Refraction drastically affects the way certain things are seen. There are many daily phenomena that are due to refraction.

1. Real and apparent depth

When looking into a pond, its bottom appears shallower than it actually is. This is because rays from the bottom bend away from the normal as they enter into air. To the eye, the rays appear to be coming from a virtual point above the bottom.

Figure (a) below shows the ray construction diagram for the eye viewing a dot at the bottom of a glass of water. The dot is seen to be elevated because the rays entering the eye appear to come from the virtual point I.

A stick partially immersed in water appears bent due to refraction and this is explained in figure (b) below. Rays from the point O bend away from the normal as they enter into air and to an outside observer; the ray appear to come from the point I. similarly, when a glass block is placed on a dot made o a piece of paper, the dot appears to be raised.

Real depth and apparent depth are related to refractive index by the equation:

Refractive index (n) = real depthapparent depth

1. Mirage ;

A mirage is an illusive appearance of a pool of water on tarred roads in a hot weather. The figure below shows a mirage;

It occurs due to the progressive refractive bending of light from the sky as it moves through air. The upper cold areas of air are optically denser than the lower warm layers. So as light moves through the different layers of air, it progressively bends away from the normal until it becomes parallel to the earth’s surface; it the proceeds to bend upwards afterwards.

To an observer, the light will seem to come the ground where he sees the image of the sky as a mirage.

Example;

A rectangular block of transparent material is 40cm in height and placed over a line on a paper. When the line is viewed vertically downwards through the block, it appears to be 15cm above the paper.

1. Draw a ray diagram which explains the observation
2. What is the name of the phenomenon which explains this observation?
3. Explain why this phenomenon occurs
4. Calculate the refractive index of the material

Solution

1. Refraction
2. It occurs because of the change in the speed of light as it moves from one medium to another
3. n= real depthapparent depth

n= 4040-15=4025=1.6