BASANT’S PHYSICS
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STUDY MATERIAL
SUB- HUMAN EYE AND COLOURFUL WORLD
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Very Short Answer Type Questions
1. What is the least distance of distinct vision of a normal human eye?
Ans. It is 25 cm.
2. Define power of accommodation.
Ans. The ability of the human eye lens to change its focal length according to the distance of the object
from the eye is known as power of accommodation of the eye.
3. Q.3. Name the muscles responsible for bringing change in the focal length of the eye.
Ans. Ciliary muscles.
4. What is the 'far point' of a normal human eye?
Ans. Infinity.
5. What is the 'near point' of a normal human eye?
Ans. 25 cm from the eye.
6. What is the type of lens used for correcting myopia (short sightedness)?
Ans. Concave lens.
7. What type of lens is used for correcting the hypermetropia?
Ans. A Convex lens is used for far-sightedness.
8. Which defect of the eye can be corrected by using a cylindrical lens?
Ans. Astigmatism, an eye defect, can be corrected by using a cylindrical lens.
9. Name the type of lens which is used for correcting the presbyopia defectof the eye.
Ans. This defect can be corrected by using a bi-focal lens which consists both concave and convex
lenses.
10. What is Cataract?
Ans. The crystalline lens of eye in older persons sometimes becomes milky and cloudy. This condition is
called cataract. This causes partial or complete loss of vision.
11. What is angle of Prism?
Ans. The angle between two lateral faces of the prism is called the angle of Prism.
12. What do you mean by angle of Deviation in prism?
Ans. The emergent ray bends at an angle to the direction of incident ray. This angle is called the
angle of deviation in the prism.
13. What do you mean by dispersion of light?
Ans. The splitting of white light into its component colours on passing through a prism is called
dispersion of light.
14. Name a natural spectrum.
Ans. A rainbow is a natural spectrum appearing in the sky.
15. What do you mean by atmospheric refraction of light?
Ans. The refraction of light rays of the Sun and other stars through the air (medium) which has
gradual changing refractive index is known as atmospheric refraction.
16. What do you mean by scattering of light or Tyndall effect?
Ans. The phenomenon of scattering of light by the colloidal particles like particles of atmosphere
is known as Tyndall effect.
17. Why are the 'danger signal lights' red in colour?
Ans. The red light is least scattered by fog or smoke. So it is visible in the same colour from quite a
large distance even through fog or smoke.
18. The change in focal length of the eye lens is caused by action of the :
(a) pupil
(b) retina
(c) ciliary muscles
(d) iris.
Ans. (c) Ciliary muscles.
19. Why does the sky appear dark instead of blue to an astronaut?
Ans. The sky appears dark to the astronauts as they are flying in the space, which does not have
an atmosphere, so scattering of light does not take place at such heights in the space.
20. What happens to the image distance in the eye when we increase the distance of an
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object from the eye?
Ans. It remains the same. As the distance between the eye lens and retina remains constant.
On which principle does cinematography work ?
Ans. It works on the principle of persistence of vision.
How much is the power of accommodation of a normal eye ?
Ans. About 4 dioptres.
Why can't we recognise the colours of objects in a dark room ?
Ans. This is because cones are not sensitive in a dim light.
In myopia what happens to the power of the eyelens. Whether it decreases or increases.
Ans. In myopia, the power of the eyelens increases.
Why the spokes of a rapidly revolving wheel cannot be distinguished ?
Ans. Due to persistence of vision, the images of various spokes on the retina get merged.
What is the cause of dispersion of light ?
Ans. When white light passes through a prism, it splits up into its constituent colours because refractive
index of glass is different for different colours.
For which colours, is the refractive index n of prism material (i) minimum and (ii) maximum ?
Ans. (i) n is minimum for red colour. (ii) n is maximum for violet colour.
Which colour is deviated (i) most (ii) least, on refraction through a prism?
Ans. (i) Violet colour is deviated the most, (ii) Red colour is deviated
the least.
A beam of light consisting of red, blue and yellow colours is
incident on prism, as shown in Fig. 11.17 (a) and (6).
Complete the diagrams to show the refracted and emergent
rays.
Ans. In Fig. (a), the ray AB is incident normally, so it does not disperse in the prism but the emergent
beam gets dispersed. In Fig. (b), dispersion occurs both inside and outside the prism.
30. Why does dispersion occur only on refraction and
not on reflection?
Ans. This is because angle of reflection is same for all the
colours (or wavelengths) of white light while the angle of
refraction is different for different colours.
31. What is monochromatic light ? Give an example of a source of such
light?
Ans. The light of single wavelength is called monochromatic light. Sodium lamp is a common source of
monochromatic light.
