In the previous section we completed a discussion on heat. In this chapter we will discuss about Refraction of light.
• When we see a pencil placed in a glass of water, the pencil appears to be broken.
• Some other such situations can be seen here. To explain this phenomenon, let us do an activity:
1. Place a glass slab on a drawing sheet and mark it's boundary.
2. Remove the glass slab. Now we have a rectangle on the drawing sheet.
• Name the rectangle as ABCD. This is shown in the fig.7.1(a) below:
3. Mark a point Q on AB. Draw the red dotted line passing through Q.
• This dotted line should be perpendicular to the side AB.
• This red dotted line is the 'normal of the side AB at Q'.
4. Draw a line PQ in such a way that, PQ makes an acute angle with the normal.
• Let this angle be 'i'.
5. Now keep the glass slab back in position.
• Pass the light from a laser torch through the glass slab.
• The ray of light should be along PQ.
6. Observe the path of light through the glass slab.
• Mark the point R at which the light comes out of the slab. This is shown in fig.b.
7. Remove the slab and join the points Q and R.
■ We can note the following points:
• The ray PQ is the incident ray
• It deviates from it's original path.
• The deviation occurs at Q, which is the boundary of the glass slab
• This deviation of the ray of light is called refraction.
■ We will write the official definition:
When a light ray is incident obliquely from one transparent medium to another, it's path undrgoes a deviation. This is refraction.
8. Note the word 'obliquely'. Why is it mentioned?
Ans: 'Obliquely' means 'at an angle'.
• For the deviation to occur, the incident ray must make an angle with the normal.
• In our present case, the angle is 'i'.
• If the incident ray is exactly along the normal, then the angle is zero.
• Then there will not be any deviation. The ray of light will pass straight through the slab.
9. The ray PQ is the incident ray
• The ray QR is the refracted ray
• The angle 'i' which the incident ray makes with the normal is called the angle of incidence
• The angle 'r' which the refracted ray makes with the normal is called the angle of refraction
• When the medium is Diamond, the speed of light is 1.25 × 108 m/s
• When the medium is Glass, the speed of light is 2.0 × 108 m/s
• When the medium is water, the speed of light is 2.25 × 108 m/s
• When there is no medium, that is., in vacuum, the speed of light is 3.0 × 108 m/s
■ From the above values, we can note that:
When the density of the medium decreases, the speed of light increases
• Optical density is a measure which shows how a medium influences the speed of light passing through it.
• When optical density decreases, the speed of light increases
Fig.7.2 below shows some ray diagrams.
• In each of them, a ray of light passes from one medium to another medium. Let us analyse each one of them in detail:
Fig.7.2(a):
1. The ray pass from air to water
2. The ray is incident obliquely on the boundary
3. There is a deviation in the path of the ray. So there is refraction
4. Which one of the following is true?
(a) The refracted ray deviates towards the normal
(b) The refracted ray deviates away from the normal
Ans: The refracted ray deviates towards the normal
5. How do we know that?
• Consider the green dashed line
• If the refracted ray is in between the green dashed line and the normal, then we say:
The refracted ray deviates towards the normal
• If the refracted ray is not in between the green dashed line and the normal, then we say:
The refracted ray deviates away from the normal
• So in the present case 4(a) is true
6. In fig.5.2(a), light passes from rarer medium to denser medium
So we can write:
■ When light passes from rarer medium to denser medium, the refracted ray deviates towards the normal
Fig.7.2(b):
1. The ray pass from glass to air
2. The ray is incident obliquely on the boundary
3. There is a deviation in the path of the ray. So there is refraction
4. Which one of the following is true?
(a) The refracted ray deviates towards the normal
(b) The refracted ray deviates away from the normal
Ans: The refracted ray deviates away from the normal
5. In fig.5.2(b), light passes from denser medium to rarer medium
So we can write:
■ When light passes from denser medium to rarer medium, the refracted ray deviates away from the normal
Fig.7.2(c):
1. The ray pass from air to water
2. The ray is incident along the normal
3. There is no deviation in the path of the ray. So there is no refraction
So we can write:
■ When the incident ray is normal to the boundary, there is no deviation
Fig.7.2(d):
