10th Class Physics 10th Lesson The Human Eye and the Colourful World Questions and Answers

10th Class Physics 10th Lesson The Human Eye and the Colourful World Questions and Answers

10th Class Physics 10th Lesson Questions and Answers (Exercise)

Question 1.

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) accommodation.

c) near-sightedness.

d) far-sightedness.

Answer:

b) accommodation.

Question 2.

The human eye forms the image of an object at its

a) cornea.

b) iris.

c) pupil.

d) retina.

Answer:

d) retina.

Question 3.

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.

Answer:

c) 25 cm.

Question 4.

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.

Answer:

c) ciliary muscles.

Question 5.

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. What is the focal length of the lens required for correcting

 (i) distant vision, and (ii) near vision?

Answer:

i) Focal length of the lens for distant vision = 1 Power  = 100−5.5 = cm = -18 cm (approx)

ii) Focal length of the lens for near vision = 1001.5 cm = 66.66 cm

Question 6.

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 ?

Answer:

The far point of a normal eye is infinity. Since the far point of the defective eye is given as 80 cm, the eye is short-sighted. To correct it, the lens should be such that an object at infinity must form its image at the far point of defective eye.

∴ u = -∝, v = -80 cm.

Using lens formula 1f=1v−1u

∴ 1f = 1−80 – 1(−∞) = 1−80

∴ Focal length of lens is – 80 cm

The correction is done by using a concave lens of focal length 80 cm.

Power of the lens = 100f( in cm) = –10080 = -1.25 D is needed.

Question 7.

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 this defect ? Assume that the near point of the normal eye is 25 cm.

Answer:

AP 10th Class Physics 10th Lesson Questions and Answers The Human Eye and the Colourful World 1

N = Near point of a hypermetropic eye

N’ = Near point of a normal eye

To correct the defect, the image of an object at 25 cm should be brought at 100 cm.

∴  = 1−100 – 1−25

1f = −1100 + 125 = −1+4100 = 3100

∴ f = + 1003 = +33.3 cm

So, a convex lens of focal length 33.3 cm is required power, P = 10033.3 = 3.0 D

Question 8.

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

Answer:

The focal length of the eye lens cannot be reduced below a certain limit.

Question 9.

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

Answer:

In eye, the image is always formed on the retina. The image distance is the distance between the eye lens and the retina. When we increase the distance of the object from the eye, the focal length of the eye lens increases due to the action of ciliary muscless so that the image of object is formed on the retina and therefore, the image distance remains the same.

Question 10.

Why do stars twinkle ?

Answer:

Stars appear to twinkle due to atmospheric refraction.

The light of stars passes through the Earth’s atmosphere, which contains air layers of varying temperatures and densities.

These variations cause the star’s light to refract and create small, rapidly changing differences in brightness and position, leading to the twinkling effect.

Question 11.

Explain why the planets do not twinkle.

Answer:

The planets are much closer to the Earth, and are thus seen as extended sources. If we consider a planet as a collection of a large number of point- sized sources of light, the total variation in the amount of light entering our eye from’all the individual, point – sized sources will average out to zero, thereby nullifying the twinkling effect.

Question 12.

Why does the sky appear dark instead of blue is an astronaut ?

Answer:

At such huge heights due to absence of atmosphere, no scattering out the light takes place. Therefore, sky appears dark.

10th Class Physics 10th Lesson The Human Eye and the Colourful World Questions and Answers