ICSE Class 8 Physics Chapter 02 Light

Light

Learning Outcomes

After studying this chapter, you will be able to understand:

Refraction of light: laws and refractive index

Refraction through a glass slab and a glass prism

Dispersion through a glass prism

Lenses: convex and concave

Optical instruments

Defects of the eye

Introduction

Look at the following figures. You might have observed such images in your day-to-day life.

In figure (a), the letters below the glass slabs appear to be raised. In figure (b), you see that the part of the pencil that is dipped in water appears to be bent. Similarly, in figure (c), you see that the swimming pool appears shallower than its actual depth. Can you tell how all this happens?

This is because of a particular property of light rays known as refraction.

In this chapter you will study what refraction is, how it is caused, what are its effects, and many more things.

Refraction

You might have studied in earlier classes that light travels in straight lines. However, when a ray of light travels from one transparent medium to another, such as from water to air; or air to glass (and vice versa), it undergoes a change in its direction at the boundary between the two mediums and bends. This phenomenon is called refraction.

Before we learn what causes the light ray to bend, let us see how the speed of light is different in different mediums.

Speed Of Light And Optical Density Of A Medium

You know that light travels with a speed of 3 - 10^8 m/s in air. However, its speed in water is 2.25 - 10^8 m/s and in glass is 1.8 - 10^8 m/s. Thus, we see that the speed of light is different in different mediums and depends on the properties of the medium. When light slows down in a medium, we say it is an optically denser medium. From the speed of light in different mediums, we can say that water is optically denser than air, and glass is optically denser than water. In other words, we say that water is optically rarer than glass, and air is optically rarer than water and glass.

The cause of bending of a light ray is due to the change in the speed of light in different materials/mediums. But the question that arises is why should a light ray bend at the boundary of the two mediums just because it cannot maintain the same speed? What happens if you roll your toy car gently from a smooth bitumen road to a grass lawn? At the boundary of the two surfaces, you will find that the wheel gets deflected from the path it follows (see Fig. 2.1). Thus, bending is a consequence of change in speed. Light rays also behave in the same way. By bending, light rays reach faster, obeying the principle of least time, which states that out of all the available paths, light takes the path that requires the shortest time.

Having understood this, let us understand why the pencil, when dipped in water, appears to be bent. Look at Figure 2.2. We have learnt in earlier classes that when light reflected from objects falls on our eyes, we see the objects. When a pencil is put in a beaker of water, light rays from the bottom of the pencil travel first through water (which is optically denser) and then through air to reach our eyes. When the light rays strike the boundary of the two mediums, they bend in such a way that they reach our eyes faster. As a result, the eyes feel as if the light rays are coming from X and not from Y. Hence, the pencil appears to be bent.

Fact File

Optical density vs. physical density - The optical density that you study here is different from physical density. The physical density of a material refers to the mass/volume ratio of that material. But, optical density tells you how fast or slow a light ray can pass through the material.

Rules Regarding Refraction

Figure 2.3 shows a single light ray travelling from air to glass, i.e., from a rarer to a denser medium. The point where the ray hits the boundary between the two mediums is called the point of incidence. A perpendicular drawn to the air-glass interface at the point of incidence is called the normal. The ray of light in the first medium is called the incident ray, and this ray of light after changing its direction (or after refraction) is called the refracted ray. The angle between the incident ray and the normal is called the angle of incidence (∠i), and the angle between the refracted ray and the normal is called the angle of refraction (∠r).

Now look at the following three figures. Do you notice any difference in the path of the light ray?

The three figures given above state the three rules regarding the refraction of light:

1. When a ray of light travels obliquely from a rarer medium to a denser medium, it bends towards the normal (Fig. 2.4 (a)).

2. When a ray of light travels obliquely from a denser medium to a rarer medium, it bends away from the normal (Fig. 2.4 (b)).

3. When a ray of light travels perpendicular to the surface of separation of the two mediums, it goes without any deviation (Fig. 2.4 (c)).

We know that the speed of light is lesser in glass than in air because glass is optically denser than air. In (a) we see that the angle of refraction is smaller than the angle of incidence. Thus, we conclude that when the angle of refraction is less, the speed of light in that medium is also less. In (b) you can see that the angle of refraction is more, which tells you that the speed of light in that medium (air) is also more than that in the first medium (which is glass).

Laws Of Refraction

The laws of refraction state that

1. The incident ray, the normal to the point of incidence, and the refracted ray all lie in the same plane; and

2. The ratio of the sine of angle of incidence to the sine of angle of refraction is a constant known as the refractive index of the medium. You will learn about this law, called Snell's law, in higher classes.

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

Refractive Index

We know that refraction results from a change in speed when light passes from one medium to another.

The ratio of the speed of light in vacuum to the speed of light in another medium is termed as the refractive index of that medium. This tells you the amount by which the speed of light gets reduced, and the amount by which the light ray is refracted or bent for a given angle of incidence.

The refractive index is denoted by the letter n. Also, since it is a ratio, it has no unit.

\[n = \frac{\text{Speed of light in vacuum (c)}}{\text{Speed of light in the medium (v)}}\]

Note that the speed of light in air and in vacuum is almost equal; therefore, for numerical purposes we take the speed of light in vacuum as that in air. Table 2.1 shows the refractive index of some materials.

Solved Example

1. The speed of light in air is 3 - 10^8 m/s. Find its speed in diamond if the refractive index of diamond is 2.42.

We know that:

\[n = \frac{\text{Speed of light in air/vacuum (c)}}{\text{Speed of light in the medium (v)}}\]

Therefore, \[2.42 = \frac{3 \times 10^8}{v}\], \[v = \frac{3 \times 10^8}{2.42} = 1.24 \times 10^8 \text{ m/s}\]

Thus, speed of light in diamond has reduced considerably.

The refractive index tells you how much light has slowed down in that medium with respect to air. Table 2.1 shows the refractive indices of some materials. You will notice that the speed of light in that medium is low whose refractive index is high. Hence, the angle of refraction will also be less than the angle of incidence. If the angle of incidence is the same as the angle of refraction, the light ray travels straight without any bending.

Refraction Through A Glass Slab

In a glass slab, there are two parallel interfaces through which a light ray passes. The first interface is the air-glass interface, and the second interface is the glass-air interface.

In order to understand refraction through a glass slab, let us first perform an activity.

Activity

Aim: To trace a ray of light through a glass slab.

Materials required: A glass slab, a thick sheet of paper, a pencil, and few thumb pins.

Procedure: 1. Keep the glass slab on a thick white sheet of paper and draw its outline. Now remove the glass slab. And draw a line AB on one side of the block.

2. Fix two thumb pins on the line, one behind the other (as shown in the figure). Now place the glass block over its outline again and look at the glass block from the opposite side.

3. By looking at the images of the two pins, place two pins on the other side such that all the four pins are in a straight line. Now remove the glass slab again and draw a line CD passing through the two pins you placed at the images of the previous two pins. Join AB to CD.

Conclusion: You will find that the light ray has bent at the two interfaces, first at the air-glass interface and second at the glass-air interface.

Teacher's Note

The practical demonstration of light bending through glass slabs helps students visualize how light changes direction at interfaces between different transparent materials, making the concept of refraction tangible and memorable.

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