Total Internal Reflection

When a ray of light passes from an optically denser medium into a rarer medium; the refracted ray is bent away from the normal. A ray of light incident normal to the surface passes without any deviation. As the angle of incidence increases, the angle of refraction also increases and at a certain angle of incidence the refracted ray just grazes surface of water. This angle of incidence within a denser medium for which angle of refraction becomes 90° is called the critical angle. If the angle of incidence is increased beyond the critical angle, the ray bends inside the denser medium. This is called total internal reflection.

Relation between critical angle and refractive index

We know that

The above implies that refractive index of any medium with respect to air is the reciprocal of the sine of the critical angle.

Example:

The critical angle of diamond is 24.4°. So C=  24.4° and µ=?

_airµ_diamond = 1/sinC

_airµ_diamond = 1/sin 24.4° = 2.42

Working of a Periscope

There are two essential conditions for Total Internal Reflection. One is that the light must proceed from denser medium to a rarer medium. Second is that the angle of incidence in the denser medium must be greater than the critical angle. For example, a prism having an angle of 90° between its two refracting surfaces and the other two angles each equal to 45° is called a totally reflecting prism, because 45° is greater than the critical angle for glass (42°). Totally reflecting prisms are used in the construction of periscope. Periscope is used in the submarines to see objects above the surface of water.

Optical Fibres

An optical fibre is a device based on total internal reflection by which a light signal can be transmitted from one place to other with negligible loss of energy. An optical fibre is a long glass rod of only a few millimeters thick and it is quite flexible. The fibre glass consists of a cylindrical inner core that carries light and an outer concentric shell called cladding. The refractive index of inner core (m = 1.7) is relatively greater than that of cladding (m = 1.5). The rays of light travelling along the fibre cannot escape because they are totally reflected from the core-cladding interface. So the fibre of solid glass can be used as a light pipe.

Optical fibres can carry light round bents. This allows doctor to see inside our body. (endoscopy). Optical fibres can also carry information in the form of a digital code of light pulses with minimum loss. They carry telephone messages and computer data. Fibre optics technique is used to destroy tumours in solid organ like liver.