# Question #ebdb2

Jun 18, 2014

It depends on two parameters: an angle of incidence and the materials of the mediums on both sides of the boundary between them.

When a ray of light falls through some medium (for example, air) onto the boundary between this medium and another medium (for example, glass), it falls at some angle to a perpendicular to a surface of the boundary. This angle is called the incidence angle.

Part of the energy of this ray of light is reflected back to the medium it came from (the air in our example) and part goes through the boundary into another medium (the glass in our example).

Amount of energy reflected back depends on the mediums and an angle of incident. The dependency on the angle of incident is always of one kind: the smaller the angle (that is, the closer the direction of a ray to a perpendicular to a boundary between mediums) - the greater amount of energy that goes through the boundary (refraction) and the less amount of energy that is reflected back.

The actual portion of reflected energy for each angle of incidence depends on the physical characteristics of both mediums.

Thus, for light falling from air onto glass, if the incidence angle is 0 degrees (that is, the light is perpendicular to a surface of glass) the amount of energy reflected back to air is 4.7% and amount of energy going through the boundary into the glass (refraction) is 95.3%.

For an incidence angle of 60 degrees the corresponding numbers are 9.8% for reflection and 90.2% for refraction.

For an incidence angle of 80 degrees the corresponding numbers are 39% for reflection and 61% for refraction.

Different mediums have these numbers different. For instance, for light going from air perpendicularly to a surface of water 2% of energy is reflected and 98% goes through, but the dependency on the incidence angle is still the same - the smaller the angle of incidence - the less reflection and more refraction.