Question

What feature best distinguishes one form of electromagnetic energy from another?

Answer 1

The family of electromagnetic waves ranges from the Long Radio waves to the gama rays.

All of that contain electromagnetic energy and move at the speed of light.

But what distinguish one from another is their wavelengh.

Or, if you like, its counterpart: their frequency. (as can be seen in the equation c = λ . f   where c is the speed of light, λ is the wavelengh and f is the frequency).

The shorter the wavelengh (the greater the frequency) more powerful (more energy it contains) is the electromagnetic wave.

From all the different kinds of electromagnetic waves of the spectrum, the less powerful (less energetic) are the Long Radio waves (λ in metres) and the more powerful ones are the gama rays (λ in $10^{-12}$ m).

:-)