Total Internal Reflection

Total Internal Reflection

A demonstration for total internal reflection

Apparatus: Small transparent bottle, Few drops of Dettol (or some salt); Pointer laser

·        Take some water in a bottle; add a few drops of Dettol or some salt.

·        Point the laser pointer at different angles and note its path

·        At some angle, you will see that the light gets reflected within the water itself. is is called total internal reflection.

1. When does total internal reflection takes place?

When light travels from denser medium into a rarer medium, it gets refracted away from the normal. We know this. While the angle of incidence in the denser medium increases the angle of refraction also increases and it reaches a maximum value of r = 90º for a particular angle of incidence value. is angle of incidence is called critical angle (Figure 26). Now the refracted ray grazes the surface of separation between the two media.

The angle of incidence at which the angle of refraction is 90º is called the critical angle.

When the angle of incidence exceeds the value of critical angle, the refracted ray is not possible, since r > 90º the ray is totally reflected back to the same medium. This is called as total internal reflection.

2. Conditions to achieve total internal reflection

·        Light must travel from denser medium to rarer medium. Example from water to air.

·        The angle of incidence inside the denser medium must be greater than that of the critical angle.

Recall

·        Write the relation between the angle of incidence and the angle of refraction.

·        What is the unit of refractive index?

·        Which has higher refractive index: water or glass?

·        When does refraction take place?

·        When does total internal reflection take place?

3. Total internal reflection in nature

Mirage: On hot summer days, when you are travelling on a straight road have you seen the patch of water on the road which keeps moving ahead as you approach it? is is an illusion sometimes in the desert or over hot roads. Especially in summer, the air near the ground becomes hotter than the air at higher levels. The refractive index of air increases with its density. Hotter air is less dense, and has smaller refractive index than the cooler air. If the air currents are small, that is, the air is still, the optical density of different layers of air increases with height. As a result, light from an object such as a car (See Photo), passes through a medium whose refractive index decreases towards the ground. us, a ray of light from such an object successively bends away from the normal and undergoes total internal reflection, if the angle of incidence for the air near the ground exceeds the critical angle.

Diamond: Diamonds are known for their spectacular brilliance. Do you know the reason for their brilliance? It is mainly due to the total internal reflection of light inside them. The critical angle for diamond – air interface (θ_c =24.4º) is very small; therefore once light enters a diamond, it is very likely to undergo total internal reflection inside it. Diamonds faces in nature rarely exhibit the brilliance for which they are known. It is the technical skill of a diamond cutter which makes diamonds to sparkle so brilliantly. By cutting the diamond suitably, multiple total internal reflections can be made to occur.

Why do stars twinkle?

Stars are very far away from us (so appear as point-like objects); light from the star passes through our atmosphere before it reaches our eyes. This light bends (refracts) due to the varying densities and temperature of atmosphere. Moreover, the atmosphere is not stable; it is very turbulent. Therefore, the light which reaches us appears to come from different points. This gives the impression that stars are twinkling. If you go above the atmosphere and see(!), stars do not twinkle. Can you find why do planets not twinkle?

Optical fibres

Optical fibres are bundles of high-quality composite glass/quartz fibres. Each fibre consists of a core and cladding. The refractive index of the material of the core is higher than that of the cladding. Optical fibres work on the phenomenon of total internal reflection. When a signal in the form of light is directed at one end of the fibre at a suitable angle, it undergoes repeated total internal reflection along the length of the fibre and finally comes out at the other end.

Optical fibres are extensively used for transmitting audio and video signals through long distances. Moreover, due to their flexible nature, optical fibers enable physicians to look and work inside the body through tiny incisions without having to perform surgery.

Recall

1.     What are the examples of total internal reflection in nature?

2.     What are the uses of total internal reflection?