Electromagnetic radiation has certain wave-like properties that are difficult to explain with a particle model. For example, it diffracts, meaning it changes speed and direction in different substances, and it refracts, meaning it bends around objects. