Question 1

Q1: The human eye can focus on objects at different distances by adjusting the focal length of the eye lens. This is due to (a) presbyopia. (b) accommodat...

Accepted Answer

Answer: (b) accommodation. Explanation: The ability of the eye lens to adjust its focal length to see both nearby and distant objects clearly is called the power of accommodation. This is achieved by the action of the ciliary muscles, which change the curvature of the lens.

Question 2

Q10: Why do stars twinkle?

Accepted Answer

Stars twinkle due to the atmospheric refraction of starlight. Stars are very distant and act as point-sized sources of light. As starlight enters the Earth's atmosphere, it passes through layers of air with continuously changing temperatures and densities. This causes the refractive index of the atm...

Question 3

Q11: Explain why the planets do not twinkle.

Accepted Answer

Planets do not twinkle because they are much closer to the Earth than stars and are therefore seen as extended sources of light, not point-sized sources. A planet can be considered as a collection of a very large number of point-sized light sources. While the light from each individual point source...

Question 4

Q12: Why does the sky appear dark instead of blue to an astronaut?

Accepted Answer

The blue colour of the sky is a result of the scattering of sunlight by the molecules of air and other fine particles in the Earth's atmosphere. This phenomenon is called Rayleigh scattering, which scatters shorter wavelengths (like blue and violet light) more effectively than longer wavelengths (li...

Question 5

Q2: The human eye forms the image of an object at its (a) cornea. (b) iris. (c) pupil. (d) retina.

Accepted Answer

Answer: (d) retina. Explanation: The human eye works like a camera. Its lens system forms a real, inverted image of an object on a light-sensitive screen called the retina. The light-sensitive cells on the retina then send electrical signals to the brain via the optic nerve.

Question 6

Q3: The least distance of distinct vision for a young adult with normal vision is about (a) 25 m . (b) 2.5 cm . (c) 25 cm . (d) 2.5 m .

Accepted Answer

Answer: (c) 25 cm . Explanation: The least distance of distinct vision, also known as the near point, is the minimum distance at which an object can be seen clearly without any strain on the eye. For a young adult with normal vision, this distance is approximately 25 cm.

Question 7

Q4: The change in focal length of an eye lens is caused by the action of the (a) pupil. (b) retina. (c) ciliary muscles. (d) iris.

Accepted Answer

Answer: (c) ciliary muscles. Explanation: The ciliary muscles are responsible for changing the shape and curvature of the eye lens. When they contract, the lens becomes thicker, decreasing the focal length to focus on nearby objects. When they relax, the lens becomes thinner, increasing the focal le...

Question 8

Q5: A person needs a lens of power -5.5 dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 dioptre....

Accepted Answer

(i) For correcting distant vision: Given: Power of the lens, $P_1 = -5.5 	ext{ D}$ To Find: Focal length of the lens, $f_1$ Formula: Power $(P)$ is the reciprocal of focal length $(f)$ in metres. $$P = \frac{1}{f(	ext{in m})}$$ Calculation: $$f_1 = \frac{1}{P_1} = \frac{1}{-5.5} = -\frac{10}{55} =...

Question 9

Q6: The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?

Accepted Answer

Given: The person suffers from myopia (near-sightedness). The far point of the myopic eye is 80 cm. This means the person can see objects clearly only up to a distance of 80 cm. Object distance, $u = -\infty$ (for viewing distant objects) Image distance, $v = -80 	ext{ cm} = -0.8 	ext{ m}$ (The co...

Question 10

Q7: Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to correct...

Accepted Answer

Diagram: The correction for hypermetropia is shown in Figure 10.3 (c) of the textbook. The diagram shows: 1.  An object placed at the normal near point, N' (25 cm from the eye). 2.  A convex lens is placed in front of the eye. 3.  This convex lens forms a virtual image of the object at the near poin...

Question 11

Q8: Why is a normal eye not able to see clearly the objects placed closer than 25 cm?

Accepted Answer

A normal eye is not able to see objects placed closer than 25 cm clearly because the eye lens has a limit to how much it can increase its curvature and decrease its focal length. To focus on a very close object, the ciliary muscles must contract significantly to make the eye lens thicker and more cu...

Question 12

Q9: What happens to the image distance in the eye when we increase the distance of an object from the eye?

Accepted Answer

When we increase the distance of an object from the eye, the image distance in the eye remains almost constant. For an object to be seen clearly, its image must be formed on the retina. The distance between the eye lens and the retina is fixed. To accommodate for the change in object distance, the e...

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