The Human Eye and the Colourful World — CBSE Class 10 Science Important Questions

Answer: (a) Ciliary muscles - they change the curvature of the eye lens, altering its focal length. This ability of the eye is called the power of accommodation.

Answer: (a) Myopia; concave lens - in myopia the image of a distant object forms in front of the retina, so a concave (diverging) lens of suitable power pushes the image back onto the retina.

Answer: (b) Cataract - the eye lens becomes cloudy (opaque). Vision can be restored by cataract surgery.

Answer: (a) $-0.5\ \text{D}$-0.5 D - the concave lens must have its focus at the far point, so $f=-2\ \text{m}$f=-2 m and $P=\frac{1}{f}=\frac{1}{-2}=-0.5\ \text{D}$P=1/f=1/-2=-0.5 D. The negative sign confirms a concave lens.

Answer: (a) Both A and R are true and R is the correct explanation of A - each raindrop refracts and disperses sunlight, reflects it internally and refracts it again on the way out, so the colours travel back towards an observer facing away from the Sun.

At sunrise and sunset the Sun is near the horizon, so sunlight travels through a greater thickness of the atmosphere to reach us. Most of the shorter-wavelength blue light is scattered away along this long path, and mainly the longer-wavelength red light reaches our eyes, so the Sun appears reddish. At noon the Sun is overhead, the path is shortest, least scattering occurs and it appears nearly white.

The phenomenon is the twinkling of stars, caused by the atmospheric refraction of starlight. Because the physical conditions (temperature and density) of the atmosphere keep changing, the amount of bending of the star's light keeps changing, so the apparent position and apparent brightness of the star fluctuate and the star seems to twinkle. Planets do not twinkle because, being much closer, they act as extended sources of light.

The Tyndall effect is the scattering of light by particles such as colloidal particles, dust or smoke, which makes the path of a beam of light visible.
Example: a beam of sunlight entering a smoke-filled or dusty room through a small hole, or sunlight passing through the canopy of a dense forest, where tiny water droplets in the mist scatter it.

Because of atmospheric refraction. When the Sun is just below the horizon, its light passes from the rarer upper layers of the atmosphere into the denser lower layers and bends towards the normal, so the Sun appears to be above the horizon before it has actually risen. The same happens at sunset. The time difference between the actual and the apparent sunrise (or sunset) is about 2 minutes.

(a) Myopia (near-sightedness).
(b) It may be caused by (i) excessive curvature of the cornea/eye lens, or (ii) elongation of the eyeball, so that parallel rays from a distant object converge in front of the retina.
(c) A concave (diverging) lens of suitable focal length is used. It diverges the incoming parallel rays before they enter the eye so that they finally focus on the retina (see the diagram).

(a) Hypermetropia (far-sightedness) is a defect in which a person can see distant objects clearly but cannot see nearby objects distinctly; the image of a nearby object forms behind the retina and the near point is farther than 25 cm.
(b) It is caused by (i) the focal length of the eye lens being too long (low converging power), or (ii) the eyeball being too short.
(c) It is corrected with a convex (converging) lens of suitable power. The convex lens adds the converging power the eye lacks, so the image of a nearby object is brought forward exactly onto the retina.

(a) Red light has the longest wavelength among visible colours, so it is scattered the least by fog, dust and smoke and can travel the largest distance without much loss. It is therefore easily visible from far away and is used for danger/stop signals.
(b) Air molecules scatter the shorter-wavelength blue light much more than red (scattered intensity $\propto \frac{1}{\lambda^4}$∝ 1/λ⁴). The scattered blue light reaching our eyes from all directions makes the sky look blue.
(c) In space (or at very high altitudes) there is no atmosphere, hence no scattering of sunlight, so the sky appears dark/black except where the Sun and stars are seen.

A rainbow is a natural spectrum that appears in the sky after a rain shower. It is caused by the dispersion of sunlight by tiny water droplets in the atmosphere, which act like small prisms.
Sunlight is (1) refracted and dispersed as it enters the drop, (2) reflected internally at the back of the drop, and (3) refracted again as it leaves the drop, so the separated colours reach the observer. A rainbow is always formed in the direction opposite to the Sun.

(a) For distant vision the defect is myopia; for near vision it is hypermetropia (together they indicate presbyopia).
(b) Distant vision needs a concave (diverging) lens (negative power); near vision needs a convex (converging) lens (positive power).
(c) Using $P=\frac{1}{f}$P=1/f so that $f=\frac{1}{P}$f=1/P (in metres):
For distant vision, $f=\frac{1}{-4.5}=-0.22\ \text{m}=-22.2\ \text{cm}$f=1/-4.5=-0.22 m=-22.2 cm (concave).
For near vision, $f=\frac{1}{+1.5}=+0.67\ \text{m}=+66.7\ \text{cm}$f=1/(+1.5)=+0.67 m=+66.7 cm (convex).
(d) A bifocal lens has two parts in each glass: the upper concave part corrects distant vision and the lower convex part corrects near vision, so a single pair of spectacles serves both needs.

(a) Dispersion is the splitting of white light into its seven constituent colours (VIBGYOR - violet, indigo, blue, green, yellow, orange, red) on passing through a prism.
(b) See the diagram: a narrow beam of white light entering the prism emerges as a band of seven colours, with red deviated least and violet most.
(c) Different colours have different wavelengths and travel with different speeds in glass, so each is refracted through a different angle. Violet (shortest wavelength) is slowed the most and bends the most; red (longest wavelength) bends the least.
(d) Newton placed a second, identical but inverted prism in the path of the spectrum from the first prism. The seven colours recombined to give white light, proving that white light is a mixture of seven colours.

(i) At the outer surface of the cornea.
(ii) The pupil becomes smaller (contracts); the iris controls it.
(iii) Least distance of distinct vision: about 25 cm; far point: infinity.
(iv) The ciliary muscles contract, the eye lens becomes thicker (more curved), and its focal length decreases.