is what we see. It can be thought of either as a particle, (the photon), or as a wave. The photon we can easily think of as a small dot travelling through space at the speed of . Each photon has a particular colour or energy. But how do we think of as a wave? For this we need to know that is also an "Electro-magnetic field" -- a combination of electric-field and magnetic-field -- the same thing that causes static electricity and magnets to attract (or repel) things. is just another way that this "field" is experienced. A wave is an oscill ....