Short Answer Type Questions
1. What is colour blindness?
2. Ans. Colour blindness is a defect of the eye due to which a person is unable to distinguish between
certain colours, "sometime even the primary colours.
3. What is presbyopia?
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Ans. It is a type of eye defect in older persons, which is due to weakening of ciliary muscles of eyes.
Due to this, the eye lens cannot properly adjust its focal length according to its distance from the
object.
A chicken can see only in the bright light. What type of cells are present in its retina?
Ans. The chicken's retina of the eye has only a few types of rod cells. Its retina does not have the cone
cells.
What is persistence of eye vision?
Ans. The image formed on eye retina is not permanent, but its impression or sensation on the retina
remains for about 1/16 part of a second. This continuance of the sensation of the image on the retina
is
known as the persistence of eye vision.
What is meant by the power of accommodation of the eye?
Ans. Human eye lens can change its focal length according to the distance of the object from the eyes.
This is done with the help of ciliary muscles of the eyes. If the object is placed at far off distance, the
eye lens becomes thin and its focal length increases. If the eye has to see the near objects clearly, then
the eye lens becomes thicker and its focal length decreases. This ability of the eye lens to change its
focal length according to the distance of the object is known as the power of accommodation of
the eye.
How do we see colours?
Ans. The retina of human eye has a large number of light sensitive cells. These cells are of two types
i.e., rod cells and cone cells. The rod shaped cells show response towards the intensity of light rays,
while the cone shaped cells respond to colours. It is these cone cells, which make it possible for a
man to see different colours and distinguish between them.
The far point of a myopic person is 80 cm in front of the eye. What is the power of the lens
required to enable him to see the distant objects clearly?
Sol. A myopic person needs a concave lens for the correction of his eyesight
So his v = -80 cm (far point) u = ∞ = (infinity)
9. What is meant by the far point, near point and the least distance of distinct vision?
10. Ans. (i) Far point. The longest distance upto which an eye can see clearly is known as Far point of the
eye. For a normal eye, that point is infinity. (ii) Near point. The smallest distance upto which the
human eye can see clearly is known as Near point of the eye. The near point of a normal eye of an
adult is 25 cm. (iii) Least distance of distinct vision of the eye (D). The minimum distance
upto which an eye can see clearly is known as its Least distance of distinct vision. It is actually the
same as the near point of an eye. So for a normal eye its value is 25 cm.
11. Why does it take some time to see objects in a dim room when you enter the room from
bright sunlight outside?
12. Ans. In the bright sunlight, iris causes the pupil to become smaller so that only a small portion of light
enters the eye and rods of the retina are also adjusted in the same way. But when a person enters into a
dimly-lit room, each iris takes some time to increase the diameter of the pupil, so that more amount of
light can enter the eyes to see the object clearly and rod cells of the retina also take some time to adjust
themselves to get the picture of the object in the dim light.
13. What is short sightedness or myopia? What causes myopia? How is myopia corrected?
14. Myopia is the defect of the eye vision due to which a person can see the near objects clearly, but he
can not see the far objects clearly. Causes of myopia. Myopia is caused:
---due to the elongation of the eye ball.
15. What is presbyopia? What causes presbyopia? How is presbyopia corrected?
16. Ans. The power of accommodation of the eye decreases with ageing. It occurs due to the gradual
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weakening of the ciliary muscles and decreasing flexibility of the crystalline lens. The near point of
the eye increases with age. It may reach even 2 metres. This defect is called presbyopia. It is mainly
caused due to the weakening of the ciliary muscles of the eyes. Such a person may suffer from myopia
and hypermetropia. This defect is then corrected by using bifocal lenses of suitable focal lengths. The
upper part of the lens is concave lens for correcting myopia to see the distant objects clearly, while the
lower part of the lens has convex lens to correct the hypermetropia to see the nearby objects clearly.
What is meant by the 'persistence of vision'? We are able to see the movie picture in a
cinema hall. How does this happen?
Ans. The ability of the human eye to continue to see the image of an object for a very short duration
even after the removal of the object is called persistence of vision. It is due to the persistence of vision
that we are able to see movie pictures in a cinema hall. The still pictures in the form of a long film are
projected on the screen at a rate of about 24 pictures per second. Under these conditions, the image of
one picture persists on the retina of the eye till the image of the next picture falls on the screen, and so on.
Due to this, the slightly different images of the successive pictures present on the film merge smoothly with
one another and give us the feeling of moving images.
Why do the stars twinkle?
Ans. The stars twinkle due to the atmospheric refraction of starlight. This atmospheric refraction
occurs in a medium of gradually increasing refractive index. Moreover the physical conditions of the
earth's atmosphere are not stationary and the stars are very distant, they act as point-sized sources of
light. The path of rays of light coming from the star goes on varying slightly, so the position of the star
appears as fluctuating and the amount of starlight entering the eye flickers, giving the twinkling effect.
