# Why total internal reflection - Slow to fast

You don't need the following in an exam it is just interesting to know.

Total internal reflection only occurs when travelling from a slow medium to a fast medium, and when the ray of light going towards the boundary is greater than the critical angle.

They call this "Total internal reflection". So does that mean that at other angles it is not all reflected?

The answer is yes. See below:

In nearly all questions in exams at basic level the reflected is not taken into account until total internal reflection occurs. In reality, depending on the angle different amounts of refraction occur as follows:

At no angle the light beam that passes into the air is very bright but 3% is reflected backwards into the glass.

At 15° angle of incidence, there is a reflected ray of 3% strength of the original ray leaving 97% to pass into the air.

 Light ray in =15^circ To the normal Reflected Ray =-15^circ To the normal = Dim reflection (3%) Refracted Ray =23^circ To the normal = Bright refraction

At 30° incidence there is a reflected ray of 4% strength.

 Light ray in =30^circ To the normal Reflected Ray =-30^circ To the normal = Still low reflection (4%) Refracted Ray =48^circ To the normal = Mid brightness refraction

At 40° incidence there is a reflected ray of 25% strength leaving 75% of the strength of the ray to pass into the air.

 Light ray in =40^circ To the normal Reflected Ray =-40^circ To the normal = Bright ray reflection (25%) Refracted Ray =75^circ To the normal = Dim refraction

When total internal reflection occurs 100% gets reflected.

 Light ray in =45^circ To the normal Reflected Ray =45^circ To the normal = Total internal reflection Refracted Ray = Zero

(Ray of light going towards the boundary is greater than the critical angle)

If you were to draw a graph of the amount of light reflected at the boundary when light travels from glass to air you get the following:

If we plot a similar graph for water to air and diamond to air you get the following graphs.