1. The ray pass from glass to water
2. The ray is incident obliquely on the boundary
3. There is a deviation in the path of the ray. So there is refraction
4. Which one of the following is true?
(a) The refracted ray deviates towards the normal
(b) The refracted ray deviates away from the normal
Ans: The refracted ray deviates away from the normal
5. In fig.5.2(d), light passes from denser medium to rarer medium
So we can write:
■ When light passes from denser medium to rarer medium, the refracted ray deviates away from the normal (this is same as the case in fig.b)
Fig.7.2(e):
1. The ray pass from glass to water
2. The ray is incident along the normal
3. There is no deviation in the path of the ray. So there is no refraction
So we can write:
■ When the incident ray is normal to the boundary, there is no deviation (this is same as the case in fig.c)
■ We can write a summary:
• Rarer to denser ➙ Light deviates towards the normal
• Denser to rarer ➙ Light deviates away from the normal
■ An easy method to remember this is to think in this way :
• Rarer to denser ➙ Light finds it difficult to travel in denser medium. It is suppressed.
♦ Suppressed towards the normal
• Denser to rarer ➙ Light finds freedom. It breaks free.
♦ Free away from the normal
Now we can explain why a pencil placed in water appears to be broken. Consider fig.7.3 below:
1. A blue rod xy is placed in water.
2. A light ray begins it's journey from the bottom end x.
3. It should have continued it's journey along the green dashed line. But instead, it deviates at the boundary.
4. So, to an observer, the ray appears to be coming from x'. Thus the rod appears to be broken
Now let us see an interesting point:
Consider fig.7.4(a) below:
1. The red dotted line is shown in fig.a. It is the normal to the boundary
2. The light ray passes from denser to rarer. So refracted ray deviates away from the normal
• We can see that the refracted ray does not lie in between the normal and the original path
3. Now consider fig.b. The refracted ray is shown to lie in between the normal and the original path
• That means, the refracted ray deviates towards the normal
4. Now, the point x' is on the right side of the rod xy. The rod appears to be 'broken inwards'.
5. But such a case will never occur.
• The ray will always deviate away from the normal when passing from water to air
• So x' will always be on the left side of x as shown in fig.a
• Indeed, we do not see an 'inward broken appearance' when a pencil or rod is placed in water
• So a 'X' mark is put on fig (b) and a 'tick' mark is put on fig.a
• When we see a pencil placed in a glass of water, the pencil appears to be broken.
• Some other such situations can be seen here. To explain this phenomenon, let us do an activity:
1. Place a glass slab on a drawing sheet and mark it's boundary.
2. Remove the glass slab. Now we have a rectangle on the drawing sheet.
• Name the rectangle as ABCD. This is shown in the fig.7.1(a) below:
 |
| Fig.7.1 |
• This dotted line should be perpendicular to the side AB.
• This red dotted line is the 'normal of the side AB at Q'.
4. Draw a line PQ in such a way that, PQ makes an acute angle with the normal.
• Let this angle be 'i'.
5. Now keep the glass slab back in position.
• Pass the light from a laser torch through the glass slab.
• The ray of light should be along PQ.
6. Observe the path of light through the glass slab.
• Mark the point R at which the light comes out of the slab. This is shown in fig.b.
7. Remove the slab and join the points Q and R.
■ We can note the following points:
• The ray PQ is the incident ray
• It deviates from it's original path.
• The deviation occurs at Q, which is the boundary of the glass slab
• This deviation of the ray of light is called refraction.
■ We will write the official definition:
When a light ray is incident obliquely from one transparent medium to another, it's path undrgoes a deviation. This is refraction.
8. Note the word 'obliquely'. Why is it mentioned?
Ans: 'Obliquely' means 'at an angle'.
• For the deviation to occur, the incident ray must make an angle with the normal.
• In our present case, the angle is 'i'.
• If the incident ray is exactly along the normal, then the angle is zero.
• Then there will not be any deviation. The ray of light will pass straight through the slab.
9. The ray PQ is the incident ray
• The ray QR is the refracted ray
• The angle 'i' which the incident ray makes with the normal is called the angle of incidence
• The angle 'r' which the refracted ray makes with the normal is called the angle of refraction
What is the reason for refraction?
