CHAPTER 11: The Human Eye and the Colourful World (Date : 31-12-2018)

Correct Answer : B

Explanation

The human eye is like a camera. Its lens system forms an image on a light-sensitive screen called the retina. Hence, option (b) is correct.

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Correct Answer : D

Explanation

• Light enters the eye, through a thin membrane called cornea. Cornea forms a transparent bulge on the front surface of the eyeball. Hence, statement 1 is correct.
• Pupil is a pigmented layer of tissues that makes up the colored portion of the eye.
• Its primary function is to control the amount of light entering in the eye. Hence, statement 2 is correct.
• The eye lens forms an inverted real image of the object on the retina. The retina is a delicate membrane having enormous number of light-sensitive cells. Hence, statement 3 is correct.

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Correct Answer : D

Explanation

• Myopia is also known as nearsightedness. A person with myopia can see nearby objects clearly but cannot see distant objects distinctly.
• Myopia is caused by two reasons:
  • Excessive curvature of eye lens.
  • Elongation of eyeball.
    Hence, statement 2 is not correct.

• A person with this defect has the far point nearer than infinity. Such a person may see clearly upto a distance of a few metres.
• In a myopic eye, the image of a distant object is formed in front of the retina and not at the retina itself. Hence, statement 1 is not correct.

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Correct Answer : B

Explanation

Myopia can be corrected by using a concave lens of suitable power. A concave lens of suitable power will bring the image back onto the retina and thus the defect gets corrected. Hence, option (b) is correct.

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Correct Answer : D

Explanation

• Hypermetropia is also known as farsightedness. A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly. Hence, Statement 1 is correct.
• The near point, for the person, is farther away from the normal near point (25 cm). Such a person has to keep a reading material much beyond 25 cm from the eye for comfortable reading. This is because the light rays from a closeby object are focussed at a point behind the retina. Hence, statement 2 is correct.
• This defect can be corrected by using a convex lens of appropriate power. Eye-glasses with converging lenses provide the additional focussing power required for forming the image on the retina. Hence, statement 3 is correct.

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Correct Answer : B

Explanation

• The power of accommodation of the eye usually decreases with ageing.
• For most people, the near point gradually recedes away. They find it difficult to see nearby objects comfortably and distinctly without corrective eye-glasses.
• This defect is called Presbyopia. It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens. Hence, option (b) is correct.

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Correct Answer : D

Explanation

• Corneal blindness can be cured through corneal transplantation of donated eyes.
• Eye donors can belong to any age group or sex. People who use spectacles, or those operated for cataract, can still donate the eyes. People who are diabetic, have hypertension, asthma patients and those without communicable diseases can also donate eyes. Hence, option (d) is correct.

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Correct Answer : A

Explanation

• Different colours of light bend through different angles with respect to the incident ray, as they pass through a prism.
• The red light bends the least while the violet the most. Lower the wavelength, Higher the Bending.
• Violet colour has less wavelength compared to the others colours, and the bending is indirectly proportional to the wavelength of a electromagnetic wave so Violet Color bends more. Hence, option (a) is correct.

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Correct Answer : B

Explanation

• The twinkling of a star is due to atmospheric refraction.
• The starlight, on entering the earth’s atmosphere, undergoes refraction continuously, before it reaches the earth. The atmospheric refraction occurs in a medium of gradually changing refractive index.
• Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position. The star appears slightly higher (above) than its actual position when viewed near the horizon.
• Further, this apparent position of the star is not stationary, but keeps on changing slightly. Since the stars are very distant, they approximate point-sized sources of light.
• As the path of rays of light coming from the star goes on varying slightly, the apparent position of the star fluctuates and the amount of starlight entering the eye flickers – the star sometimes appears brighter, and at some other time, fainter, which gives the twinkling effect. Hence, option (b) is correct.

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Correct Answer : B

Explanation

The time difference between actual sunset and the apparent sunset is about 2 minutes because of atmospheric refraction. During sunrise, the light rays bend due to our atmosphere and we see the sun early even though the sun is just below the horizon. Similarly, at sunset, due to the same bending of light rays, we see the apparent position of the sun, not the actual position. Hence, option (b) is correct.

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Correct Answer : B

Explanation

When a beam of light strikes fine particle such as smoke, water droplets, suspended particles of dust and molecules of air; the path of the beam becomes visible. The light reaches us, after being reflected diffusely by these particles.

• The phenomenon of scattering of light by the colloidal particles gives rise to Tyndall effect. Hence, statement 1 is not correct.
• Tyndall effect can be observed when sunlight passes through a canopy of a dense forest. Here, tiny water droplets in the mist scatter light. Hence, statement 2 is correct.

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Correct Answer : D

Explanation

• The sky appears blue during a clear cloudless day because the molecules in the air scatter blue light from the sun more than they scatter red light. Hence, statement 1 is correct.
• Twinkling of stars is caused by the atmospheric refraction of light. Hence, statement 2 is not correct.
• Light from the sun near the horizon passes through thicker layers of air and travel larger distance in the earth’s atmosphere before reaching our eyes. Near the horizon, most of the blue light and other lights with shorter wavelengths are scattered away by the particles.
• Therefore, the light that reaches our eyes is of longer wavelength.
• This gives rise to the reddish appearance of the Sun. Hence, statement 3 is correct.

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Correct Answer : B

Explanation

If the earth had no atmosphere, there would not have been any scattering. Then, the sky would have looked dark. The sky appears dark to passengers flying at very high altitudes, as scattering is not prominent at such heights. Hence option (b) is correct.

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Correct Answer : A

Explanation

'Danger' signal lights are red in colour. This is because the red has highest wavelength so it is least scattered by fog or smoke. Therefore, it can be seen in the same colour at a distance. Hence, option (a) is correct.

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