What is meant by total reflection?

Refraction of light - you need this prior knowledge

Light spreads in a straight line when "nothing is put in its way". The best example of this are not only laser beams, but also the rays of the sun through a forest of leaves.

• However, if light gets into another (transparent) medium, be it water, glass or diamond, the direction of propagation of a light beam changes.
• You are probably all familiar with the associated school experiment in which a light beam was "bent down" when it passed from air to glass or water. And you've probably seen this phenomenon on your legs in shallow water.
• Incidentally, the physical reason for this refraction lies in the speed at which the light propagates in the various substances. So it is slower in water (and most glasses) than in air or the vacuum.
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• By the way, when the light beam is bent, the angle smaller at the point of incidence - this is the best way to understand the (mathematically formulated) law of refraction.


The critical angle - a simple explanation

The critical angle is a quantity that is related to the phenomenon of total reflection. …

Total reflection - that's what it means

• Now turn the light path around in your mind's eye: The light beam should get from the water into the air, for example from lighting in an aquarium or swimming pool.
• As a reverse of the law of refraction described, the "kink angle" increases.
• Now gradually increase the angle at which the light comes out of the water.
• At some point you will reach a striking borderline case: the light beam does not pass into the adjacent air, but rather brushes along the surface of the water.
• If you choose an even more slanted light beam, it can no longer leave the water and is reflected at the interface. Exactly this case is understood by the term "total reflection".
• Total reflection is only possible if light wants to pass from an (optically) denser material such as water or glass into an (optically) thinner material such as air or other gases. From a certain critical angle, this is no longer possible, and total reflection occurs at the interface.
• One of the most important applications for total reflection can be found in light guides or Fiber optic cables. In such cables, information can be transmitted over long distances using laser light, since the light cannot (or can hardly) leave the conductor due to total reflection. Endoscopes are also used in medicine.