Advanced Applications of Total Internal Reflection

Diamonds don't shine, they reflect

The diamonds are usually cut in a particular way

So that whenever the ray of light enters it, it suffers total internal reflections at various faces

Total internal reflection refers to the complete reflection of a ray of light within a given medium from the surrounding surface

This happens only when the angle of incidence exceeds the critical angle

Total Internal Reflection has a number of applications

Total internal reflection is used in a right-angled isosceles prism

These prisms can turn light through 90 and 180 degrees based on internal reflection

For example, the prism is placed in the way as shown in the image above

A totally reflecting prism is that, which has one of its angle equal to 90° and each of the remaining two angles equal to 45°

If the light ray is incident on the surface of the prism at $90^o$,

The light travelling inside the glass prism suffers total internal reflection and reflects back to the medium

The light rays that come out of the prism are now deviated by $90^o$

Hence two right angled prisms can be used to make a periscope

The Total internal application finds its application in diamonds as well

For diamonds the refractive index ${\mu }_d=2.4$

The corresponding critical angle comes out to be approximately $24^o$

So, the diamonds are usually cut in such a way that whenever the ray of light enters it, it suffers total internal reflections at various faces

When the angle of incidence at any face is greater than 24º, the light comes out from it and diamond appears to be bright

Total internal reflection is also used in optical fibres

Let's consider an optical fibre having a core of refractive index ${\mu }_1$

The optical fibre is covered with a material called cladding whose refractive index(${\mu }_2$) is less than that of the core

If the ray is travelling from the core to the cladding, it is moving from the denser medium to the rarer medium

The ray of light suffers multiple reflections within the optical core

As a result of which, there is no loss of light

Therefore, the intensity of light from the entry junction and the exit junction of the optical fibre will be almost the same

For total internal reflection to occur light has to enter the core at an angle greater than the critical angle

When all the possibilities are taken into account we get a cone known as the acceptance cone

Revision

The phenomenon of Total internal reflection is used in optical instruments such as binoculars, periscopes and microscopes

Essentially, the faces of a diamond are cut in a manner that allows the light entering it to result in multiple total internal reflections at the same time. This causes the diamond to sparkle

Optical fibres use the principle of Total Internal Reflection

The End