minutes.
In simple words: The atmosphere adds about 4 minutes to the apparent length of the day.

Teacher's Note:
1. Add the two gains of about 2 minutes.
2. The cause is atmospheric refraction.

Question 12. A beam of blue, green and yellow light passes through the Earth's atmosphere. Name the colour which is scattered (i) the most, (ii) the least.
Answer:
Scattering is greater for shorter wavelengths.
(i) Blue has the shortest wavelength of the three and is scattered the most.
(ii) Yellow has the longest wavelength of the three and is scattered the least.
In simple words: The shorter the wavelength, the more it is scattered.

Teacher's Note:
1. Order of wavelength: blue < green < yellow.
2. Scattering is inversely related to wavelength.

Question 13. A person is able to see objects clearly only when these are lying at distances between 50 cm and 300 cm from his eyes. Name the kind of defect of vision he is suffering from.
Answer:
His near point (50 cm) is farther than the normal 25 cm, and his far point (300 cm) is nearer than infinity.
So he has both the defects of hypermetropia and myopia, which is the condition seen in presbyopia in old age. Bifocal lenses correct it.
In simple words: His clear-vision range is short at both ends, so he needs bifocal lenses.

Teacher's Note:
1. Check both the near point and the far point.
2. The upper part of a bifocal lens is concave and the lower part is convex.

Question 14. Explain why the change in the direction of violet light is greater than the change in the direction of red light when white light passes through a prism.
Answer:
In glass, violet light travels slower than red light, so the refractive index of glass is greater for violet than for red.
A larger refractive index means greater bending. So violet is bent more (deviated more) than red, which is deviated the least.
In simple words: Glass slows violet most, so violet bends most.

Teacher's Note:
1. Link speed, refractive index and bending.
2. Red is deviated the least.

Question 15. Nanda saw rays of sunlight entering a dark room. (A) Name the phenomenon involved. (B) He did something to the air in the room after which he was not able to see the rays of sunlight in the room. What could he have done? Justify your answer.
Answer:
(A) The Tyndall effect (scattering of light by dust and fine particles in the air).
(B) He could have removed the dust and smoke particles from the air (for example by cleaning the room or filtering the air).
Without particles to scatter the light, the beam goes straight and no light reaches the eye from the sides, so the path of the beam is not visible.
In simple words: We see the sunbeam only because dust scatters the light; remove the dust and the beam disappears.

Teacher's Note:
1. The beam is seen by the scattered light.
2. Name the phenomenon first.

Question 16. Give reasons: (i) There is no dispersion of light refracted through a rectangular glass slab. (ii) There is no dispersion of light when a monochromatic light having only one wavelength passes through a prism.
Answer:
(i) The opposite faces of a rectangular slab are parallel. The colours that are separated at the first face are bent back by an equal amount at the second face and emerge parallel to one another, so they overlap and appear as white light.
(ii) Dispersion means separation of colours. Monochromatic light has only one colour (one wavelength), so there is nothing to separate. It only deviates as a whole.
In simple words: A slab undoes its own bending, and a single colour cannot be split.

Teacher's Note:
1. Parallel faces cancel the separation.
2. Monochromatic light shows deviation but not dispersion.

Long Answer and Case Based HOTS Questions

Question 17. What happens to a beam of white light when it gets refracted through a glass prism? Which colour deviates the most and the least after refraction through a prism? What is likely to happen if a second identical prism is placed in an inverted position with respect to the first prism? Justify your answer.
Answer:
White light splits into its constituent colours (violet, indigo, blue, green, yellow, orange, red) because each colour bends through a different angle. This is dispersion.
Violet deviates the most and red the least.
If a second identical prism is placed in an inverted position, it bends each colour in the opposite direction by the same amount, so the colours recombine and white light emerges, parallel to the incident beam.
In simple words: One prism splits white light; a second, inverted prism joins it back.

Teacher's Note:
1. The two prisms must be identical.
2. This shows that white light is a mixture of colours.

