Textbook: Refraction of Light | Science and Technology Class 10 (Maharashtra SSC Board) PDF Download

 Page 1


73
	Ø  Refraction of light   Ø  Laws of refraction           
	Ø  Refractive index  Ø  Dispersion of light
6. Refraction of light 
1. What is meant by reflection of light?
2. What are the laws of reflection?
 We have seen that, generally light travels in a straight line.  Because of this, if an 
opaque object lies in its path, a shadow of the object is formed. We have also seen in 
previous classes how these shadows change due to the change in relative positions of the 
source of light and the object. But light can bend under some special circumstances as we 
will see below
Activity 1:
 1. Take a transparent glass and fill it with 
water.
 2. Dip some portion of a pencil vertically  
in water and observe the thickness of 
the portion of  the pencil, in water. 
 3. Now keep the pencil inclined to water  
surface and observe its thickness.
 In both cases, the portion of the pencil 
inside water appears to be thicker than the 
portion above water. In the second case, 
the pencil appears to be broken near the 
surface of water. Why does it happen?
 In both the above activities the  observed effects are created due to the change in 
the direction of light while coming out of water.  Light changes its direction when going 
from one transparent medium to another transparent medium. This is called the 
refraction of light.
Activity 3:
1. Keep a glass slab on a blank paper and draw its outline PQRS as shown in figure 6.1. 
2. Draw an inclined straight line on the side of PQ so that it intersects PQ at N. Pierce two 
pins vertically at two points A and B along the line. 
3. Look at the pins A and B from the opposite side of the slab and pierce pins C and D 
vertically so that the images of A and B are in line with C and D. 
4. Now remove the chip and  the pins and draw a straight line going through points C and 
D so that it intersects SR at M. 
5. Join points M and N. Observe the incident ray AN and emergent ray MD.
Refraction of light
Activity 2: 
1. keep a 5 rupee coin in a metallic vessel. 
2. Slowly go away from the vessel
3. Stop at the place when the coin   
    disappears.
4. Keep looking in the direction of the coin.
5. Ask a friend to slowly fill water in the 
vessel. You will be able to see the coin 
once the  level of water reaches a 
certain height. Why does it happen?
Material: Glass, 5 rupee coin, Pencil, metallic vessel etc.
Can you recall?
Try this.
Page 2


73
	Ø  Refraction of light   Ø  Laws of refraction           
	Ø  Refractive index  Ø  Dispersion of light
6. Refraction of light 
1. What is meant by reflection of light?
2. What are the laws of reflection?
 We have seen that, generally light travels in a straight line.  Because of this, if an 
opaque object lies in its path, a shadow of the object is formed. We have also seen in 
previous classes how these shadows change due to the change in relative positions of the 
source of light and the object. But light can bend under some special circumstances as we 
will see below
Activity 1:
 1. Take a transparent glass and fill it with 
water.
 2. Dip some portion of a pencil vertically  
in water and observe the thickness of 
the portion of  the pencil, in water. 
 3. Now keep the pencil inclined to water  
surface and observe its thickness.
 In both cases, the portion of the pencil 
inside water appears to be thicker than the 
portion above water. In the second case, 
the pencil appears to be broken near the 
surface of water. Why does it happen?
 In both the above activities the  observed effects are created due to the change in 
the direction of light while coming out of water.  Light changes its direction when going 
from one transparent medium to another transparent medium. This is called the 
refraction of light.
Activity 3:
1. Keep a glass slab on a blank paper and draw its outline PQRS as shown in figure 6.1. 
2. Draw an inclined straight line on the side of PQ so that it intersects PQ at N. Pierce two 
pins vertically at two points A and B along the line. 
3. Look at the pins A and B from the opposite side of the slab and pierce pins C and D 
vertically so that the images of A and B are in line with C and D. 
4. Now remove the chip and  the pins and draw a straight line going through points C and 
D so that it intersects SR at M. 
5. Join points M and N. Observe the incident ray AN and emergent ray MD.
Refraction of light
Activity 2: 
1. keep a 5 rupee coin in a metallic vessel. 
