Reference: Postulate Mechanics
The Radiation
Radiation is the short expression for electromagnetic radiation. The foremost example of radiation as substance is the light from the sun. We can sense it on our skin. It helps us see things. It has a visible spectrum of colors. It has a much larger spectrum from radio waves to the gamma rays. Radiation is seen to have an extremely high speed but no mass.
We differentiate radiation from energy, where energy is a capacity for action or, more exactly, for interaction. Energy is the measure of that which passes from one quantum to another in the course of their transformations. Thus, all substance, whether it is thought, radiation or matter, displays energy as kinetic or potential.
For a long time radiation has been visualized as a wavelike disturbance in a material medium (ether), similar to the ripples in water; but radiation is now found to be substantial, being made of quanta that occupies space.
The concepts of wavelength and frequency are still used; but they mean something very different in the context of radiation. For example, radiation is viewed as a field; and higher frequency may denote higher concentration of this field, while larger wavelength may denote much larger spread of the field.
.
