CBSE Class 10 Science The Human Eye and the Colourful World MCQs Set 09

Answer: (c) 25 cm

Explanation:
1. The minimum distance at which the eye sees an object clearly without strain is the least distance of distinct vision (near point).
2. For a young adult with normal vision it is about 25 cm.

Teacher's Note:
1. Remember the pair: near point 25 cm and far point at infinity.
2. The range of vision of a normal eye is from 25 cm to infinity.

Answer: (b) outer surface of the cornea

Explanation:
1. Light bends most when it passes from air into the cornea, because the change in refractive index is largest there.
2. The eye lens only provides the fine adjustment for focusing objects at different distances.

Teacher's Note:
1. Cornea does the bulk of the bending; the lens does the fine focusing.
2. Air to cornea is the largest jump in refractive index.

Answer: (c) focus objects at different distances

Explanation:
1. The ciliary muscles change the curvature, and so the focal length, of the eye lens.
2. This ability to focus objects at different distances is the power of accommodation.

Teacher's Note:
1. Accommodation means adjusting the focal length of the eye lens.
2. It is not the same as the change in pupil size.

Answer: (b) contract, decrease

Explanation:
1. For a near object the lens must be more strongly curved.
2. The ciliary muscles contract, the lens becomes thicker and its focal length decreases.

Teacher's Note:
1. Rule: near object means thick lens and small focal length.
2. Long reading tires the eyes because the muscles stay contracted.

Answer: (b) short-sightedness or myopia

Explanation:
1. In myopia (short-sightedness) the image of a distant object forms in front of the retina.
2. Nearby objects are seen clearly but distant ones look blurred.

Teacher's Note:
1. Myopia: near is clear. Hypermetropia: far is clear.
2. The name tells what you can see: short-sighted sees short distances.

Answer: (c) presbyopia

Explanation:
1. With age the ciliary muscles weaken and the lens becomes less flexible.
2. The eye cannot then adjust its focal length properly; this defect is presbyopia.

Teacher's Note:
1. Presbyopia is an old-age defect and may affect both near and far vision.
2. It is often corrected with bifocal lenses.

Answer: (c) using bifocal lenses

Explanation:
1. A bifocal lens has a concave upper part for distant vision and a convex lower part for near vision.
2. Such spectacles help a person with presbyopia see objects at all distances.

Teacher's Note:
1. Bifocal means two focal lengths in one lens.
2. The upper part is for far and the lower part for near.

Answer: (b) myopia

Explanation:
1. Negative power means a concave (diverging) lens.
2. Concave lenses are used to correct myopia.

Teacher's Note:
1. A minus sign on a prescription means a concave lens.
2. Concave lens corrects near-sightedness.

Answer: (a) myopia

Explanation:
1. He sees nearby objects but not distant ones.
2. This is myopia (short-sightedness).

Teacher's Note:
1. Not seeing far objects is the sign of myopia.
2. A concave lens corrects it.

Answer: (b) −0.5 D

Explanation:
1. The person is myopic with far point 2 m. The lens must form the image of a distant object at 2 m, so f = −2 m.
2. Power P = 1/f = 1/(−2) = −0.5 D.

Teacher's Note:
1. For myopia the focal length of the corrective lens equals the far point, with a negative sign.
2. Convert distance to metres before using P = 1/f.

Answer: (d) −1.0 D

Explanation:
1. The man is short-sighted with far point 100 cm = 1 m, so the lens must have f = −1 m.
2. P = 1/f = −1.0 D.

Teacher's Note:
1. The source lists the answer as positive, but a short-sighted person needs a concave lens of negative power.
2. Far point equals the focal length (with a negative sign).

Answer: (a) −0.25 D

Explanation:
1. Far point = 4 m, so f = −4 m for the corrective lens.
2. P = 1/(−4) = −0.25 D.

Teacher's Note:
1. Short-sighted: concave lens, negative power.
2. Do the division carefully: 1 ÷ 4 = 0.25.

Answer: (b) −1/3 D and +2 D

Explanation:
1. (i) For distant objects f = −far point = −3 m, so P = −1/3 D.
2. (ii) For reading, u = −0.25 m and v = −0.5 m: 1/f = −2 + 4 = 2, so P = +2 D.

Teacher's Note:
1. Two defects are present, so two different lenses are needed (a bifocal).
2. The first part is a concave lens and the second a convex lens.

Answer: (c) dispersion

Explanation:
1. A glass prism bends light of different colours by different amounts.
2. The splitting of white light into its component colours is called dispersion.

Teacher's Note:
1. Dispersion = splitting into colours.
2. Deviation = turning of the ray from its original path.

Answer: (d) gets deviated and bends towards the thicker part (base) of the prism

Explanation:
1. The ray refracts at both faces of the prism, which are inclined to each other.
2. Each refraction turns the ray towards the base, so the emergent ray bends towards the thicker part.

Teacher's Note:
1. A prism always deviates light towards its base.
2. In a glass slab the emergent ray is parallel instead.

Answer: (b) Twice

Explanation:
1. The ray refracts once when it enters the prism and once when it leaves it.
2. So it bends twice.

Teacher's Note:
1. Count the surfaces the ray crosses: two refractions.
2. Both bendings turn the ray towards the base.

Answer: (b) r > v

Explanation:
1. Violet has a higher refractive index than red in glass, so violet bends more towards the normal.
2. Hence its angle of refraction is smaller: r > v.

Teacher's Note:
1. More bending towards the normal means a smaller angle of refraction.
2. Violet bends more than red.

