Ray Optics and Optical Instruments

Total Internal Reflection

You must have seen diamonds sparkling. Do you know why do the diamonds sparkle? Or suppose you put a coin below the glass and put some water in the glass. You cannot see the coin from the sideways but the same coin is visible to you if you look at it from the top. Why does this happen? The answer to this is an internal reflection. Let us study the total internal reflection in detail.

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Total Internal Reflection

To understand the concept of total internal reflection let us carry out a small activity. Take a coin and a glass. Now put the coin below the glass and fill the glass with water. Why did the coin disappear? We can see the coin from the top of the glass but if we try to see it sideways, we cannot see it. The coin is invisible to us. Why did this happen?

This is because when there is water in the glass, light from the coin travels through the glass to our eyes at a particular angle. Further on adding water to the glass the light from the coil hits the inside of the glass at an angle greater than the critical angle. All the light from the glass is internally reflected. This is what internal reflection is.

Total internal reflection is defined as the complete reflection of a light ray at the boundary of two media when the ray is in the medium with greater refractive index. Total internal reflection takes place in two following conditions:

  • When the light is in the more dense medium and approaching the less dense medium.
  • If the angle of incidence is greater than the critical angle.

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How is the sparkling of diamond-related to total internal reflection?

A diamond sparkles as when the rays of light enter the diamond they suffer total internal reflection at its various faces. The critical angle for the ray of light traveling from the diamond to air is very low about 24º. Therefore most of the incident rays suffer total internal reflections.

Also, 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 less than 24º, the light comes out from it and diamond appears to be bright. Total internal reflection is also used in optical fibres. Optical fibres are used in transmitting video and audio signals to a long distance.

Questions For You

Q. A ray of light, traveling in water, is incident on its surface open to air. The angle of incidence θ which is less than the critical angle. Then there will be:

  1. only a reflected ray and no refracted ray
  2. only a refracted ray and no reflected ray
  3. a reflected ray and refracted ray and the angle between them would less than π- 2θ
  4. a reflected ray and refracted ray and the angle between them would greater than π- 2θ

Answer: C

The ray will be partly reflected and partly refracted. ∠MOB = π – 2θ. But the angle between refracted and reflected ray is ∠POB. Clearly, ∠POB is less than ∠MOB that means less than π – 2θ

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