Why do the planets not appear twinkling like the stars?
Ans. The planets are much closer to the earth. A planet can be considered as a collection of a large
number of point-sized sources of light. So 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.
21. A person with a defective eye-vision is unable to see the objects nearer than 1.5 m. He
wants to read books at a distance of 30 cm. Find the nature, focal length and power
of the lens he needs in his
spectacles.
22.
Far point of a myopic
person is 40 cm. What type
of lens should he wear so
as to see the distant
objects clearly? Calculate
the focal length and the
power of the lens he
should use.
(-) sign of power shows that he
use a concave lens.
myopic person has been using
spectacles of power -1.00 D for clear
vision.
During old age he also needs to use
separate reading glasses of power + 2.00 D. Explain what may have happened to his
eye-sight?
24. Ans. When he was using only lenses for myopic defect, his near point was normal i.e. equal to
25 cm. His far off point was 100 cm according to the power of lens he was using. His eye lens had
the normal power of accommodation. Due to old age, his ciliary muscles became weaker and he
did not have a normal power of accommodation. So his near point of vision also increased and
reached upto 50 cm as
shown below and he
needed a lens of 2.00
D
power
for
that
purpose. P = + 2.00 D
should
23. A
.'. The near point of this person is 50 cm away from his eyes.
25. Why does the colour of the sky appear blue?
Ans. The molecules of air and other fine particles in the atmosphere have a size smaller than the
wave-length of visible light. So these particles scatter more effectively the light rays of shorter
wave-length at the blue end than light of longer wave-lengths at the red end. When the scattered
blue light enters our eyes, it gives us the feeling of a blue sky.
26. Why do the colour of the Sun appears red at sunrise and sun-set?
27. Ans. Sun at sunrise and at sun-set time is more near the horizon, and near the horizon
most of the blue light of shorter wave-lengths is scattered, away by the particles.
Therefore, the light that reaches our eyes is of longer wave lengths that gives rise to the
reddish appearance of the Sun.
28. Draw a labelled diagram of human eye.
29. What is long sightedness or hypermetropia? What causes long sightedness?
How is long sightedness corrected?
Ans. Long sightedness or hypermetropia. Due to this defect, a person is not able to
see the nearby objects clearly but can see the distant objects clearly.
Causes of long sightedness. It is caused due to the following reasons:
— Normal increase in the focal length of the eye lens. The lens becomes less
convergent.
— Shortening of the eyeball size.
Long sightedness can be corrected by using convex lens of suitable focal length in
the spectacles of such a person.
30. Why is
the sunlight
reaching the
earth,
yellowish?
Ans.
When the white
light (coming from Sun) passes through upper atmosphere, the violet, indigo and blue colours
seatter. However, after the scattering of these colours the white light is deficient of violet, indigo and
blue. Thus, it appears yellowish instead of white.
31. Why does Sun appear bigger during sunset or sunrise?
Ans. The atmosphere consists of a number of layers of air of varying densities, such that the
densest layeris close to the surface of earth and rarest layer far away from the surface of the
earth.
The rays of light coming from the heavenly bodies such as Sun, Moon and stars on passing
through the atmosphere, bend towards the normal and hence when they reach the eye, then the
heavenly bodies appear at some other point than their actual position and also closer to the earth.
During sunset or sunrise, the rays coming from the Sun pass through maximum depth of atmosphere
and hence suffer maximum refraction. Thus, the apparent image of the Sun is formed very close to
the earth. Thus, the Sun appears bigger, during sunrise or sunset.
32. How does atmospheric refraction increases the light of day time?
When the Sun is below the horizon early in the morning, the rays of hght coming from it, on
passing through atmosphere bend inward, i.e., towards the normal and hence manage to reach
earth. This results in formation of dawn.
It has been found that Sun is visible to us 2 minutes before the actual sunrise and 2 minutes after
the actual sunset on account of refraction. Thus, the formation of twilight (during dawn and dusk)
and appearance of Sun 2 minutes earlier during sunrise and 2 minutes latter during sunset
increase the light of day time.
33. Why do the distant hills covered with trees appear blue?
In the thick growth of trees on the hills there are always present tiny droplets of water in air.
When the white light passes through moisture laden air, the blue light is scattered. When this
scattered blue light reaches our eyes, the hills appear blue.
34. Why is red light used as universal danger signal?
Ans. The red light has the longest wavelength amongst the spectral colours and hence is least
scattered by the atmosphere. Thus, red hght can easily pass through fog or mist, without getting
scattered and hence is visible from a long distance. Thus, it is used as universal danger signal.