Answer: (c) (I), (II) and (III)

Explanation:
1. The correct observation is (IV): a band of seven colours with red and violet at the two ends.
2. Statements (I), (II) and (III) describe things that do not happen, so they are incorrect.

Teacher's Note:
1. Read whether the question asks for correct or incorrect statements.
2. The spectrum always has red at one end and violet at the other.

Answer: (a) Red > Green > Blue

Explanation:
1. In glass red light travels fastest and violet slowest.
2. So the correct order is red > green > blue (the source key listed the reverse order, which is wrong).

Teacher's Note:
1. Higher refractive index means lower speed.
2. Blue has a higher refractive index than green, and green than red.

Answer: (a) Refraction, total internal reflection, refraction

Explanation:
1. Light first refracts (and disperses) when it enters the raindrop.
2. It then undergoes internal reflection at the back of the drop and finally refracts again as it leaves.

Teacher's Note:
1. Order inside the drop: in, reflect, out.
2. This single reflection gives the primary rainbow.

Answer: (b) refraction of light by different layers of varying refractive indices

Explanation:
1. The atmosphere has layers of air of different density and refractive index, which keep changing.
2. Starlight passing through them bends by a changing amount, so the apparent position and brightness of the star flicker.

Teacher's Note:
1. Twinkling is caused by atmospheric refraction, not scattering.
2. Planets do not twinkle because they are nearer and act as extended sources.

Answer: (d) atmospheric refraction

Explanation:
1. Air density changes with height and temperature, so its refractive index changes.
2. The resulting bending of light is called atmospheric refraction.

Teacher's Note:
1. Atmospheric refraction explains twinkling, advanced sunrise and delayed sunset.
2. It is different from scattering.

Answer: (d) Atmospheric refraction alone explains the rainbow formation

Explanation:
1. Atmospheric refraction explains twinkling of stars, non-twinkling of planets and the wavering of objects seen through hot air.
2. A rainbow needs refraction, dispersion and internal reflection inside water droplets, so refraction in the atmosphere alone does not explain it.

Teacher's Note:
1. Look for the word 'alone' in options.
2. Rainbow formation involves water drops, not the atmosphere's layers.

Answer: (c) violet and blue lights get scattered more than lights of the other colours by the atmosphere

Explanation:
1. Air molecules and fine particles are smaller than the wavelength of visible light.
2. They scatter shorter wavelengths (violet and blue) more strongly, and the scattered blue light reaches our eyes from all directions.

Teacher's Note:
1. Shorter wavelength means stronger scattering.
2. The sky looks blue because we see scattered light.

Answer: (a) the longest distance through the atmosphere

Explanation:
1. At sunrise and sunset the Sun is near the horizon.
2. Its light therefore has to pass through the greatest thickness of air.

Teacher's Note:
1. Low Sun means a long path through air.
2. More air means more scattering of blue light.

Answer: (b) scattering

Explanation:
1. When light falls on tiny particles it is sent off in all directions.
2. This is called scattering of light.

Teacher's Note:
1. Scattering = spreading of light in all directions by particles.
2. The Tyndall effect is one example, not the general term.

Answer: (d) Red is the colour of courage and fear

Explanation:
1. The first three statements are physical facts: red light has the longest wavelength, is scattered the least and so can be seen from far.
2. The fourth is not a scientific reason for using red in danger signals, so it is the incorrect statement.

Teacher's Note:
1. The choice of red for danger signals is based on physics.
2. Symbolic meanings of colours are not scientific reasons.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. The eye is valuable and sensitive precisely because it lets us see the world and colours.
2. Both statements are true and R explains A.

Teacher's Note:
1. Check each statement first, then see whether R explains A.
2. Here R gives the reason for A.

Answer: (c) A is true, but R is false.

Explanation:
1. Cataract is clouding of the eye lens and can cause partial or complete loss of vision, so A is true.
2. The eyeball is approximately spherical, not elliptical, so R is false.

Teacher's Note:
1. Learn the shape of the eyeball: nearly spherical.
2. Cataract: lens becomes cloudy.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. Both definitions are correct.
2. Neither explains the other, so R is not the explanation of A.

Teacher's Note:
1. The source called the near point the 'least point', which is not a standard term.
2. Far point and near point are separate definitions.

Answer: (d) A is false, but R is true.

Explanation:
1. The image must always fall on the retina, so the image distance is fixed and does not increase; A is false.
2. R correctly states that the image distance is fixed and only the focal length changes, so R is true.

Teacher's Note:
1. Image distance in the eye is constant.
2. The eye adjusts focal length, not image distance.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. The reddish Sun at sunrise is due to scattering of light in the atmosphere.
2. The Moon has no atmosphere, so no scattering occurs and this effect is not seen. R explains A.

Teacher's Note:
1. No atmosphere, no scattering.
2. Scattering explains red sunrise on earth.

Answer: (c) A is true, but R is false.

Explanation:
1. A ray refracts at both inclined faces and each refraction turns it towards the base, so A is true.
2. The faces refract the light; they do not scatter it, so R is false.

Teacher's Note:
1. Refraction and scattering are different phenomena.
2. A prism deviates by refraction.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. A myopic person sees nearby objects clearly but not distant ones, so A is true.
2. An elongated eyeball (one cause of myopia) makes the image form in front of the retina, which explains A.

Teacher's Note:
1. Myopia can also arise from excessive lens curvature.
2. Both are causes of the same defect.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. A prism deviates light towards its base, so A is true.
2. R describes the process of refraction through a prism and is true, but it does not explain why the bending is towards the base.

Teacher's Note:
1. Why towards the base: inclined faces and refraction at both faces.
2. R only repeats what happens.