Human Eye and the Colorful World ||10th Science || Intext & Exercise Question || CBSE
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Intext
Question
Question
1:_ What is meant by power of
accommodation of the eye?
Answer:_ The ability of eye
lens to adjust its focal length to view both distant and nearby objects on the
retina is called the power of accommodation of the eyes.
Question
2:_ A person with a myopic eye cannot see
object beyond 1.2 m distinctly. What should be the types of the corrective lens
used to restore proper vision?
Answer:_ The person is able
to see nearby objects clearly but he is unable to see objects beyond 1.2 m.
To correct this defect of vision he must
use a concave lens.
Question
3:_ What is the far point and near point
of human eye with normal vision?
Answers:_ The near point of
the eye is the minimum distance of the object from the eye, which can be seen
distinctly without strain. For normal eye this distance is 25 cm.
The far point of the eye is the maximum
distance to which the eye can see the objects clearly. The far point of the
normal human eye is infinity.
Question
4:_ 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:_ If a student has
difficulty in reading the blackboard while sitting in the last row. It shows
that he is unable to see distant object clearly. He is suffering from myopia.
This defect can be corrected by using concave lens.
Exercise
Question
1:_ The human eye can focus object at
different distance by the focal length of the eyes. This is due to
(a) Presbyopia
(b) Accommodation
(c) Near-sightedness
(d) Far-sightedness
Answer:_ (b) This is due to
the power of accommodation of eyes.
Question
2:_ The human eye form 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 m
(c) 25 cm
(d) 2.5 cm
Answer:
(d) 25 cm
Question
4:_ The change in focal length of an Eyes
lens is caused by the action of the:-
(a) Pupil
(b) Retina
(c) Cilliary muscles
(d) Iris
Answer:_ Cilliary muscles.
Question
5:_ A person needs a lens of power ─ 5.5 dioptre
for correcting his vision distant vision. For correcting his near vision he
need a lens of power + 1.5 dioptre. What is the focal length of the lens
required for correcting (i) distant vision (ii) near vision?
Answer:_ (i) We know that
the power of the lens of the focal length f is a given by
The focal length of a lens for correcting
vision is ─ 0•181 m.
(ii) Power of lens = + 1•5 D
The focal length of the lens for correcting
vision is 0•667m
Question
6:_ The far point of a myopic person is
80 cm in front of the eyes. What is the nature and power of the lens required
to correct the problem?
Answer:_ The person is
suffering from an eye defect called myopia.
In this defect the image is formed in front of the retina. Hence a concave
lens is used to correct this defect of vision.
Object
distance u = ∞
Image distance v = ─ 80 cm
Focal length = f
According to Lens formula
\ A
concave lens of power ─ 1.25 D is required.
Question
7:_ Make a diagram to show how hyper-metropia
is corrected. The near point of hypermetropia eye is 1 m. What is the power of
lens required to correct the defect. Assume that the near point of the normal
eyes is 25 cm.
Answer:_ The defect of
vision is collected by using a convex lens. A convex lens of suitable focal
length converge the incoming light in such a way that the image is formed on
the retina as shown in figure
Here: Object distance u =
Image distance v =
Let focal length = f
Using the lens formula
A convex lens of power + 3.0 D is required
to correct the defect.
Question
9:_ What happens to the image distance in
the eye when we increase the distance of an object from the eyes?
Answer:_ Since the size of
eyes cannot change i.e. cannot increase or decrease, the image distance remains
constant. When we increase the distance of an object from the eye. The image
distance in the eye does not change. The increase in the object distance is a
compensated by the change in the focal length of the eye lens. The focal length
of the eye lens is a change in such a way that the image is formed at the
retina of the eyes.
Question
10:_ Why do star Twinkle?
Answer:_ Star emits their
own light and they twinkle due to atmospheric refraction of light. Stars are
very far away from the earth and they are considered.
Question
11:_ Why planets do not twinkle?
Answer:_ Planet do not twinkle
because they appear larger in size then the star as they are relatively close
to earth. Planet can be considered as a collection of large number of point sizes
source of light. The different part of these planets produces either brighter
or dimmer effect in such a way that the average of brighter and dimmer effect
is zero. Hence the twinkling effects of the planets are nullified and they do
not twinkle.
Question
12:_ Why does the sun appear reddish early
in the morning?
Answer:_ During sunrise,
the light comings from the sun have to travel a greater distance in the earth’s
atmosphere before reaching our eyes. In this journey, the shorter wavelength of
light are scattered out and only longer wavelength able to reach our eyes. Since
blue colour has a shorter wavelength and red colour have a longer wavelength. The
red colour is able to reach our eyes after the atmospheric scattering of light.
Therefore, the sun appears reddish early in the morning.
Question
13:_ Why does the sky appear dark instead
of blue to the astronaut?
Answer:_ The sky appears
dark instead of blue to an astronaut because they are there is no atmosphere in the
outer space that can scatter the sunlight. As the sunlight is not scattered. No
scattered light reach the eyes of astronaut and the sky appear black to them.