Reflection

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Basic

Law of reflection (click to enlarge)


Reflection is the return by a surface of some of the light which falls on that surface (other parts get absorped or refracted).

There are two main laws that play a role in this (called "Snell's Laws").

  1. The angle of reflection of a ray (r) of light is equal to the ray of incidence (i).
  2. The incident ray (i), the reflected ray (r) and the normal (NO) all lie in the same plane.


The angle of incidence (ION) and the angle of reflection (NOR) are both measured by reference to an imaginary line called the normal (NO), which is perpendicular to the reflecting surface (HP), at the point of incidence (O).
The point of incidence is the point at which the incident ray strikes the reflecting surface.

Understanding these basic principles are important in order to understand the many gemological terms that are used to describe optical effects caused by reflection.

Amongst those effects are:

Advanced

Brewster Angle

Brewster Angle

In 1812 Sir David Brewster (1781-1868) deposited a new phenomanom which showed that, at a certain angle, light falling on an optically denser object (like water) the reflected component of the light would be completely polarised in the plane of of the surface it reflects of. In this case horizontal.
He also observed that the angle of the refracted ray was at 90° to the reflected ray (at this specific angle).

This angle was named The Brewster Angle and it varies for every material, relating to the refractive index of the material.

The latter makes it possible to use reflection as an aid of measuring the refractive index of polished stones.
As the reflected ray is completely polarised in the horizontal plane at that specific angle, one could insert a vertically orientated polarizing filter to block all the reflected light (similar to how sunglasses work).

By measuring the angle of the incident light, at which there passes no light through the polarizing filter, you could calculate the refractive index. This is the principle of "The Brewster Angle Meter" developed by Peter Read.