2. Slowly go away from the vessel
3. Stop at the place when the coin   
    disappears.
4. Keep looking in the direction of the coin.
5. Ask a friend to slowly fill water in the 
vessel. You will be able to see the coin 
once the  level of water reaches a 
certain height. Why does it happen?
Material: Glass, 5 rupee coin, Pencil, metallic vessel etc.
Can you recall?
Try this.
74
The first refraction occurs when light ray 
enters the glass from air at N on the side PQ. The 
second refraction occurs when light enters air 
through glass at point M on the side SR. For the 
first refraction the angle of incidence is i while 
for the second it is i
1
. The angle of refraction at N 
is r.
Note that i
1
 = r. In the second refraction, the 
angle of refraction is e which is equal to i. On 
both parallel sides PQ and RS of the glass slab, 
the change in direction of light ray is equal but in 
opposite directions. 
Thus, the light ray MD emerging from the 
glass slab is parallel to the incident ray AN on 
the side PQ of the slab. But the emergent ray is  
somewhat displaced with respect to the incident 
ray. 
6.1 Refraction of light passing 
through a glass slab
P
S
Q
R
Refraction 
of light
M
C
D
A
B
N
  r
i
i
1
e
Glass
Air
Air
A
C
N
B
r
i
6.2 Light ray entering a glass slab from air
D
Glass
Air
Incident ray 
Refracted 
ray
sin i
sin r
= constant = n
  n is called the refractive index 
of the second medium with respect 
to the first medium. This second law 
is also called Snell’s law. A ray 
incident along the normal (i = 0) 
goes forward in the same direction 
(r = 0).
1. Will light travel through a glass slab with the same 
velocity as  it travels in air?
2.  Will the velocity of light be same in all media?
 
Laws of refraction
Let us study the light ray entering a glass 
slab from air as shown in the figure 6.2. Here AN 
is the incident ray and NB is the refracted ray. 
1. Incident ray and refracted ray at the point of      
incidence N are on the opposite sides of the 
normal to the surface of the slab at that point 
i.e. CD, and the three, incident ray, refracted 
ray and the normal, are in the same plane. 
2. For a given pair of media, here air and glass, 
the  ratio of sin i to sin r  is a constant. Here, i 
is the angle of incidence and r is the angle of 
refraction. 
Refractive index
The change in the direction of a light ray 
while entering different media is different. It is 
related to the refractive index of the medium. 
The value of the refractive index is different for 
different media and also for light of different 
colours for the same medium. The refractive 
indices of some substances with respect to 
vacuum are given in the table. The refractive 
index of a medium with respect to vacuum is 
called its absolute refractive index.
Refractive index depends on the velocity 
of light in the medium.
Use your brain power !
Page 3


73
	Ø  Refraction of light   Ø  Laws of refraction           
	Ø  Refractive index  Ø  Dispersion of light
6. Refraction of light 
1. What is meant by reflection of light?
2. What are the laws of reflection?
 We have seen that, generally light travels in a straight line.  Because of this, if an 
opaque object lies in its path, a shadow of the object is formed. We have also seen in 
previous classes how these shadows change due to the change in relative positions of the 
source of light and the object. But light can bend under some special circumstances as we 
will see below
Activity 1:
 1. Take a transparent glass and fill it with 
water.
 2. Dip some portion of a pencil vertically  
in water and observe the thickness of 
the portion of  the pencil, in water. 
 3. Now keep the pencil inclined to water  
surface and observe its thickness.
 In both cases, the portion of the pencil 
inside water appears to be thicker than the 
portion above water. In the second case, 
the pencil appears to be broken near the 
surface of water. Why does it happen?
 In both the above activities the  observed effects are created due to the change in 
the direction of light while coming out of water.  Light changes its direction when going 
from one transparent medium to another transparent medium. This is called the 
refraction of light.
Activity 3:
1. Keep a glass slab on a blank paper and draw its outline PQRS as shown in figure 6.1. 
2. Draw an inclined straight line on the side of PQ so that it intersects PQ at N. Pierce two 
pins vertically at two points A and B along the line. 
