CBSE Class 10 Science The Human Eye and the Colourful World MCQs Set 12

Answer: (b) Iris. The iris changes the size of the pupil and so regulates the light entering the eye.

Teacher's Note:
1. The iris is a muscular diaphragm with the pupil at its centre.
2. Do not confuse it with ciliary muscles, which change the lens shape.

Answer: (c) real and inverted. The eye lens forms a real, inverted image on the retina, and the brain interprets it as erect.

Teacher's Note:
1. The eye behaves like a convex lens forming a real image on a screen.
2. Remember that the brain, not the lens, makes the image appear upright.

Answer: (c) 25 cm. A normal eye can see objects clearly without strain when they are at about 25 cm or more.

Teacher's Note:
1. This is also called the near point of the eye.
2. The far point of a normal eye is at infinity.

Answer: (d) infinity. A normal eye can see very distant objects clearly, so its far point is at infinity.

Teacher's Note:
1. The range of vision is from 25 cm to infinity.
2. A defect appears when either limit shifts.

Answer: (a) nearby objects clearly but not distant ones. In myopia the image of a distant object forms in front of the retina, so only near objects are seen clearly.

Teacher's Note:
1. Myopia is also called short-sightedness.
2. The far point of such an eye is nearer than infinity.

Answer: (b) concave lens. A concave (diverging) lens of suitable power moves the image back onto the retina.

Teacher's Note:
1. The lens must form a virtual image of a distant object at the far point of the myopic eye.
2. A negative power value indicates a concave lens.

Answer: (c) convex lens. A convex (converging) lens brings the image of a near object, formed behind the retina, forward onto it.

Teacher's Note:
1. Hypermetropia is long-sightedness and its near point is farther than 25 cm.
2. A positive power value indicates a convex lens.

Answer: (c) behind the retina. The eye lens is too weak, or the eyeball too short, so the rays focus behind the retina.

Teacher's Note:
1. Compare this with myopia, where the image falls in front of the retina.
2. Draw both cases once to remember them.

Answer: (c) bifocal lenses. Presbyopia may combine myopia and hypermetropia, so bifocal lenses with concave upper and convex lower parts are used.

Teacher's Note:
1. It occurs because the ciliary muscles weaken and the lens loses flexibility.
2. The upper part of a bifocal lens is for distant vision.

Answer: (b) −2 D. The lens must have f = −50 cm = −0.5 m, so P = 1/f = −2 D.

Teacher's Note:
1. Apply the sign convention carefully because a concave lens has negative focal length.
2. Always convert centimetres to metres before finding power.

Answer: (a) +2 D. Using u = −25 cm and v = −50 cm, 1/f = 1/v − 1/u = −1/50 + 1/25 = 1/50, so f = +50 cm and P = +2 D.

Teacher's Note:
1. The lens forms a virtual image at the near point of the eye.
2. A positive result confirms a convex lens.

Answer: (c) dispersion. Different colours bend by different amounts, so white light spreads into a spectrum; this is dispersion.

Teacher's Note:
1. Violet bends the most and red the least.
2. The order VIBGYOR is from the most deviated to the least deviated colour.

Answer: (d) Red. Red has the longest wavelength and the smallest refractive index in glass, so it bends the least.

Teacher's Note:
1. Violet has the shortest wavelength and bends the most.
2. A colour with a higher refractive index in glass bends more.

Answer: (b) the emergent rays from the two parallel faces recombine into white light. The second face undoes the dispersion caused by the first face, as the faces are parallel, and the colours emerge together.

Teacher's Note:
1. The slab does disperse light slightly at each face.
2. A prism does not undo it because its faces are inclined to each other.

Answer: (a) dispersion, refraction and internal reflection of sunlight in water droplets. Sunlight is refracted and dispersed on entering a droplet, internally reflected, and refracted again as it leaves.

Teacher's Note:
1. A rainbow needs sunshine and water droplets in the air at the same time.
2. Name all three processes in a descriptive answer.

Answer: (b) with the Sun behind the observer. Light from the Sun behind the observer enters the droplets in front and returns to the eye after refraction and reflection.

Teacher's Note:
1. A rainbow therefore always appears in the direction opposite to the Sun.
2. It is seen after rain or near waterfalls and fountains.

Answer: (b) scattering of light by air molecules. Air molecules and fine particles scatter shorter wavelengths such as blue far more strongly than longer ones.

Teacher's Note:
1. The scattering is strongest for blue and violet, but our eyes are more sensitive to blue.
2. Scattering is inversely related to a power of the wavelength.

Answer: (b) is scattered the least by fog and smoke. Red has the longest wavelength among visible colours, so it is scattered the least and can be seen from a long distance.

Teacher's Note:
1. The same reason is behind red tail lamps and warning lamps.
2. Mention scattering in your answer, not absorption.

Answer: (b) light has to travel a longer path through the atmosphere and blue light is scattered away. On the long slanting path most of the blue light is scattered out, so mainly red light reaches the eye.

Teacher's Note:
1. At noon the path through the air is shorter and the Sun looks white.
2. Use the idea of path length through the atmosphere.

Answer: (b) Tyndall effect. Fine particles scatter the light sideways and make its path visible; this is the Tyndall effect.

Teacher's Note:
1. Sunlight entering a dense forest through the canopy is a good example.
2. The effect depends on the size of the particles.

Answer: (b) atmospheric refraction of starlight. Starlight passes through layers of air with changing refractive index, so its apparent position and brightness keep fluctuating.

Teacher's Note:
1. A star is a point source, so the effect is easily noticeable.
2. This is a very frequently asked question.

Answer: (b) they are nearer and act as extended sources of many point sources. A planet is a collection of many point sources, so the variations in the light from them average out.

Teacher's Note:
1. Planets shine by reflecting sunlight.
2. The nearer distance makes them appear as tiny discs, not points.

Answer: (a) 2 minutes. Refraction bends the rays towards the normal, so the Sun appears above the horizon about 2 minutes before it actually rises.

Teacher's Note:
1. The same effect delays the apparent sunset by about 2 minutes.
2. This makes the day about 4 minutes longer.

Answer: (b) outer surface of the cornea. The cornea has a large difference in refractive index from air, so it does most of the bending of light.

Teacher's Note:
1. The lens only fine-tunes the focus.
2. Do not give the lens as the answer when the question asks for the major refraction.

Answer: (c) ciliary muscles. The ciliary muscles change the curvature, and hence the focal length, of the eye lens; this is called accommodation.

Teacher's Note:
1. The lens becomes thinner for distant objects and thicker for near objects.
2. Do not confuse this with the pupil adjusting.

Answer: (a) there is no atmosphere to scatter sunlight. Without air, the sunlight is not scattered, so the sky looks dark instead of blue.

Teacher's Note:
1. This supports the idea that scattering by the atmosphere causes the blue sky on Earth.
2. Keep this link in mind for reasoning questions.

Answer: (a) Both Assertion (A) and Reason (R) are true and R is the correct explanation of A. Both statements are true, and the image forming in front of the retina is exactly why the distant object looks blurred.

Teacher's Note:
1. In such questions check each statement alone first.
2. Then decide whether the reason really explains the assertion.

Answer: (b) Both A and R are true but R is not the correct explanation of A. Both are true, but the real reason is that a planet is an extended source made of many point sources whose fluctuations average out; being closer is not the explanation.

Teacher's Note:
1. Choose option (b) when the reason is a true fact but not the right explanation.
2. Know the correct explanation yourself.