To begin the study of Optics, we must know what definition light receives in its study. Because the study of optics is directly linked to the cognitive nature of light (Physical Optics) and its propagation and phenomena that occur during its trajectory.
In optics, light is considered radiant energy. But what is radiant energy? It is a type of energy that propagates through electromagnetic waves, such as: radio waves, TV and even LIGHT.
Radiant energy has an enormous speed of propagation. The medium it travels with the greatest speed is the vacuum, which reaches a speed of
3.00. 105 km/s.
Light has a different frequency from other radiant energies that ranges from red to violet.
With all the conclusions above we can make a definition for LIGHT within optics.
Do you know why we see objects? Objects send our eyes a light that allows us to see them. So we can understand why we have difficulty seeing black bodies, because they do not emit any light.
We conclude that every body capable of emitting light is a SOURCE OF LIGHT.
Primary light source
The sun, the flame of a candle are considered primary light sources because they emit their own light and do not need other bodies to propagate light.
secondary light source
Moon, tree and clouds are examples of secondary light sources, as they do not emit light of their own. For the emission of light by them to occur, diffusion must occur, other bodies reflect light to all sides of the bodies of secondary light source so they emit the light received from other bodies.