Question 1:
The human
eye can focus 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
(b) Human eye can change the focal length of the eye lens to see the
objects situated at various distances from the eye. This is possible
due to the power of accommodation of the eye lens.
Question 2:
The human
eye forms the image of an object at its
(a) cornea (b) iris (c) pupil (d) retina
(d) The
human eye forms the image of an object at its 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
(c) The least distance of distinct vision is the minimum distance of
an object to see clear and distinct image. It is 25 cm for a young
adult with normal visions.
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
(c) The relaxation or contraction of ciliary muscles changes the
curvature of the eye lens. The change in curvature of the eye lens
changes the focal length of the eyes. Hence, the change in focal
length of an eye lens is caused by the action of 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?
For
distant vision = −0.181 m, for near vision = 0.667 m
The power
P of a lens of focal length f is given by the relation
(i) Power
of the lens used for correcting distant vision = −5.5 D
Focal length of the required lens, f =
The focal length of the lens for correcting distant vision is −0.181 m.
(ii) Power
of the lens used for correcting near vision = +1.5 D
Focal length of the required lens, f =
The focal length of the lens for correcting near vision is 0.667 m.
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?
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 = infinity =
Image
distance, v = −80 cm
Focal
length = f
According
to the lens formula,
We know,
A concave
lens of power −1.25 D is required by the person to correct his
defect.
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.
A person
suffering from hypermetropia can see distinct objects clearly but
faces difficulty in seeing nearby objects clearly. It happens because
the eye lens focuses the incoming divergent rays beyond the retina.
This defect of vision is corrected by using a convex lens. A convex
lens of suitable power converges the incoming light in such a way
that the image is formed on the retina, as shown in the following
figure.
The convex
lens actually creates a virtual image of a nearby object (N’ in
the figure) at the near point of vision (N) of the person suffering
from hypermetropia.
The given
person will be able to clearly see the object kept at 25 cm (near
point of the normal eye), if the image of the object is formed at his
near point, which is given as 1 m.
Object
distance, u = −25 cm
Image
distance, v = −1 m = −100 m
Focal
length, f
Using the
lens formula,
A convex
lens of power +3.0 D is required to correct the defect.
Question 8:
Why is a normal eye not able to see clearly the objects placed closer
than 25 cm?
A normal
eye is unable to clearly see the objects placed closer than 25 cm
because the ciliary muscles of eyes are unable to contract beyond a
certain limit.
If the
object is placed at a distance less than 25 cm from the eye, then the
object appears blurred and produces strain in the eyes.
Question 9:
What
happens to the image distance in the eye when we increase the
distance of an object from the eye?
Since the
size of eyes 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 compensated by the change in the focal length of
the eye lens. The focal length of the eyes changes in such a way that
the image is always formed at the retina of the eye.
Question 10:
Why do stars twinkle?
Stars emit
their own light and they twinkle due to the atmospheric refraction of
light. Stars are very far away from the earth. Hence, they are
considered as point sources of light. When the light coming from
stars enters the earth’s atmosphere, it gets refracted at
different levels because of the variation in the air density at
different levels of the atmosphere. When the star light refracted by
the atmosphere comes more towards us, it appears brighter than when
it comes less towards us. Therefore, it appears as if the stars are
twinkling at night.
Question 11:
Explain why the planets do not twinkle?
Planets do
not twinkle because they appear larger in size than the stars as they
are relatively closer to earth. Planets can be considered as a
collection of a large number of point-size sources of light. The
different parts of these planets produce 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?
During
sunrise, the light rays coming from the Sun have to travel a greater
distance in the earth’s atmosphere before reaching our eyes. In
this journey, the shorter wavelengths of lights are scattered out and
only longer wavelengths are able to reach our eyes. Since blue colour
has a shorter wavelength and red colour has 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 an astronaut?