35. Why do motorists use orange lights, rather than normal white light on a foggy day?
Ans. If a motorist uses normal white light while driving in fog, then tiny droplets of water in air
scatter large amount of blue light. This scattered blue light on reaching the eyes, decreases
visibility and hence driving becomes extremely difficult. However, when orange lights are used,
they do not get scattered on account of their longer wavelengths. Thus, the driver can see clearly
in the fog.
A ray of white light passes through an equilateral prism and breaks up into component colours. Draw a
neat ray diagram to show the path of rays and state which colour : (i) deviates least, (ii) deviates
most. (i) Red colour deviates the least, (ii) Violet colour deviates the most.
36. Give an explanation of dispersion of light.
Ans .When the rays of light of different colours, and hence different wavelengths pass through
the prism, they deviate through different angles. The light of larger wavelength deviates through
small angle, whereas the light of smaller wavelength deviates through larger angle. Thus, when
white light is incident on the face of a prism, the waves of different wavelengths (colours)
deviate through different angles and in the process get dispersed. While passing out from the
opposite face of the prism they do not suffer any dispersion, but suffer refraction.
37. (i) When is a rainbow formed in the sky? (ii) Relative to the position of the Sun. in
which direction rainbow is formed? (iii) Draw ray diagram to show how a droplet of
water produces a rainbow? (iv) Which colour is on the outer arc of rainbow?
Ans . (i) Rainbow is formed just after the rain when a
bright Sun appear in the sky. (ii) The rainbow is
formed in the direction, opposite to the direction of
the Sun. (iv) Red colour is on the outer arc of the
rainbow.
38. Why is no rainbow formed on the Moon?
39. Ans. The rainbow is formed only, if the atmosphere contains large number of tiny droplets of
water suspended in the air. As the Moon has neither atmosphere nor water, therefore, no
rainbow is formed.
40. Why does the sky appear dark, instead of blue, to an astronaut?
Ans. The sky will appear blue only, if there is an atmosphere around the spaceship. As
there is complete vacuum around the spaceship, therefore, no scattering of light takes place.
Thus, the space around the spaceship (sky) appears dark.
41. Why do the faces of persons sitting opposite to you around a camp fire appear to
shimmer?
Ans. This happens due to the refraction of light. The rays of light reflected from the face of a
person; sitting opposite to you, on passing through the hot air (produced by the burning of
wood), get refracted. Since the air is rapidly moving and its optical density is continuously
changing, therefore, the path of refracted rays passing through it also changes. This gives rise to
shimmering effect.
42. Explain, why the planets do not twinkle?
Ans. Compared to stars, the planets are very close to us. The light coming from the planets on
passing through atmosphere does suffer refraction, with the result the apparent position of the planets
change. However, the size of apparent image of the planets is fairly large, such that it seldom falls
outside the line of sight of the observer. Hence, the planets do not appear to twinkle.
43. Explain why sky appears dark on the surface of Moon.
Ans .The sky will appear blue only, if there is an atmosphere around the heavenly body. As there
is no atmosphere around the Moon, therefore, no scattering of light takes place, and hence, sky
appears dark.
44. What are colloids? Give some important properties of colloidal solutions.
Ans. Colloids. A colloid is an intermediate state of true solution and suspension. A colloid is a
heterogeneous system in which one substance is dispersed (disperse phase) as very fine particles
in another substance called dispersion medium. Colloidal particles have sizes between 1 and 1000
nm (10-9 to 10-6 m). For example, in milk, water is dispersion medium while fats, proteins etc.
constitute disperse phase.
Properties of colloidal solutions:
1.
They are heterogeneous mixtures.
2.
The size of the colloidal particles ranges between 1 nm to 1000 nm.
The path of a beam of light becomes visible while passing through a colloidal solution (Tyndal effect).
Colloidal particles cannot be separated by simple sedimentation or filtration.
45. What is Tyndal effect? What is its cause? Name two phenomena observed in daily life
which are based on Tyndal effect.
46. Ans. Tyndal effect. When a beam of light is passed through a colloidal solution, placed in a dark
room, the path of beam becomes illuminated (or visible), when observed through a microscope
placed perpendicular to the path of light. This effect is called Tyndal effect.
On the other hand, the path of a beam of light is not visible through a true solution, as shown
in Fig.
3.
4.
Cause of Tyndal effect. The size of the colloidal
particle is relatively larger than the solute particle
of a true solution. The colloidal particles first absorb
energy from the incident light and then scatter a
part of this energy from their surfaces. Thus
Tyndal effect is due to scattering of light by the
colloidal particles and the colloidal particles are
seen to be moving as points of light moving
against a dark backgrou