Question 1.
What is meant by power of accommodation of the eye?
Answer.
The images of objects at different
distances from the eye are brought to focus on retina by changing the focal
length of the eyelens. This is known as power of accommodation of the eye.
Question 2.
What is the near point and the far point of
a normal human eye?
Answer.
Near point is 25 cm and far point is
infinity.
Question 3.
A student has difficulty reading the
blackboard while sitting in the last row. What could be
the defect the child is suffering from? How
can it be corrected?
Answer.
The child is suffering from myopia. It can
be corrected by using spectacles with concave lenses of suitable focal length.
Question 4.
A person with a myopic eye cannot see
objects beyond 1.2 m distinctly. What should be the type of the corrective
lens used to restore proper vision?
Answer.
A concave lens of focal length 1.2 m should
be used to restore proper vision.
Chapter End Questions
Question 1.
The human eye can, focus objects at
different distances by adjusting the focal length of the eyelens. 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 eyelens 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 D for
correcting his distant vision. For correcting his near vision, he needs a lens
of power +1.5 D. What is the focal length of the lens required for correcting
(a) distant
vision?
(b) near
vision?
Answer.
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.
Given: Distance of far point = 80 cm, P= ?
For viewing far-off objects, the focal
length of corrective lens,
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.
The required diagram is shown below:
Question 8.
Why is a normal eye not able to see clearly
the objects placed closer than 25 cm?
Answer.
A normal eye is not able to see clearly the
objects placed closer than 25 cm because thefocal length of eyelens cannot be
decreased below a certain minimum 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 order to focus the eye on objects
situated at varying distances, the focal length of the eye lens is changed by
the action of ciliary muscles.
Question 10.
Why do stars twinkle?
Answer.
The rays of starlight pass through many
layers of atmosphere. These layers have different densities and hence,
different refractive indices. So the directions of rays reaching the eye keep
changing causing the image of the star to shift in random directions or eyen
disappear for an instant. Thus, the brightness of the star seems to change.
Question 11.
Explain why the planets do not twinkle.
Answer.
Planets are comparatively much closer to
the earth and are thus seen as extended sources of light. Being of larger
size, planets can be taken as a collection of a number of point-sized sources
of light which nullify the twinkling effect of each other.
Question 12.
Why does the sun appear reddish early in
the morning?
Answer.
At the time of shnrise, the rays of the sun
have to travel a larger atmospheric distance. As the wavelength of red colour
is the largest of all the colours of sunlight, most of the blue colour and
other colours are scattered away. Only red . colour which is,,least scattered,
enters into our eyes. Hence, the sun appears reddish at the time of sunrise.
Question 13.
Why does the sky appear dark instead of
blue to an astronaut?
Answer.
This is because there is no scattering of
light due to the absence of atmosphere in the free space.