A double refraction is a dual refraction that happens at a single interface. Some mediums called anisotropic mediums can polarize a ray into two orthogonal rays as shown below.
In the above diagram the rectangle represents the face of an anisotropic medium. The optical axis of the crystal is the line ‘x’ shown in blue. If a ray enters the face in the direction of line ‘x’, then the ray passes as it is. However if a ray R enters in any other direction, say at A, as shown in the diagram, then it is split into a set of two rays AO and AE in orthogonal directions. The ray AO is called as ordinary ray and emerges out following Snell’s law. The ray AE is called extraordinary ray and emerges without following Snell’s law. Thus an observer on the right is able to see two images for the same object. This is called double refraction.
What Causes Refraction of Light? A wave travels at different velocities in different mediums. But the frequency of the motion is not altered. Since the phase velocity is frequency times the wave length, a change in phase velocity means a change in wave length. The change in velocity also results change in direction of the wave propagation which we defined as the wave refraction or refraction of a light ray.
When a light ray passes from one medium to another medium, the light ray may be totally absorbed by the boundary if it is an opaque surface. In such a case, no light is seen on the other medium. In some cases the light ray may be sent back, following a rule, to the same medium if the boundary has the ability to do so. This phenomenon of light ray ‘being sent back’ is called ‘Reflection’ of light ray and the ‘ability’ of the boundary is called as ‘reflective property’ of the medium. However, in most cases, the boundary is transparent and the light ray is able to enter the second medium. However, the path of the light ray will no longer be the same. Depending on many factors, it may bend towards the normal to the surface, away from the normal to the surface or, in exceptional cases, the ray be even be reflected back. In any case the phenomenon of any of such things happening is called the ‘Refraction’ of the light ray. The refraction of light ray also follows a certain rule.
The law relating to reflection and refraction is called Snell’s law which is one of the important concepts in optics. When a light hits a surface, it is totally absorbed if the surface is opaque. But if the surface is reflective, the light ray is ‘reflected’ or if the surface is transparent and if the medium of the other side is different the light tray undergoes ‘refraction’.
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