In 1900 Max Planck proposed that radiation was emitted not in a continuous stream of energy but in bundles of energy called quanta. He related the energy of a quantum to its frequency by the formula:

where h is a constant known as Planck's constant. This constant is 6.626 x 10^-34 Js and this formula applies to all electromagnetic radiation.

Although all energy is quantised we are not aware of this in everyday life because of the very small value of Planck’s constant.

The energy of a quantum of yellow light, wavelength 600 nm, is about 3.3 x 10^-19 J
Now this is a very small energy and so we often use a smaller unit called the electron volt (eV). An electron volt is defined as the energy gained by an electron when it is accelerated through a potential difference of 1 Volt.