Chapter 11
NCERT Solutions for Class 10
Science
“The Human Eye and the Colourful World”
1. What is
meant by the power of accommodation of the eye?
Answer:
When
the ciliary muscles are relaxed, the eye lens becomes thin, the focal length
increases, and the distant objects are clearly visible to the eyes. The ciliary muscles contract to see the nearby objects clearly, making the eye lens
thicker. Thus, the focal length of the eye lens decreases and the nearby
objects become visible to the eyes. Hence, the human eye lens is able to adjust
its focal length to view both distant and nearby objects on the retina. This
ability is called the power of accommodation of the eyes.
2. A person
with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be
the corrective lens used to restore proper vision?
Answer:
A
person with a myopic eye should use a concave lens of focal length 1.2 m so as
to restore proper vision.
3. What is
the human eye's far and near point with normal vision?
Answer:
For
a human eye with normal vision, the far point is at infinity and the near point
is at 25 cm from the eye.
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:
The
student is suffering from myopia or short sightedness. The defect can be
corrected by the use of concave lens of suitable power.
5. 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
Answer:
(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.
6. The
human eye forms the image of an object at its
(a) Cornea
(b) Iris
(c) Pupil
(d) Retina
Answer:
(d)
retina
7. The
least distance of distinct vision for an eye lens is caused by the action of
the
(a) 25 m
(b) 2.5 cm
(c) 25 cm
(d) 2.5 m
Answer:
(c) 25 cm
8. The
change in focal length of an eye lens is caused by the action of the
(a) Pupil
(b) Retina
(c) Cilliary muscles
(d) Iris
Answer:
(c) Ciliary muscles
9. A person
needs a lens of power -5.5 dioptre for correcting his distinct vision. For
correcting his near vision he needs a lens +1.5 dioptre. What is the focal
length of the lens required for correcting (i) distinct vision, and (ii) near vision?
Answer:
(i) Power of lens needed for correction of distant vision of the person (P) = -5.5 D
The focal length of the lens
required for correcting distant vision
(f) = 1/P = 1/-5.5 m = 0.18 m = 18 cm.
(ii) For correcting near
vision the power of lens required (P) = +1.5 D
Focal length of lens
required for correcting near vision
(f) = 1/P = 1/1.5 m = 0.67 m = 66.7 cm.
10. 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:
To
correct myopia the person concerned should use concave lens of focal length
(f) = -80 cm = -0.80 m
Power of lens (P) =
1/f(m) = 1/-0.80 = 100/-80 = -1.25 D.
11. 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 near point of defective
eye is 1 m and that of normal eye is 25 cm.
Here, u = -25 cm, v = -1m
= 100 cm.
Using lens formula
1/f = 1/v – 1/u
1/f = 1/-100 + 1/25 =
3/100
f = 100/3 cm = 1/3m.
P = 1/f(m) = 1/0.33 =
+3.0 D.
12. Why is
a normal eye not able to see clearly the objects placed closer than 25 cm?
Answer:
Due
to limit of power of accommodation, the focal length of the eye lens cannot be
decreased below certain minimum limit. So, a normal eye cannot see clearly the
objects placed closer than minimum distance, called near point of the eye.
13. What
happen to the image distance in the eye when we increase the distance of an
object from the eye?
Answer:
The
image is formed on the retina even on increasing the distance of an object from
the eye. In fact, the eye lens becomes thinner and its focal length increases
as the object is moved away from the eye and consequently image is formed on
the retina.
14. Why do stars twinkle?
Answer:
Stars
twinkle due to the atmospheric refraction of starlight. As the stars are very away
they behave as almost point sources of light. As on account of atmospheric
refraction, the path of rays of light coming from the star goes on varying
slightly, the apparent position of the star fluctuates and the amount of light
entering the eye flickers, so sometimes the star appear brighter and at some
other time, fainter. Thus the stars twinkle.
15. Explain
why the planets do not twinkle.
Answer:
Planets
are much closer to the earth and are seen as extended source. So, a planet may
be considered as a collection of a large number of point-sized light sources.
Although light coming from individual point-sized sources flickers but the
total amount of light entering our eye from all the individual point-sized
sources average out to be constant. Thereby, planets appear equally brighter
and there is no twinkling of planets.
16. Why
does the Sun appear reddish early in the morning?
Answer:
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.
17. Why
does the sky appear dark instead of blue to astronaut?
Answer:
Blue
colour of the sky is on account of scattering of light of shorter wavelength by
particles in the atmosphere of earth. If the earth had no atmosphere, there
would not have been any scattering and sky would have looked dark. When
astronaut in his spacecraft goes above the atmosphere of earth, sky appears
dark to him because there is no scattering of light.
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