■ Light travels at different speeds through different media. Some examples are given below:• When the medium is Diamond, the speed of light is 1.25 × 108 m/s
• When the medium is Glass, the speed of light is 2.0 × 108 m/s
• When the medium is water, the speed of light is 2.25 × 108 m/s
• When there is no medium, that is., in vacuum, the speed of light is 3.0 × 108 m/s
■ From the above values, we can note that:
When the density of the medium decreases, the speed of light increases
• Optical density is a measure which shows how a medium influences the speed of light passing through it.
• When optical density decreases, the speed of light increases
Fig.7.2 below shows some ray diagrams.
 |
| Fig.7.2 |
Fig.7.2(a):
1. The ray pass from air to water
2. The ray is incident obliquely on the boundary
3. There is a deviation in the path of the ray. So there is refraction
4. Which one of the following is true?
(a) The refracted ray deviates towards the normal
(b) The refracted ray deviates away from the normal
Ans: The refracted ray deviates towards the normal
5. How do we know that?
• Consider the green dashed line
• If the refracted ray is in between the green dashed line and the normal, then we say:
The refracted ray deviates towards the normal
• If the refracted ray is not in between the green dashed line and the normal, then we say:
The refracted ray deviates away from the normal
• So in the present case 4(a) is true
6. In fig.5.2(a), light passes from rarer medium to denser medium
So we can write:
■ When light passes from rarer medium to denser medium, the refracted ray deviates towards the normal
Fig.7.2(b):
1. The ray pass from glass to air
2. The ray is incident obliquely on the boundary
3. There is a deviation in the path of the ray. So there is refraction
4. Which one of the following is true?
(a) The refracted ray deviates towards the normal
(b) The refracted ray deviates away from the normal
Ans: The refracted ray deviates away from the normal
5. In fig.5.2(b), light passes from denser medium to rarer medium
So we can write:
■ When light passes from denser medium to rarer medium, the refracted ray deviates away from the normal
Fig.7.2(c):
1. The ray pass from air to water
2. The ray is incident along the normal
3. There is no deviation in the path of the ray. So there is no refraction
So we can write:
■ When the incident ray is normal to the boundary, there is no deviation
Fig.7.2(d):
1. The ray pass from glass to water
2. The ray is incident obliquely on the boundary
3. There is a deviation in the path of the ray. So there is refraction
4. Which one of the following is true?
(a) The refracted ray deviates towards the normal
(b) The refracted ray deviates away from the normal
Ans: The refracted ray deviates away from the normal
5. In fig.5.2(d), light passes from denser medium to rarer medium
So we can write:
■ When light passes from denser medium to rarer medium, the refracted ray deviates away from the normal (this is same as the case in fig.b)
Fig.7.2(e):
1. The ray pass from glass to water
2. The ray is incident along the normal
3. There is no deviation in the path of the ray. So there is no refraction
So we can write:
■ When the incident ray is normal to the boundary, there is no deviation (this is same as the case in fig.c)
■ We can write a summary:
• Rarer to denser ➙ Light deviates towards the normal
• Denser to rarer ➙ Light deviates away from the normal
■ An easy method to remember this is to think in this way :
• Rarer to denser ➙ Light finds it difficult to travel in denser medium. It is suppressed.
♦ Suppressed towards the normal
• Denser to rarer ➙ Light finds freedom. It breaks free.
♦ Free away from the normal
Now we can explain why a pencil placed in water appears to be broken. Consider fig.7.3 below:
 |
| Fig.7.3 |
2. A light ray begins it's journey from the bottom end x.
3. It should have continued it's journey along the green dashed line. But instead, it deviates at the boundary.
4. So, to an observer, the ray appears to be coming from x'. Thus the rod appears to be broken
Now let us see an interesting point:
Consider fig.7.4(a) below:
 |
| Fig.7.4 |
2. The light ray passes from denser to rarer. So refracted ray deviates away from the normal
• We can see that the refracted ray does not lie in between the normal and the original path
3. Now consider fig.b. The refracted ray is shown to lie in between the normal and the original path
• That means, the refracted ray deviates towards the normal
4. Now, the point x' is on the right side of the rod xy. The rod appears to be 'broken inwards'.
5. But such a case will never occur.
• The ray will always deviate away from the normal when passing from water to air
• So x' will always be on the left side of x as shown in fig.a
• Indeed, we do not see an 'inward broken appearance' when a pencil or rod is placed in water
• So a 'X' mark is put on fig (b) and a 'tick' mark is put on fig.a
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