3. Look at the pins A and B from the opposite side of the slab and pierce pins C and D 
vertically so that the images of A and B are in line with C and D. 
4. Now remove the chip and  the pins and draw a straight line going through points C and 
D so that it intersects SR at M. 
5. Join points M and N. Observe the incident ray AN and emergent ray MD.
Refraction of light
Activity 2: 
1. keep a 5 rupee coin in a metallic vessel. 
2. Slowly go away from the vessel
3. Stop at the place when the coin   
    disappears.
4. Keep looking in the direction of the coin.
5. Ask a friend to slowly fill water in the 
vessel. You will be able to see the coin 
once the  level of water reaches a 
certain height. Why does it happen?
Material: Glass, 5 rupee coin, Pencil, metallic vessel etc.
Can you recall?
Try this.
74
The first refraction occurs when light ray 
enters the glass from air at N on the side PQ. The 
second refraction occurs when light enters air 
through glass at point M on the side SR. For the 
first refraction the angle of incidence is i while 
for the second it is i
1
. The angle of refraction at N 
is r.
Note that i
1
 = r. In the second refraction, the 
angle of refraction is e which is equal to i. On 
both parallel sides PQ and RS of the glass slab, 
the change in direction of light ray is equal but in 
opposite directions. 
Thus, the light ray MD emerging from the 
glass slab is parallel to the incident ray AN on 
the side PQ of the slab. But the emergent ray is  
somewhat displaced with respect to the incident 
ray. 
6.1 Refraction of light passing 
through a glass slab
P
S
Q
R
Refraction 
of light
M
C
D
A
B
N
  r
i
i
1
e
Glass
Air
Air
A
C
N
B
r
i
6.2 Light ray entering a glass slab from air
D
Glass
Air
Incident ray 
Refracted 
ray
sin i
sin r
= constant = n
  n is called the refractive index 
of the second medium with respect 
to the first medium. This second law 
is also called Snell’s law. A ray 
incident along the normal (i = 0) 
goes forward in the same direction 
(r = 0).
1. Will light travel through a glass slab with the same 
velocity as  it travels in air?
2.  Will the velocity of light be same in all media?
 
Laws of refraction
Let us study the light ray entering a glass 
slab from air as shown in the figure 6.2. Here AN 
is the incident ray and NB is the refracted ray. 
1. Incident ray and refracted ray at the point of      
incidence N are on the opposite sides of the 
normal to the surface of the slab at that point 
i.e. CD, and the three, incident ray, refracted 
ray and the normal, are in the same plane. 
2. For a given pair of media, here air and glass, 
the  ratio of sin i to sin r  is a constant. Here, i 
is the angle of incidence and r is the angle of 
refraction. 
Refractive index
The change in the direction of a light ray 
while entering different media is different. It is 
related to the refractive index of the medium. 
The value of the refractive index is different for 
different media and also for light of different 
colours for the same medium. The refractive 
indices of some substances with respect to 
vacuum are given in the table. The refractive 
index of a medium with respect to vacuum is 
called its absolute refractive index.
Refractive index depends on the velocity 
of light in the medium.
Use your brain power !
75
Let the velocity of light in medium 1 be v
1
 and in 
medium 2 be v
2
 as shown in figure 6.3. The refractive 
index of the second medium with respect to the first 
medium, 
1
n
2
 is equal to the ratio of the velocity of light 
in medium 1 to that in medium 2.
Substance
Refractive 
index
Substance 
Refractive 
index
Substance
Refractive 
index
Air 1.0003 Fused Quartz 1.46
Carbon 
disulphide
1.63
Ice 1.31 Turpentine oil 1.47 Dense flint glass 1.66
Water 1.33 Benzene 1.50 Ruby 1.76
Alcohol 1.36 Crown glass 1.52 Sapphire 1.76
Kerosene 1.39 Rock salt 1.54 Diamond 2.42
Absolute refractive indices of some media
2
n
1
 =
v
2
v
1
6.3 Light ray going from 
 medium 1 to medium 2
Medium 1
 Air  
Medium 2
Glass
Ray
Similarly, the refractive index of medium 1 
with respect to medium 2 is 
If the refractive index of second medium with respect to first 
medium is 
1
n
2
and that of third medium with respect to second 
medium is 
2
n
3
 , what and how much is 
1
n
3 
?
