Causes of Refraction of Light

The speed of light in air, water, and glass is 3×10⁸ m/s, 2.2 ×10⁸ m/s and 2×10⁸ m/s respectively. The change in speed of light when it passes from one medium to another is the cause of the refraction of light. The medium, through which the light passes is called optical medium. The optical medium, in which the velocity of light is more, is called rarer medium. The medium in which the velocity of light is comparatively less is called denser medium.

Some examples of refraction of light:

1. The apparent upward bending of a pencil when placed in water:

The given figure shows a pencil ABC placed in a trough of water. The pencil appears bent upwards from the point B. Rays CE and CD from the end C of the pencil pass from water to air, and are bent away from the normal along EE’ AND DD’ respectively, since they are passing in rarer medium. To the eye, they appear to come from point C’ above C. C’ is thus the image of C as a result of the refraction. The same reasoning applies to any point on the immersed portion of pencil BC so that the observer sees an image apparently in the position BC.

2. Image of coin in water:

In the given figure, a coin ‘o’ is placed at the bottom of a beaker containing water. Two rays OA and OD from O are refracted along OA and OD from O are refracted along AB and DE respectively. These divergent rays appear to come from ‘I’ which is the virtual image of ‘O’. Clearly, the image ‘I’ is at a less depth than ‘O’. Thus, the coin at ‘O’ appears to rise at ‘I’ when the coin is kept in water. The Same phenomenon explains why a swimming pool appears shallower than it really is.

3. Twinkling of stars:

The light from the stars passes through the atmosphere before it reaches an observer on the earth. Stars twinkles due to change in density whichis in a small range, the image of the star appears to move within a small area in the space and this gives to twinkling effect. Stars do not appear twinkling if they are viewed from above the earth’s atmosphere. Twinkling effect is observed if the stars are viewed from earth’s surface.

Real and apparent depth

Let a point object O be placed just below a rectangular glass slab ABCD of thickness AD or BC as shown in the figure. A ray OP incident normally on the surface CD passes to air without deviation along PQ. Ray OS in the glass is refracted along SR in the air. The two rays meet at I when produced back. Thus, ‘I’ is the virtual image of the object ‘O’. Hence, the object at ‘O’ appears raised at ‘I’. When viewed directly the object O through the glass slab. Let ∠OSN = ∠i be the angle of the incidence in the glass and ∠MSR = ∠r be the angle of refraction in the air. Here, PO is real depth and PI is medium, the object appears to be in less depth due to refraction. Such less depth is called apparent depth and the actual depth is called real depth.