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Propagation of light

Rectilinear propagation of light

1) What is propagation of light ?

As light isn't made of matter ( we can't touch it ! ) it would be unsuitable to say light moves: in order to describe its  travel through space we can say it " propagates ".
We can also use the term " propagation " for sounds and waves because there's no motion of matter in these cases.

2) Light's trajectory

Firstly, light can propagate only in transparent mediums like air, glass or emptiness.
In this kind of medium, light has a rectilinear propagation: it means that light propagates in straight line.
Furthermore, this rectilinear propagation requires an homogenous medium that has same physical properties ( temperature, density, pressure etc ) and chemical properties ( composition ) at each of its points.

3) What is a ray of light ?

The path of light can be represented drawing a ray of light.
As for light has a rectilinear propagation, a ray of light can be drawn as a line with an arrow that shows direction of propagation.
Ray of light from a flashlight

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