 When a light ray 
passes from a rarer 
medium to a denser a 
medium, it bends towards 
the normal.  
 When a light ray 
passes from a denser 
medium to  a rarer 
medium, it bends away 
from the normal.
 When a light ray is incident 
normally at the boundary 
between two media, it does not 
change its direction and hence 
does not get refracted.
If the first medium is vacuum then the refractive index of medium 2 
is called absolute refractive index and it is written as n. 
Denser medium   
Rarer medium  
Denser medium  
Rarer medium
Denser medium  
i 
r
i 
r
Rarer medium 
v
1
v
2
6.4  Refraction of light in different media
Refractive index 
1
n
2
 =
Velocity of light in medium 1 (v
1
) 
Velocity of light in medium 2 (v
2
)
Can you tell?
Page 4


73
	Ø  Refraction of light   Ø  Laws of refraction           
	Ø  Refractive index  Ø  Dispersion of light
6. Refraction of light 
1. What is meant by reflection of light?
2. What are the laws of reflection?
 We have seen that, generally light travels in a straight line.  Because of this, if an 
opaque object lies in its path, a shadow of the object is formed. We have also seen in 
previous classes how these shadows change due to the change in relative positions of the 
source of light and the object. But light can bend under some special circumstances as we 
will see below
Activity 1:
 1. Take a transparent glass and fill it with 
water.
 2. Dip some portion of a pencil vertically  
in water and observe the thickness of 
the portion of  the pencil, in water. 
 3. Now keep the pencil inclined to water  
surface and observe its thickness.
 In both cases, the portion of the pencil 
inside water appears to be thicker than the 
portion above water. In the second case, 
the pencil appears to be broken near the 
surface of water. Why does it happen?
 In both the above activities the  observed effects are created due to the change in 
the direction of light while coming out of water.  Light changes its direction when going 
from one transparent medium to another transparent medium. This is called the 
refraction of light.
Activity 3:
1. Keep a glass slab on a blank paper and draw its outline PQRS as shown in figure 6.1. 
2. Draw an inclined straight line on the side of PQ so that it intersects PQ at N. Pierce two 
pins vertically at two points A and B along the line. 
3. Look at the pins A and B from the opposite side of the slab and pierce pins C and D 
vertically so that the images of A and B are in line with C and D. 
4. Now remove the chip and  the pins and draw a straight line going through points C and 
D so that it intersects SR at M. 
5. Join points M and N. Observe the incident ray AN and emergent ray MD.
Refraction of light
Activity 2: 
1. keep a 5 rupee coin in a metallic vessel. 
2. Slowly go away from the vessel
3. Stop at the place when the coin   
    disappears.
4. Keep looking in the direction of the coin.
5. Ask a friend to slowly fill water in the 
vessel. You will be able to see the coin 
once the  level of water reaches a 
certain height. Why does it happen?
Material: Glass, 5 rupee coin, Pencil, metallic vessel etc.
Can you recall?
Try this.
74
The first refraction occurs when light ray 
enters the glass from air at N on the side PQ. The 
second refraction occurs when light enters air 
through glass at point M on the side SR. For the 
first refraction the angle of incidence is i while 
for the second it is i
1
. The angle of refraction at N 
is r.
Note that i
1
 = r. In the second refraction, the 
angle of refraction is e which is equal to i. On 
both parallel sides PQ and RS of the glass slab, 
the change in direction of light ray is equal but in 
opposite directions. 
Thus, the light ray MD emerging from the 
glass slab is parallel to the incident ray AN on 
the side PQ of the slab. But the emergent ray is  
somewhat displaced with respect to the incident 
ray. 
