The person is able to see nearby objects clearly, but he is unable to see
objects beyond 1.2 m. This happens because the image of an object beyond 1.2 m
is formed in front of the retina and not at the retina, as shown in the given
figure.
To correct this defect of vision, he must use a concave lens. The concave lens
will bring the image back to the retina as shown in the given figure.
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,
P= 1 / f (metres)
P = 1/ -0.8 = -1.25
A concave lens of power - 1.25 D is required by the person to correct his
defect.
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,
P= 1 / f (metres)
P = 1/ 0.33 m = + 3.0 D
A convex lens of power +3.0 D is required to correct the defect.
1. A person cannot see distinctly objects kept beyond 2 m. This defect can be
corrected by using a lens of power
(a) + 0.5 D
(b) – 0.5 D
(c) + 0.2 D
(d) – 0.2 D
Ans. (b) -0.5 D
Explanation: This person is suffering from myopia. He needs a concave lens and
hence power would be in negative.
P = 1 /f = 1/2m =0.5D
2. A student sitting on the last bench can read the letters written on the
blackboard but is not able to read the letters written in his text book. Which
of the following statements is correct?
(a) The near point of his eyes has receded away
(b) The near point of his eyes has come closer to him
(c) The far point of his eyes has come closer to him
(d) The far point of his eyes has receded away
Ans. (a) The near point of his eyes has receded away
Explanation: In hypermetropia the near point of eye moves away from 25cm. Due to
this, the person needs to keep a book at more than 25 cm to read it properly.
3. A prism ABC (with BC as base) is placed in different orientations. A narrow beam of white light is incident on the prism as shown in Figure 11.1. In which of the following cases, after dispersion, the third colour from the top corresponds to the colour of the sky?
(a) (i)
(b) (ii)
(c) (iii)
(d) (iv)
Ans. (b) (ii)
Explanation: If prism is kept with base BC at bottom, then the emergent band of
colour would show violet at the bottom. If prism is kept with base BC at top,
then violet would be at top; followed by indigo and blue.
4. At noon the sun appears white as
(a) light is least scattered
(b) all the colours of the white light are scattered away
(c) blue colour is scattered the most
(d) red colour is scattered the most
Ans. (b) all the colours of the white light are scattered away
Explanation: Sky will appear dark in case of option ‘a’. It will appear blue in
case of option ‘c’ and will appear red in case of option ‘d’.
5. Which of the following phenomena of light are involved in the formation of a
rainbow?
(a) Reflection, refraction and dispersion
(b) Refraction, dispersion and total internal reflection
(c) Refraction, dispersion and internal reflection
(d) Dispersion, scattering and total internal reflection
Ans. (c) Refraction, dispersion and internal reflection
Explanation: Dispersion results in white light getting segregated into its
component colours. Refraction bends the incident light to an angle that is
causes internal reflection; and finally rainbow is formed.
6. Twinkling of stars is due to atmospheric
(a) dispersion of light by water droplets
(b) refraction of light by different layers of varying refractive indices
(c) scattering of light by dust particles
(d) internal reflection of light by clouds
Ans. (b) refraction of light by different layers of varying refractive indices
Explanation: Due to refraction of light by different layers of varying
refractive indices, the apparent position of source of light keeps on changing.
Due to this, stars appear to twinkle.
7. The clear sky appears blue because
(a) blue light gets absorbed in the atmosphere
(b) ultraviolet radiations are absorbed in the atmosphere
(c) violet and blue lights get scattered more than lights of all other colours
by the atmosphere
(d) light of all other colours is scattered more than the violet an blue colour
lights by the atmosphere
Ans. (c) violet and blue light get scattered more than lights of all other
colours by the atmosphere
8. Which of the following statements is correct regarding the propagation of
light of different colours of white light in air?
(a) Red light moves fastest
(b) Blue light moves faster than green light
(c) All the colours of the white light move with the same speed
(d) Yellow light moves with the mean speed as that of the red and the violet
light
Ans. (c) All the colours of the white light move with the same speed
9. The danger signals installed at the top of tall buildings are red in colour.
These can be easily seen from a distance because among all other colours, the
red light
(a) is scattered the most by smoke or fog
(b) is scattered the least by smoke or fog
(c) is absorbed the most by smoke or fog
(d) moves fastest in air
Ans. (b) is scattered the least by smoke or fog
10. Which of the following phenomena contributes significantly to the reddish
appearance of the sun at sunrise or sunset?
(a) Dispersion of light
(b) Scattering of light
(c) Total internal reflection of light
(d) Reflection of light from the earth
Ans. (b) Scattering of light
Explanation: Red colour scatters the least and hence travels the farthest.
During sunset or sunrise, light has to travel a longer distance to reach us.
Hence, only red light reaches to us and the sky appears reddish.
