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

Answer: (c) ciliary muscles

Explanation:
1. The ciliary muscles hold the eye lens and change its curvature.
2. A change in curvature changes the focal length, so the image always falls on the retina.

Teacher's Note:
1. Do not confuse the iris (pupil size) with the ciliary muscles (lens shape).
2. Ciliary muscles change focal length; iris changes the amount of light.

Answer: (a) The pupil becomes larger

Explanation:
1. In dim light the iris enlarges the pupil.
2. A larger pupil lets more light enter the eye.

Teacher's Note:
1. The pupil is large in the dark and small in bright light.
2. It takes a few moments for the iris to adjust.

Answer: (b) infinity and 25 cm

Explanation:
1. A normal eye can see very distant objects clearly, so its far point is at infinity.
2. The nearest point it can focus on without strain is about 25 cm.

Teacher's Note:
1. Far point is the farthest clear point; near point is the nearest clear point.
2. The range of vision of a normal eye is 25 cm to infinity.

Answer: (a) The near point of his eyes has receded away.

Explanation:
1. He sees distant objects clearly but not nearby ones.
2. So his near point has moved away from the normal 25 cm; this is hypermetropia.

Teacher's Note:
1. Cannot see near: near point has moved away.
2. Cannot see far: far point has come closer.

Answer: (a) convex lens

Explanation:
1. In hypermetropia the image of a nearby object forms behind the retina.
2. A convex lens converges the rays a little before they enter the eye, so the image falls on the retina.

Teacher's Note:
1. Hypermetropia needs a converging (convex) lens.
2. Myopia needs a diverging (concave) lens.

Answer: (c) A person with myopia can see nearby objects clearly

Explanation:
1. A myopic eye sees near objects clearly and distant objects blurred.
2. A hypermetropic eye sees distant objects clearly and near objects blurred. Only the third statement is correct.

Teacher's Note:
1. Make a small two-row table of the two defects to avoid mixing them up.
2. Near-sighted sees near; far-sighted sees far.

Answer: (c) a concave lens as the upper lens and a convex lens as the lower lens

Explanation:
1. Distant objects are seen through the upper part and nearby objects through the lower part of the spectacles.
2. Near-sightedness needs a concave lens (upper part) and far-sightedness needs a convex lens (lower part).

Teacher's Note:
1. Upper part is for distance, lower part for reading.
2. Upper concave, lower convex.

Answer: (d) Both (II) and (III)

Explanation:
1. Adi sees near objects but not distant ones, so he is short-sighted (myopic).
2. His eyeball may be elongated and he needs a diverging (concave) lens. Statements (II) and (III) are therefore incorrect.

Teacher's Note:
1. Read carefully: the question asks for the incorrect statements.
2. Cannot see far means myopia, which needs a diverging lens.

Answer: (d) −0.4 dioptres

Explanation:
1. Far point = 250 cm = 2.5 m, so the corrective lens has f = −2.5 m.
2. P = 1/(−2.5) = −0.4 D.

Teacher's Note:
1. Nearsightedness needs a concave lens (negative power).
2. P = 1/f with f in metres.

Answer: (c) convex lens of focal length 50 cm

Explanation:
1. The lens must make an object at u = −25 cm appear at v = −50 cm.
2. 1/f = −1/50 + 1/25 = 1/50, so f = +50 cm: a convex lens of focal length 50 cm.

Teacher's Note:
1. A long-sighted person always needs a convex lens.
2. Check the sign of f to confirm the type of lens.

Answer: (a) 60 cm

Explanation:
1. The lens must have f = −300 cm (far point 300 cm to infinity).
2. For the new near point, the lens forms a virtual image at 50 cm: −1/300 = −1/50 + 1/d, so 1/d = 1/50 − 1/300 = 5/300 and d = 60 cm.

Teacher's Note:
1. The prescribed lens is concave, so it pushes the near point slightly farther away.
2. Work with magnitudes carefully when using the lens formula.

Answer: (c) Bifocal

Explanation:
1. His near point is beyond 25 cm (hypermetropia) and his far point is nearer than infinity (myopia).
2. With both defects he needs a bifocal lens: concave in the upper part and convex in the lower part.

Teacher's Note:
1. Both defects together call for a bifocal lens.
2. A single convex or concave lens corrects only one of them.

Answer: (d) violet light

Explanation:
1. Violet has the shortest wavelength and the largest refractive index in glass.
2. So violet bends the most and has the largest angle of deviation.

Teacher's Note:
1. Violet bends most among the visible colours.
2. Red bends least.

Answer: (c) All the colours of the white light move with the same speed.

Explanation:
1. In air (nearly the same as vacuum) all colours of light travel at the same speed.
2. Speeds differ only inside media such as glass or water.

Teacher's Note:
1. Same speed in air or vacuum; different speed in glass.
2. This is why dispersion needs a medium like glass.

Answer: (c) 7

Explanation:
1. White light such as sunlight splits into seven colours: violet, indigo, blue, green, yellow, orange and red.

Teacher's Note:
1. Remember the seven colours by VIBGYOR.
2. Newton showed that the colours are already present in white light.

Answer: (b) deviated and dispersed

Explanation:
1. A prism turns the ray towards its base (deviation).
2. Because sunlight is made of colours that bend by different amounts, it is also dispersed.

Teacher's Note:
1. Sunlight in a prism is both deviated and dispersed.
2. Single-colour light would only be deviated.

Answer: (c) black

Explanation:
1. The red object reflects only red light.
2. Blue glass transmits only blue light and blocks red, so no light reaches the eye and the object looks black.

Teacher's Note:
1. A coloured filter passes only its own colour.
2. Red light cannot pass through blue glass.