6.1 Refraction of light passing 
through a glass slab
P
S
Q
R
Refraction 
of light
M
C
D
A
B
N
  r
i
i
1
e
Glass
Air
Air
A
C
N
B
r
i
6.2 Light ray entering a glass slab from air
D
Glass
Air
Incident ray 
Refracted 
ray
sin i
sin r
= constant = n
  n is called the refractive index 
of the second medium with respect 
to the first medium. This second law 
is also called Snell’s law. A ray 
incident along the normal (i = 0) 
goes forward in the same direction 
(r = 0).
1. Will light travel through a glass slab with the same 
velocity as  it travels in air?
2.  Will the velocity of light be same in all media?
 
Laws of refraction
Let us study the light ray entering a glass 
slab from air as shown in the figure 6.2. Here AN 
is the incident ray and NB is the refracted ray. 
1. Incident ray and refracted ray at the point of      
incidence N are on the opposite sides of the 
normal to the surface of the slab at that point 
i.e. CD, and the three, incident ray, refracted 
ray and the normal, are in the same plane. 
2. For a given pair of media, here air and glass, 
the  ratio of sin i to sin r  is a constant. Here, i 
is the angle of incidence and r is the angle of 
refraction. 
Refractive index
The change in the direction of a light ray 
while entering different media is different. It is 
related to the refractive index of the medium. 
The value of the refractive index is different for 
different media and also for light of different 
colours for the same medium. The refractive 
indices of some substances with respect to 
vacuum are given in the table. The refractive 
index of a medium with respect to vacuum is 
called its absolute refractive index.
Refractive index depends on the velocity 
of light in the medium.
Use your brain power !
75
Let the velocity of light in medium 1 be v
1
 and in 
medium 2 be v
2
 as shown in figure 6.3. The refractive 
index of the second medium with respect to the first 
medium, 
1
n
2
 is equal to the ratio of the velocity of light 
in medium 1 to that in medium 2.
Substance
Refractive 
index
Substance 
Refractive 
index
Substance
Refractive 
index
Air 1.0003 Fused Quartz 1.46
Carbon 
disulphide
1.63
Ice 1.31 Turpentine oil 1.47 Dense flint glass 1.66
Water 1.33 Benzene 1.50 Ruby 1.76
Alcohol 1.36 Crown glass 1.52 Sapphire 1.76
Kerosene 1.39 Rock salt 1.54 Diamond 2.42
Absolute refractive indices of some media
2
n
1
 =
v
2
v
1
6.3 Light ray going from 
 medium 1 to medium 2
Medium 1
 Air  
Medium 2
Glass
Ray
Similarly, the refractive index of medium 1 
with respect to medium 2 is 
If the refractive index of second medium with respect to first 
medium is 
1
n
2
and that of third medium with respect to second 
medium is 
2
n
3
 , what and how much is 
1
n
3 
?
 When a light ray 
passes from a rarer 
medium to a denser a 
medium, it bends towards 
the normal.  
 When a light ray 
passes from a denser 
medium to  a rarer 
medium, it bends away 
from the normal.
 When a light ray is incident 
normally at the boundary 
between two media, it does not 
change its direction and hence 
does not get refracted.
If the first medium is vacuum then the refractive index of medium 2 
is called absolute refractive index and it is written as n. 
Denser medium   
Rarer medium  
Denser medium  
Rarer medium
Denser medium  
i 
r
i 
r
Rarer medium 
v
1
v
2
6.4  Refraction of light in different media
Refractive index 
1
n
2
 =
Velocity of light in medium 1 (v
1
) 
Velocity of light in medium 2 (v
2
)
Can you tell?
76
Twinkling of stars
1. Have you seen a mirage which is an illusion of the appearance 
of water on a hot road or in a desert?
2. Have you seen that objects beyond and above a holi fire appear 
to be  shaking? Why does this happen?
Effect of atmospheric conditions on refraction of light can be seen in the twinkling of 
the stars.  
Stars are self-luminous and can be seen at night in the absence of sunlight. They 
appear to be point sources because of their being at a very large distance from us.  As the 
desity of air increases with  lowering height above the surface of the earth, the refractive 
index also increases. Star light coming towards us travels from rarer medium to denser 
medium and constantly bends towards the normal. This makes the star appear to be higher 
in the sky as compared to its actual position as shown in the figure, 6.6. 
