Maharashtra Board Class 10 Science Part 1 Chapter 6 Refraction of light PDF Download

Refraction Of Light

Can You Recall?

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.

Refraction Of Light

Try This

Material: Glass, 5 rupee coin, Pencil, metallic vessel etc.

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?

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?

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.

Teacher's Note

When you dip a pencil in water, it looks broken. This happens because light bends when going from water to air. In India, fishermen see fish at a different place than where the fish actually are because of this bending.

Exam Trick

Remember: Light bends when it goes from one medium to another. Like when you look at your feet in a swimming pool, they appear closer to the surface than they really are.

Points To Remember

Light bends when going from one medium to another.
This bending is called refraction.
Light bends because it travels at different speeds in different materials.
A pencil in water looks bent because of refraction.
We can see a coin in water that was hidden before because of refraction.

Refraction Of Light Passing Through A Glass Slab

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 i₁. The angle of refraction at N is r.

Note that i₁ = 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.

Teacher's Note

When light passes through a glass slab, it bends twice but comes out parallel to how it went in. Like when you look through a glass window, you see things but they are slightly displaced.

Exam Trick

Remember: Light in = Light out parallel. The light ray comes out parallel but displaced. Like a train going through a tunnel comes out on the same track.

Points To Remember

Light bends when entering glass and bends back when leaving.
The emergent ray is parallel to the incident ray.
But the emergent ray is displaced sideways.
The two bending angles are equal but opposite.

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.

\[\frac{\sin i}{\sin r} = \text{constant} = n\]

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).

Use Your Brain Power!

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?

Teacher's Note

Light travels slower in glass than in air. Like a car slows down when it goes from a smooth road to a rough road.

Exam Trick

Remember the formula: sin i / sin r = n. The ratio of sines is constant for any two materials. Write this formula first in your exam.

Points To Remember

There are two laws of refraction.
The first law says incident ray and refracted ray are on opposite sides of normal.
The second law is Snell's law: sin i / sin r = constant.
This constant is called refractive index n.

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.

Let the velocity of light in medium 1 be v₁ and in medium 2 be v₂. The refractive index of the second medium with respect to the first medium, ₂n₁ is equal to the ratio of the velocity of light in medium 1 to that in medium 2.

\[\,_2n_1 = \frac{v_1}{v_2}\]

Similarly, the refractive index of medium 1 with respect to medium 2 is

\[\,_1n_2 = \frac{v_2}{v_1}\]

If the first medium is vacuum then the refractive index of medium 2 is called absolute refractive index and it is written as n.

Absolute Refractive Indices Of Some Media

Can You Tell?

If the refractive index of second medium with respect to first medium is ₂n₁ and that of third medium with respect to second medium is ₃n₂, what and how much is ₃n₁?

When a light ray passes from a rarer medium to a denser 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.

Teacher's Note

Refractive index tells us how much light bends in a material. Diamond has the highest refractive index, which is why it sparkles so much.

Exam Trick

Remember: Higher refractive index = more bending = more slowing down of light. Diamond (2.42) bends light the most, water (1.33) bends it less.

Points To Remember

Refractive index is the ratio of velocities of light in two materials.
The formula is n = v₁ / v₂.
Diamond has the highest refractive index (2.42).
Air has the lowest refractive index (1.0003).
Higher the refractive index, more the light bends.

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