Answer: (d) violet

Explanation:
1. Violet light has the highest refractive index in glass.
2. Since n = c/v, the speed of violet light is the smallest.

Teacher's Note:
1. Highest refractive index means lowest speed.
2. Red has the highest speed in glass.

Answer: (d) both (a) and (b)

Explanation:
1. The angle of deviation depends on the refractive index of the prism material and on the angle of incidence (and on the angle of the prism).
2. It does not depend on the size of the prism.

Teacher's Note:
1. The source listed 'size of the prism' as the answer, which is wrong.
2. Bigger or smaller prism of the same shape and material gives the same deviation.

Answer: (b) Violet

Explanation:
1. Frequency increases from red to violet.
2. So violet has the highest frequency among these colours.

Teacher's Note:
1. Short wavelength means high frequency.
2. Violet: shortest wavelength, highest frequency.

Answer: (a) 2 min

Explanation:
1. Because of atmospheric refraction the Sun is seen about 2 minutes before the actual sunrise and about 2 minutes after the actual sunset.
2. The difference at sunset is therefore about 2 minutes.

Teacher's Note:
1. Both ends of the day gain about 2 minutes, a total of about 4 minutes.
2. The question asks only for one end.

Answer: (c) the earth has an atmosphere

Explanation:
1. Due to atmospheric refraction the Sun is visible about 2 minutes before actual sunrise and 2 minutes after actual sunset.
2. This lengthens the apparent day by about 4 minutes, and it happens because the earth has an atmosphere.

Teacher's Note:
1. Without an atmosphere there would be no such extra time.
2. The cause is the atmosphere, not the rotation.

Answer: (a) they are very very close to earth

Explanation:
1. Planets are much nearer to the earth than stars.
2. They behave like a collection of many point sources, so the flickering of each tiny part averages out and the planet does not twinkle.

Teacher's Note:
1. Stars are point sources far away; planets are extended sources.
2. Comparing the distances is the key.

Answer: (c) atmospheric particles scatter red light more than blue light

Explanation:
1. At present the atmosphere scatters blue light more than red light, which gives a blue sky.
2. If it scattered red light more strongly, the scattered light reaching us would be red and the sky would look red.

Teacher's Note:
1. The colour of the sky is the colour that is scattered the most.
2. Equal scattering would give a white sky.

Answer: (a) scattering of light is not enough at such heights

Explanation:
1. At great heights the air is very thin, so there are few particles to scatter sunlight.
2. Not enough scattered light reaches the eyes, so the sky looks dark.

Teacher's Note:
1. Thin air means little scattering.
2. There is still some atmosphere at those heights, so option (b) is too strong.

Answer: (c) Scattering of light takes place as various colours of white light travel with different speed in air.

Explanation:
1. All colours travel with the same speed in air.
2. Scattering is caused by particles in the path of light, not by different speeds, so statement (c) is false.

Teacher's Note:
1. Differences in speed explain dispersion in glass, not scattering in air.
2. Check each statement separately.

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

Explanation:
1. Like a camera lens, the eye lens forms a real inverted image on the retina, which acts like the film.
2. Both statements are true and R explains A.

Teacher's Note:
1. Compare parts: eye lens with camera lens, retina with film.
2. R states exactly why the eye is like a camera.

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

Explanation:
1. In a myopic eye the image of a distant object forms in front of the retina, so A is true.
2. Excessive curvature of the lens shortens its focal length, which is a cause of this, so R explains A.

Teacher's Note:
1. Strong lens curvature focuses light too early.
2. Both statements describe myopia.

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

Explanation:
1. A rainbow has seven colours, written VIBGYOR from violet to red.
2. The colours appear because white light is dispersed by water droplets, so R explains A.

Teacher's Note:
1. Rainbow colours arise from dispersion.
2. VIBGYOR is the name of the order.

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

Explanation:
1. Red light is used for signals, so A is true.
2. Red light is scattered the least, not the most; that is why it is visible from far. R is false.

Teacher's Note:
1. Red has the longest wavelength and the least scattering.
2. Be careful with 'most' and 'least'.

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

Explanation:
1. Air molecules are indeed smaller than the wavelength of visible light, so A is true.
2. Shorter wavelengths are scattered more, so R is true; but R does not explain why the particles are small, so it does not explain A.

Teacher's Note:
1. Both statements are about scattering but answer different things.
2. R explains why blue scatters more.

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

Explanation:
1. The sky appears blue because the atmosphere scatters blue light more than other colours, so A is true.
2. Blue light has a short wavelength; the longest is red. R is false.

Teacher's Note:
1. Shorter wavelength is scattered more.
2. Red has the longest wavelength.

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

Explanation:
1. A myopic eye can see nearby objects clearly, so its near point is not farther than 25 cm; A is false.
2. Myopia is corrected with concave lenses, so R is true.

Teacher's Note:
1. It is the far point of a myopic eye that is nearer than infinity.
2. A far-sighted eye has a near point beyond 25 cm.

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

Explanation:
1. In myopia the eye lens is too converging, so the image forms in front of the retina; A is true.
2. A concave lens corrects it, so R is true, but it does not explain the cause.

Teacher's Note:
1. A gives the cause, R gives the cure.
2. The cure does not explain the cause.

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

Explanation:
1. Atmospheric refraction makes the Sun visible about 2 minutes before actual sunrise and 2 minutes after actual sunset, so A is true.
2. The cause is refraction in the atmosphere, not the earth's rotation and revolution, so R is false.

Teacher's Note:
1. The source printed A with 'after sunrise' and 'before sunset', which is reversed.
2. Advanced sunrise and delayed sunset: atmospheric refraction.