The apparent position of the star keeps changing a bit. This is because of the motion 
of atmospheric air and changing air density and temperature. Because of this, the refractive 
index of air keeps changing continuously. Because of this change, the position and 
brightness of the star keep changing continuously and the star appears to be twinkling. 
Local atmospheric conditions affect the refraction of light to some extent. In both the 
examples above, the air near the hot road or desert surface and near the holi flames is hot 
and hence rarer than the air above it.  The refractive index of air keeps increasing as we 
go to increasing heights. In the first case above, the direction of light rays, coming from a 
distance, keeps changing according to the laws of refraction. 
The light rays coming from a 
distant object appear to be coming 
from the image of the object inside 
the ground as shown in figure 6.5. 
This is called a mirage.  
In the second example, the 
direction of light rays coming from 
objects beyond the holi fire changes 
due to changing refractive index 
above the fire. Thus, the objects 
appear to be moving.
Cold air
Hot surface
Hot air
6.5   Mirage
6.6  Apparent position of a star 
Star
Apparent position of a star 
increasing 
refractive 
index
6.7 Effect of atmospheric refraction
Apparent position
Horizon
Earth
Real position
Atmospheric layers
Can you tell?
Page 5


73
	Ø  Refraction of light   Ø  Laws of refraction           
	Ø  Refractive index  Ø  Dispersion of light
6. Refraction of light 
1. What is meant by reflection of light?
2. What are the laws of reflection?
 We have seen that, generally light travels in a straight line.  Because of this, if an 
opaque object lies in its path, a shadow of the object is formed. We have also seen in 
previous classes how these shadows change due to the change in relative positions of the 
source of light and the object. But light can bend under some special circumstances as we 
will see below
Activity 1:
 1. Take a transparent glass and fill it with 
water.
 2. Dip some portion of a pencil vertically  
in water and observe the thickness of 
the portion of  the pencil, in water. 
 3. Now keep the pencil inclined to water  
surface and observe its thickness.
 In both cases, the portion of the pencil 
inside water appears to be thicker than the 
portion above water. In the second case, 
the pencil appears to be broken near the 
surface of water. Why does it happen?
 In both the above activities the  observed effects are created due to the change in 
the direction of light while coming out of water.  Light changes its direction when going 
from one transparent medium to another transparent medium. This is called the 
refraction of light.
Activity 3:
1. Keep a glass slab on a blank paper and draw its outline PQRS as shown in figure 6.1. 
2. Draw an inclined straight line on the side of PQ so that it intersects PQ at N. Pierce two 
pins vertically at two points A and B along the line. 
3. Look at the pins A and B from the opposite side of the slab and pierce pins C and D 
vertically so that the images of A and B are in line with C and D. 
4. Now remove the chip and  the pins and draw a straight line going through points C and 
D so that it intersects SR at M. 
5. Join points M and N. Observe the incident ray AN and emergent ray MD.
Refraction of light
Activity 2: 
1. keep a 5 rupee coin in a metallic vessel. 
2. Slowly go away from the vessel
3. Stop at the place when the coin   
    disappears.
4. Keep looking in the direction of the coin.
5. Ask a friend to slowly fill water in the 
vessel. You will be able to see the coin 
once the  level of water reaches a 
certain height. Why does it happen?
Material: Glass, 5 rupee coin, Pencil, metallic vessel etc.
Can you recall?
Try this.
74
The first refraction occurs when light ray 
enters the glass from air at N on the side PQ. The 
second refraction occurs when light enters air 
through glass at point M on the side SR. For the 
first refraction the angle of incidence is i while 
for the second it is i
1
. The angle of refraction at N 
is r.
Note that i
1
 = r. In the second refraction, the 
angle of refraction is e which is equal to i. On 
both parallel sides PQ and RS of the glass slab, 
the change in direction of light ray is equal but in 
opposite directions. 
Thus, the light ray MD emerging from the 
glass slab is parallel to the incident ray AN on 
the side PQ of the slab. But the emergent ray is  
somewhat displaced with respect to the incident 
ray. 