Question 18. Read the passage and answer the questions.
The cause of myopia is that the eyeball gets elongated and the focal length of the eye lens becomes too short, or the power of the eye lens becomes too high. By elongation of the eyeball we mean that the distance between the eye lens and the retina increases. When this distance becomes more than the focal length of the lens, the eye lens cannot form a sharp image of a distant object on the retina but forms the image in front of it.
(i) Is the eye lens made of glass? (ii) A myopic person uses spectacles of power −0.2 D. What is the distance of the far point of his eye? (iii) Which lens is used for correcting a myopic eye: convex, concave or bifocal? (iv) At which part does the human eye form the image of an object?
Answer:
(i) No. The eye lens is made of a transparent, fibrous, jelly-like material.
(ii) f = 1/P = 1/(−0.2) = −5 m, so the far point is 5 m.
(iii) A concave lens.
(iv) On the retina.
In simple words: A myopic eye needs a concave lens, and a −0.2 D lens means a far point of 5 m.

Teacher's Note:
1. Far point = magnitude of focal length.
2. The image is formed on the retina in a normal eye.

Question 19. Read the passage and answer the questions.
White light is a mixture of seven colours: violet, indigo, blue, green, yellow, orange and red. Every colour has its own characteristic wavelength: red 7900 Å, orange 6000 Å, yellow 5800 Å, green 5400 Å, blue 4800 Å, indigo 4500 Å, violet 4000 Å. The splitting of white light into seven colours when it passes through a glass prism is called dispersion.
(i) Name the phenomenon occurring in nature due to dispersion of light. (ii) Light of two colours A and B passes through a glass prism. A deviates more than B from its path of incidence. Which colour has a higher speed in the prism? (iii) Do all colours of light travel with the same speed in a given medium such as glass?
Answer:
(i) The rainbow.
(ii) Colour B has the higher speed. The colour that deviates more has the larger refractive index and so moves slower in the prism.
(iii) No. In a given medium each colour travels with a different speed, which is why the colours bend by different amounts.
In simple words: The more a colour bends, the slower it travels in glass.

Teacher's Note:
1. Greater deviation means greater refractive index and lower speed.
2. In vacuum all colours have the same speed.

Question 20. Read the passage and answer the questions.
The ciliary muscles of the eye control the curvature of the lens and hence alter the effective focal length of the system. When the muscles are fully relaxed, the focal length is maximum. When the muscles are strained, the curvature of the lens increases and the focal length decreases. For clear vision the image must be on the retina, so the image distance is fixed and equals the distance of the retina from the eye lens. It is about 2.5 cm for a grown-up person. The smallest distance at which a person can see clearly is related to the minimum possible focal length. For an average grown-up person the minimum distance of an object is around 25 cm.
(i) What do you mean by the ciliary muscles? (ii) What is the minimum focal length of the eye lens of a normal person? (iii) What is the maximum focal length of the eye lens of a normal person? (iv) A near-sighted man can clearly see objects only up to a distance of 100 cm and not beyond this. What is the power of the spectacle lens necessary for the remedy of this defect?
Answer:
(i) The ciliary muscles are the muscles that hold the eye lens and change its curvature, thereby changing its focal length.
(ii) The image distance is 2.5 cm and the nearest object is at 25 cm. 1/f = 1/v − 1/u = 1/2.5 + 1/25 = 0.44, so f ≈ 2.27 cm.
(iii) For a very distant object the image is formed at the focus, so the maximum focal length is 2.5 cm.
(iv) The far point is 100 cm, so f = −100 cm = −1 m and P = −1 D (concave lens, 'number' −1).
In simple words: The eye lens has focal length between about 2.27 cm and 2.5 cm, and a far point of 1 m needs a −1 D lens.

Teacher's Note:
1. Use the lens formula with the fixed image distance 2.5 cm.
2. The number of the spectacle lens equals its power with sign.