Applications of Refraction and Total Internal Reflection

Mirage

Mirage is an optical illusion which is observed in deserts or on tarred roads in hot summer days. This creates an illusion of water, which actually is not there.

Due to excessive heat, the road gets very hot and the air in contact with it also gets heated up. The densities and the refractive indices of the layers immediately above the road are lower than those of the cooler higher layers. Since there is no abrupt change in medium, a ray of light from a distant object, such as a tree, bends more and more as it passes through these layers. And when it falls on a layer at an angle greater than the critical angle for the two consecutive layers, total internal reflection occurs. This produces an inverted image of the tree giving an illusion of reflection from a pool of water.

Totally Reflecting Prism

A prism with right angled isosceles triangular base or a totally reflecting prism with angles of 90°, 45° and 45° is a very useful device for reflecting light. The symmetry of the prism allows light to be incident on O at an angle of 45°, which is greater than the critical angle for glass (42°). As a result, light suffers total internal reflection and is deviated by 90°.

Optical Fibres

An optical fibre is a hair-thin structure of glass or quartz. It has an inner core which is covered by a thin layer (called cladding) of a material of slightly lower refractive index. For example, the refractive index of the core is about 1.7 and that of the cladding is 1.5. This arrangement ensures total internal reflection.

When light is incident on one end of the fibre at a small angle, it undergoes multiple total internal reflections along the fibre. The light finally emerges with undiminished intensity at the other end. Even if the fibre is bent, this process is not affected.