6.1 Refraction of light passing 
through a glass slab
P
S
Q
R
Refraction 
of light
M
C
D
A
B
N
  r
i
i
1
e
Glass
Air
Air
A
C
N
B
r
i
6.2 Light ray entering a glass slab from air
D
Glass
Air
Incident ray 
Refracted 
ray
sin i
sin r
= constant = n
  n is called the refractive index 
of the second medium with respect 
to the first medium. This second law 
is also called Snell’s law. A ray 
incident along the normal (i = 0) 
goes forward in the same direction 
(r = 0).
1. Will light travel through a glass slab with the same 
velocity as  it travels in air?
2.  Will the velocity of light be same in all media?
 
Laws of refraction
Let us study the light ray entering a glass 
slab from air as shown in the figure 6.2. Here AN 
is the incident ray and NB is the refracted ray. 
1. Incident ray and refracted ray at the point of      
incidence N are on the opposite sides of the 
normal to the surface of the slab at that point 
i.e. CD, and the three, incident ray, refracted 
ray and the normal, are in the same plane. 
2. For a given pair of media, here air and glass, 
the  ratio of sin i to sin r  is a constant. Here, i 
is the angle of incidence and r is the angle of 
refraction. 
Refractive index
The change in the direction of a light ray 
while entering different media is different. It is 
related to the refractive index of the medium. 
The value of the refractive index is different for 
different media and also for light of different 
colours for the same medium. The refractive 
indices of some substances with respect to 
vacuum are given in the table. The refractive 
index of a medium with respect to vacuum is 
called its absolute refractive index.
Refractive index depends on the velocity 
of light in the medium.
Use your brain power !
75
Let the velocity of light in medium 1 be v
1
 and in 
medium 2 be v
2
 as shown in figure 6.3. The refractive 
index of the second medium with respect to the first 
medium, 
1
n
2
 is equal to the ratio of the velocity of light 
in medium 1 to that in medium 2.
Substance
Refractive 
index
Substance 
Refractive 
index
Substance
Refractive 
index
Air 1.0003 Fused Quartz 1.46
Carbon 
disulphide
1.63
Ice 1.31 Turpentine oil 1.47 Dense flint glass 1.66
Water 1.33 Benzene 1.50 Ruby 1.76
Alcohol 1.36 Crown glass 1.52 Sapphire 1.76
Kerosene 1.39 Rock salt 1.54 Diamond 2.42
Absolute refractive indices of some media
2
n
1
 =
v
2
v
1
6.3 Light ray going from 
 medium 1 to medium 2
Medium 1
 Air  
Medium 2
Glass
Ray
Similarly, the refractive index of medium 1 
with respect to medium 2 is 
If the refractive index of second medium with respect to first 
medium is 
1
n
2
and that of third medium with respect to second 
medium is 
2
n
3
 , what and how much is 
1
n
3 
?
 When a light ray 
passes from a rarer 
medium to a denser a 
medium, it bends towards 
the normal.  
 When a light ray 
passes from a denser 
medium to  a rarer 
medium, it bends away 
from the normal.
 When a light ray is incident 
normally at the boundary 
between two media, it does not 
change its direction and hence 
does not get refracted.
If the first medium is vacuum then the refractive index of medium 2 
is called absolute refractive index and it is written as n. 
Denser medium   
Rarer medium  
Denser medium  
Rarer medium
Denser medium  
i 
r
i 
r
Rarer medium 
v
1
v
2
6.4  Refraction of light in different media
Refractive index 
1
n
2
 =
Velocity of light in medium 1 (v
1
) 
Velocity of light in medium 2 (v
2
)
Can you tell?
76
Twinkling of stars
1. Have you seen a mirage which is an illusion of the appearance 
of water on a hot road or in a desert?
2. Have you seen that objects beyond and above a holi fire appear 
to be  shaking? Why does this happen?
Effect of atmospheric conditions on refraction of light can be seen in the twinkling of 
the stars.  
