total light reflection

Total light reflection is an optical phenomenon that occurs when light strikes a surface at the limiting angle L and is completely reflected.

The impression we have of seeing wet asphalt on hot days is an example of a mirage, one of the consequences of the total reflection of light.
When light falls on a surface separating two media, one part of it undergoes refraction and the other, reflection , as shown in the following figure:

 the reflection and refraction when light passes from a less refractive to a more refractive medium, for example, from air to water.

Now see what happens when light passes from a more refractive medium to a less refractive one. We can consider as an example the light passing from water to air. Notice what happens in the picture, where the light source is placed under an aquarium filled with water:

From point P, light is fully reflected

Note that all light rays passing through the aquarium, at points to the left of point P, have both a reflected and a refracted part. But when the angle between the incident light rays and the normal line to the aquarium surface is equal to L, the limiting angle , refraction no longer occurs, but total reflection of light .

Here is a diagram representing the total reflection of light:

The threshold angle is defined as the “ smallest angle of incidence of light on a surface separating two media from which it is fully reflected ”. It can be calculated from the Snell-Descartes law :

No. 1 . Sin i = n 2 . Sen r


1 – refractive index of medium 1;
2 – refractive index of medium 2;
i – angle of incidence;
r – angle of reflection.

In the case of total reflection of light, we know that n 1 > n 2 , since this phenomenon occurs only when light passes from a more refractive medium to a less refractive one. Furthermore, the angle of incidence i is the limiting angle L, and the angle of reflection is 90º:

i = L and r = 90

Substituting these data into the Snell-Descartes Law, we have:

No. 1 . Sin L = n 2 . Sen 90º

Sin 90º = 1, so:

No. 1 . Sin L = n 2 . 1
Sen L = 2
 n 1

A consequence of the total reflection of light is the impression we have that the asphalt surface is wet on hot days, a fact that characterizes a type of mirage. Light from the Sun passes through several layers of air with different temperatures. The air that is close to the asphalt is warmer and, above it, there is another layer of air with a slightly lower temperature. This temperature difference causes the air to have different densities and, consequently, the two layers will have different refractive indices.

Light rays fall on the warmest layer of air, passing first through the coldest layer with the highest refractive index. Depending on the angle of view of the observer, the light will reflect on the surface separating these two layers the image of the sky, giving the impression that the asphalt is wet.

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