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Gateway to Spectroscopy > Physical Background > Optics > Reflection and refraction

Reflection and refraction

at the interface between to optical media.

Let us consider the interface between air and water. From experiment we know :

When a light beam is directed towards the interface between two media of different "refraction index" some of the light beam may be reflected. For example when we see the mirror image of the moon in the surface of a river, lake or the open sea. The light of the moon is directed towards the water, reflected at the boundary between the air and water and finally reaches our eye. That is reflection.

Refraction is what happens to the light beam when it actually enters the second medium. When trying to catch a fish in the aquarium one is sometimes seriously mistaken about the actual position of the fish. When you hold a stick into the aquarium and observe it from a somewhat tilt position, the stick apears bent. When you slowly move the stick towards the fish, and the fish is sufficiently cooperative, you may understand what is happening when you miss the position of the fish. Refraction changes the ligth on its way from the water (medium 1) to the air (medium 2). Our day to day experience told us that light moves in straight lines. When we see a fish in a certain direction, our brain assumes it actually is in that direction, but it is not. Let us continue the

The result is a cange in speed and direction, given by Snell's law (or in France by Descartes' law):

[Snell's law]where v1 and v2 are the speeds in the first and second media, respectively, n21 is the refractive index of the second medium compared with the first medium, and q1 and q2 are the incident and refracted angles, relative to the normal (i.e. perpendicular).

In going from air into glass, the photon speed is reduced. Hence the refractive index of glass is greater than 1.

First published on the web: 15 February 2000.

Authors: Richard Payling and Thomas Nelis