Stars are self-luminous and can be seen at night in the absence of sunlight. They 
appear to be point sources because of their being at a very large distance from us.  As the 
desity of air increases with  lowering height above the surface of the earth, the refractive 
index also increases. Star light coming towards us travels from rarer medium to denser 
medium and constantly bends towards the normal. This makes the star appear to be higher 
in the sky as compared to its actual position as shown in the figure, 6.6. 
The apparent position of the star keeps changing a bit. This is because of the motion 
of atmospheric air and changing air density and temperature. Because of this, the refractive 
index of air keeps changing continuously. Because of this change, the position and 
brightness of the star keep changing continuously and the star appears to be twinkling. 
Local atmospheric conditions affect the refraction of light to some extent. In both the 
examples above, the air near the hot road or desert surface and near the holi flames is hot 
and hence rarer than the air above it.  The refractive index of air keeps increasing as we 
go to increasing heights. In the first case above, the direction of light rays, coming from a 
distance, keeps changing according to the laws of refraction. 
The light rays coming from a 
distant object appear to be coming 
from the image of the object inside 
the ground as shown in figure 6.5. 
This is called a mirage.  
In the second example, the 
direction of light rays coming from 
objects beyond the holi fire changes 
due to changing refractive index 
above the fire. Thus, the objects 
appear to be moving.
Cold air
Hot surface
Hot air
6.5   Mirage
6.6  Apparent position of a star 
Star
Apparent position of a star 
increasing 
refractive 
index
6.7 Effect of atmospheric refraction
Apparent position
Horizon
Earth
Real position
Atmospheric layers
Can you tell?
77
We do not see twinkling of planets. This is because, planets are much closer to us as 
compared to stars. They, therefore, do not appear as point sources but appear as a collection 
of point sources. Because of changes in atmospheric refractive index the position as well 
as the brightness of individual point source change but the average position and total 
average brightness remains unchanged and planets do not twinkle. 
By Sunrise we mean the appearance of the Sun above the horizon.  But when the Sun 
is somewhat below the horizon, its light rays are able to reach us along a curved path due 
to their refraction through earth’s atmosphere as shown in the figure 6.7. Thus, we see the 
Sun even before it emerges above the horizon. Same thing happens at the time of Sunset 
and we keep seeing the Sun for a short while even after it goes below the horizon.
Dispersion of light 
Hold the plastic scale in your compass in front of your eyes and see through it while 
turning it slowly. Y ou will see light rays divided into different colours. These colours appear 
in the following order: violet, indigo, blue, green, yellow, orange and red. You know that 
light is electromagnetic radiation. Wavelength is an important property of radiation. The 
wavelength of radiation to which our eyes are sensitive is between 400  and 700 nm. In this 
interval, radiation of different wavelengths appears to have different colours mentioned 
above. The red light has maximum wavelength i.e. close to 700 nm while violet light has 
the smallest wavelength, close to 400 nm. Remember that 1 nm = 10
-9
 m.
In vacuum, the velocity of light rays of all frequencies is the same. But the velocity of 
light in a medium depends on the frequency of light and thus different colours travel with 
different velocity. Therefore, the refractive index of a medium is different for different 
colours. Thus, even when white light enters a single medium like glass, the angles of 
refraction are different for different colours. So when the white light coming from the Sun 
through air, enters any refracting medium, it emerges as a spectrum of seven colours. 
The process of separation of light into its component colours while passing 
through a medium is called the dispersion of light.
Sir Isaac Newton was the first person to 
use a glass prism to obtain Sun’s spectrum. 
When white light is incident on the prism, 
different colours bend through different angles. 
Among the seven colours, red bends the least 
while violet bends the most. Thus, as shown in 
figure 6.8, the seven colours emerge along 
different paths and get separated and we get a 
spectrum of seven colours. 
6.8  Dispersion of light
Sun light
R
O
Y
G
B
I
V
Glass Prism
1. From incident white light how will you obtain white 
emergent light by making use of two prisms?
2. You must have seen chandeliers having glass prisms. The light from a tungsten bulb 
gets dispersed while passing through these prisms and we see coloured spectrum. If we 
use an LED light instead of a tungsten bulb, will we be able to see the same effect?
Use your brain power !