CBSE Class 10 Physics Human Eye and the Colourful World MCQs Set 29

Answer: (c) green

Explanation:
1. Colour depends on frequency, which does not change when light enters another medium.
2. So the green object still looks green to the diver.

Teacher's Note:
1. Wavelength changes in a medium; frequency (colour) does not.

Answer: (c) v_red > v_violet

Explanation:
1. Violet is slowed more in glass than red (higher refractive index).
2. Hence the speed of red light is greater than that of violet light.

Teacher's Note:
1. Red bends least and travels fastest in glass.

Answer: (b) green

Explanation:
1. The colours of the spectrum in order: violet, indigo, blue, green, yellow, orange, red.
2. The middle (fourth) colour is green.

Teacher's Note:
1. VIBGYOR: green is in the centre.

Answer: (c) take off the spectacles

Explanation:
1. The microscope has a small eyepiece and the eye is placed close to it.
2. Spectacles would only get in the way, so they are removed and the eyepiece is focused.

Teacher's Note:
1. The microscope can be focused for the user's own eye.

Answer: (d) violet

Explanation:
1. Violet has the highest refractive index in glass.
2. So it bends the most towards the base of the prism.

Teacher's Note:
1. Red deviates least, violet most.

Answer: (c) f₀ + fₑ

Explanation:
1. For normal adjustment the final image is at infinity.
2. The tube length = \( f_0 + f_e \).

Teacher's Note:
1. Normal adjustment: the objective's image lies at the eyepiece's focus.

Answer: (d) refraction of light

Explanation:
1. The atmosphere has layers of changing density, so the refractive index varies with time.
2. The star's light bends differently from moment to moment, so it appears to twinkle.

Teacher's Note:
1. Twinkling is due to atmospheric refraction.

Answer: (d) scattering of light

Explanation:
1. Air molecules scatter short wavelengths (blue) much more than long wavelengths.
2. So scattered blue light reaches our eyes from all directions.

Teacher's Note:
1. Rayleigh scattering varies as \( 1/\lambda^4 \).

Answer: (b) concave lens, −1.33 D

Explanation:
1. \( P = -1/\text{far point} = -1/0.75 = -1.33 \) D.
2. A negative power means a concave (diverging) lens.

Teacher's Note:
1. A myopic eye needs a concave lens whose focal length equals the far point.

Answer: (c) scattering of seven colours

Explanation:
1. Cloud droplets are large compared with the wavelength of light and scatter all seven colours nearly equally.
2. Equal mixture of all colours looks white.

Teacher's Note:
1. Large particles scatter all colours.

Answer: (a) scattering of light does not take place

Explanation:
1. Sky looks blue because molecules scatter sunlight.
2. At a great height the air is very thin, so there is almost no scattering and the sky looks dark.

Teacher's Note:
1. No scattering means no blue sky.

Answer: (c) scattering

Explanation:
1. In the Tyndall effect, particles in a colloid scatter the light and make its path visible.

Teacher's Note:
1. Scattering makes sunbeams visible in a dusty room.

Answer: (c) accommodation

Explanation:
1. The eye lens changes its curvature by the action of ciliary muscles to focus on near and far objects.
2. This ability is called accommodation.

Teacher's Note:
1. Presbyopia is the loss of accommodation.

Answer: (c) the outer surface of the cornea

Explanation:
1. Most of the bending of light occurs where light passes from air into the cornea (large change in refractive index).
2. The lens only fine-tunes the focus.

Teacher's Note:
1. Cornea provides about two-thirds of the eye's focusing power.

Answer: (d) 4000 Å

Explanation:
1. The visible range is about 4000 Å (violet) to 7800 Å (red).
2. So violet is about 4000 Å.

Teacher's Note:
1. Red has the longest wavelength.

Answer: (b) λ_r > λ_v

Explanation:
1. Red has the longest and violet the shortest wavelength in the visible spectrum.
2. So the wavelength of red is greater.

Teacher's Note:
1. Red 7800 Å; violet 4000 Å.

Answer: (b) a concave lens

Explanation:
1. Myopia: image forms in front of the retina.
2. A concave (diverging) lens moves it back onto the retina.

Teacher's Note:
1. Diverging lens: negative power.

Answer: (a) white

Explanation:
1. The primary colours of light are red, green and blue.
2. White is a mixture of all of them.

Teacher's Note:
1. Primary colours cannot be made by mixing others.

Answer: (c) the Tyndall effect

Explanation:
1. Scattering of light by small particles makes the path of light visible in a colloid.
2. This is the Tyndall effect.

Teacher's Note:
1. Mirage: total internal reflection; twinkling: refraction.

Answer: (d) concave

Explanation:
1. Myopia is corrected by a concave (diverging) lens.

Teacher's Note:
1. Hypermetropia is corrected by a convex lens.

Answer: (c) −1.25 D, concave

Explanation:
1. \( P = -1/0.80 = -1.25 \) D.
2. Negative power means concave.

Teacher's Note:
1. Far point 80 cm needs a lens of f = −80 cm.

Answer: (a) dispersion and total internal reflection

Explanation:
1. Raindrops disperse sunlight and reflect it internally before it emerges.
2. So a rainbow is dispersion plus total internal reflection.

Teacher's Note:
1. Refraction, one total internal reflection and another refraction.

Answer: (a) red

Explanation:
1. Red has the lowest refractive index in glass and so deviates the least.

Teacher's Note:
1. Violet deviates the most.

Answer: (b) red light has a longer wavelength and is scattered the least

Explanation:
1. Red has the longest wavelength and is scattered least by fog and dust.
2. So it is visible from the farthest distance.

Teacher's Note:
1. Scattering varies as \( 1/\lambda^4 \).