Light is a type of energy called electromagnetic radiation. It travels through space in tiny bundles called photons. All photons travel space at the same speed but the electromagnetic field of some photons fluctuate faster than of others, therefore having differentials of energy and speed of fluctuation. These differences in energy levels on the photons are perceived by human eye as colors. This electromagnetic radiation can travel through vacuum and some forms of matter. Luckily for us light travels through glass and water.

Electromagnetic radiation is different from other forms of energy that can only travel through matter (namely sound or heat). Infrared radiation can be confused with heat because both forms of energy travel together and can be converted into one or the other. The force exerted by light in vacuum is similar to the gravitational force.

Most films are sensitive to a wider range of energy frequencies that the ones perceived by the human eye. That's why film can be ruined by UV-light or wrecked by X-rays at the airports (in the security systems).