Light is energy that we can see. It is given off (emitted) by matter when it is hot or in some way excited. The colours human eyes can see show how much energy is in the light (ignoring how bright it is), ranging from low energy red to high energy blue. This scale stretches further in both the high energy and low energy directions than the human eye can detect. Generally the term light refers to the range of energy’s we can see, but across all energy ranges electromagnetic radiation or just radiation is a better term. That said, I’m going to keep calling it light because it’s easier.
This radiation/light can be thought of as both a wave and a particle. Sometimes it behaves like one, at other times it behaves like the other. This may seem very odd, and to be honest it is, at least compared to other things in day to day life. I’m not going to go into how it can be both; that can be explained better in the quantum mechanics section.
Anyway, if we think about light as a wave, how does some light have more energy than other light? Imagine holding one end of a string, dangling it. If you move your hand back and forth, that back and forth motion will travel along the string as a wave. There are two ways of putting more energy into this. One is to move your hand farther in the same amount of time, increasing the amplitude (tallness) of the wave (which is like the brightness of the light). The other is to move your hand back and forth the same distance faster, causing waves to travel down the string more often. As you probably know, this will tire you more than moving your hand slowly, and this is because you’re putting more energy into the wave. This decreases the length of the waves travelling along the string, and increases the number of waves that pass a certain point on the length of the string each second. These two things are known as wavelength and frequency. Here is a diagram of the whole range of energy’s electromagnetic radiation comes in, called the spectrum.