Refraction Huygens’ principle

Here, cq is the speed of light in vacuum. According to Huygens principle, the rays are refracted as described by Snellius law of refraction ... 

They allow us to illustrate the refraction of light using Huygens principle (Fig. 4.26). 

Fig. 4.26. Refraction according to Huygens principle the wave fronts and the light rays in a uniaxial crystal...

In some physical circumstances, we can derive familiar elements of the less fundamental theory from the more fundamental by straightforward limiting inferences. Thus if ordinary cases of refraction and reflection are what is being considered, we may simply derive a geometrical law of reflection or a Snell law of refraction from wave optics and Huygens principle, with, perhaps, applications of the method of going to the limit where wavelengths are small relative to macroscopic relevant features of the experimental setup. 

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