Different chemicals are used to produce different coloured fireworks. Therefore, in order to produce a firework of a certain colour, the correct corresponding chemical or as the case may be, mixture of chemicals can be used. These release excess energy in the form of light (hv) at the broad end of the spectrum. The higher the temperature, the shorter the wavelength at which light is emitted, and the nearer it tends toward the blue end of the coloured spectrum. This is why blue coloured fireworks are so hard to synthesise, as they only occur at very high temperatures.