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Reflection and Absorption

What does reflection mean?

Reflection means that something is thrown back. If, for example, a ball is thrown against a wall and it comes back, one could say that it has been reflected. The same happens with sunlight meeting the surface of the earth: It is being reflected and can thus be collected by a remote sensing sensor.
If the light makes contact with a smooth surface such as a mirror the equation is: angle of incidence = angle of reflection. The principle of the specular (mirror-like) reflection is illustrated in the animation below.

 

 


Click on the laser in order to point it in a new direction and see how both the angle of incidence and the angle of reflection change.

 

 

There are generally three types of light reflections. However, please note that the reflection depends upon the surface roughness of an object.

1. Specular (mirror-like) reflection: The light ray meets a smooth surface and the angle of incidence is identical with the angle of reflection.
2. Diffuse reflection: The light ray meets a rough surface and is equally reflected in all directions.
3. Mixed reflection: The light ray meets a very rough surface and is unequally reflected in all directions. This type of reflection is the most common one in nature!

 

Arten von Reflexion

Three types of light reflection: The mixed reflection (right) is the most common one in nature.

 

 

If light makes contact with a body, it doesn't mean it is being reflected in any case. It can pass through the body (transmission) or can be refracted (refraction). In case of refraction, the light passes through the body but changes its propagation direction. The most important processes are reflection and absorption.

 

What is meant by absorption?

Surfaces do not only reflect light; they also absorb it. The light energy is absorbed by the molecules of a body and is then transformed into kinetic energy. The rise in movement of the molecules produces heat which is radiated to the surroundings. The animation below shows the process of absorption in a simplified manner.  

 

 

The darker a surface, the more sunlight is being absorbed and, thus, more kinetic energy is received by the molecules. The area warms up!


Albedo

The principles of absorption can be transferred to daily life: A black t-shirt absorbs more sunlight than a white one. This is the reason why we are sweating more wearing a black shirt in summer. The albedo of a body is crucial for the percentage of absorbed sunlight. It measures the degree of reflection of materials in different spectral ranges. An albedo of 100% indicates than no absorption takes place; accordingly, an albedo of 0% stands for no reflection. The table below lists values of albedo for different materials in the visible range of light.

Material

Albedo

snow

80-90%

clouds

60-90%

desert

30%

meadow

20%

forest

5-18%

concrete

15%

water bodies

(sun is low)

22%

water bodies
(sun is high)
5%

With the help of reflection and absorption behaviour in the range of the electromagnetic spectrum, the spectral fingerprint of objects can be characterised.

 

 


Conclusion:
Electromagnetic waves like the sunlight can be reflected and absorbed. Reflection means that they are thrown back from a surface; absorption means that they are incorporated by a surface and transformed into heat energy. Different surfaces reflect and absorb differently. The ratio between absorption and reflection is called albedo.