If you are lying on a raft, and you notice that the number of waves that go past the raft increases, you also find that the distance between each crest .

Aform of energy that is reflected or emitted from objects in the form of electrical and magnetic waves that can travel through space. there are many forms of  electromagnetic  energy including gamma rays, x rays, ultraviolet radiation, visible light, infrared radiation, microwaves and radio waves.

It is the areas with lines. this may sound a little confusing, but i hope it !