11. The bluish colour of water in deep sea is due to
(a) the presence of algae and other plants found in water
(b) reflection of sky in water
(c) scattering of light
(d) absorption of light by the sea
Ans. (b) Reflection of sky in water
Explanation: Water is colourless. Its colour appears to be same as the object
reflected by it.
12. When light rays enter the eye, most of the refraction occurs at the
(a) crystalline lens
(b) outer surface of the cornea
(c) iris
(d) pupil
Ans. (b) Outer surface of the cornea
13. The focal length of the eye lens increases when eye muscles
(a) are relaxed and lens becomes thinner
(b) contract and lens becomes thicker
(c) are relaxed and lens becomes thicker
(d) contract and lens becomes thinner
Ans. (a) are relaxes and lens becomes thinner
14. Which of the following statement is correct?
(a) A person with myopia can see distant objects clearly
(b) A person with hypermetropia can see nearby objects clearly
(c) A person with myopia can see nearby objects clearly
(d) A person with hypermetropia cannot see distant objects clearly
Ans. (c) A person with myopia can see nearby objects clearly
Explanation: This is the reason; myopia is also known as near sightedness.
This student is unable to see far off objects. This means that the student is suffering from myopia. Doctor will prescribe a concave lens of suitable focal length.
For this, one prism is placed near another prism so that one prism is in erect position and another prism is in inverted position. When ray of white light enters the first prism, dispersion of light takes place. When lights of different colours pass through the second prism, they recombine to make a ray of white light
Order of colours from bottom to top: Violet, Indigo, Blue, Green, Yellow, Orange and Red.
The human eye has following main parts:
Cornea: Human eye is spherical in shape. It has tough white
coat which protects the interior of the eye. The front portion of this coat is
transparent and is called cornea.
Iris: This is a dark muscular structure behind the cornea.
Unique colour of a person�s eye is because of colour of iris.
Pupil: The small opening in the iris is called pupil. Iris
controls the size of the pupil and thus controls the amount of light entering
the eye. Light enters the eye through pupil.
Lens: Lens is thicker at the middle and is made of transparent
material. Lens focuses the light on the back of the eye; called retina.
Retina: The back of the eye is called retina. It works like a
screen; on which image is formed. There are numerous light-sensitive nerve cells
on retina. These nerve cells are connected to optic nerve.
Formation of Image in Eye: Light rays enter the eye through
pupil and pass through lens. Lens focuses light rays on retina. Real, inverted
and smaller image is formed on retina. Optic nerve carries the message to the
brain. The brain interprets the message and we get the sense of vision.
When a person is unable to clearly see distant objects, he is considered a myopic person. Such a person is suffering from myopia.
When a person is unable to clearly see a nearby object, he is considered a hypermetropic person. Such a person is suffering from hypermetropia.
Correction of Myopia: A person suffering from myopia needs to use a concave lens of suitable focal length. The concave lens diverges the rays coming from infinity. After refraction from the concave lens, the rays appear to be coming from the far point of this person’s eye. Due to this, a clear image of distant object is made on the retina of that person. That is how a myopic person is able to clearly see distant objects; with the help of suitable concave lens.
Correction of Hypermetropia: A hypermetropic person needs to use a convex lens of suitable focal length. The convex lens converges the light rays coming from a nearby object. As a result, these light rays appear to be coming from the near point of this person’s eyes. Due to this, a clear image of nearby object is made on the retina of that person. That is how a hypermetropic person is able to clearly see nearby objects; with the help of suitable convex lens.
• ABC is prism with base BC
• PE is incident ray on surface AB. It makes < PEN with the normal NE. This angle is angle of incidence.
• After entering the prism, the light ray bends towards normal. In this case, EF is refracted ray. <N' EF is angle of refraction.
• Once the refracted ray emerges from prism into air, it bends away from normal. In this firgure, FS is emergent ray. <SFM is angle of emergence.
Angle of Deviation: The angle between incident ray and emergent ray is called angle of deviation. Here, <SGH is angle of deviation or <D.
Colours near the red end of the spectrum scatter the least. This happens because of short wavelength of reddish colours. During sunset and sunrise, sunlight needs to travel more distance to reach us. Red colour is able to reach us because it is scattered the least. Hence, sky appears reddish during sunrise/sunset.
The sky appears reddish during sunrise/sunset but it appears white at noon. During noon, the sunlight has to travel less distance to reach us. Most of the colours reaching us get scattered. Due to this, sky appears white at noon
When ray of light enters a prism, it bends because of refraction of light. When the ray of light finally emerges out of the prism, it deviates drastically from its original path. This happens because of unique shape of prism. Different colours in the visible spectrum have different speeds. Due to this, different colours bend at different angles of deviation. As a result, the emergent light appears as a band of seven colours; the colours which are the components of white light. These colours are Violet, Indigo, Blue, Green, Yellow, Orange and Red. Segregation of white light into its different components is called dispersion of light.
