Laws of Refraction of Light

We call refraction of light the phenomenon in which it is transmitted from one medium to another.

In this change of means the frequency of the lightwave is not changed, although its speed and wavelength are.

Changing the propagation speed causes a deviation from the original direction.

To better understand this phenomenon, imagine a ray of light passing from one medium to another on a flat surface, as shown in the figure below:


  • Radius 1 is the incident radius, with characteristic speed and wavelength;
  • Radius 2 is the refracted radius, with characteristic speed and wavelength;
  • The dashed line is the line normal to the surface;
  • The angle formed between radius 1 and the normal line is the angle of incidence;
  • The angle formed between radius 2 and the normal line is the refractive angle;
  • The boundary between the two media is a flat diopter.

Knowing the elements of a refraction we can understand the phenomenon through the two laws that govern it.

1st Law of Refraction

The first law of refraction says that the incident radius (radius 1), the refracted radius (radius 2) and the normal line to the point of incidence (dashed line) are contained in the same plane, which in the case of the above drawing is the plane of screen.

Refraction Law 2 - Snell's Law

The 2nd law of refraction is used to calculate the deviation of light rays when changing medium, and is expressed by:

However, we know that:

Besides that:

By grouping this information together, we come to a complete form of Snell's Law: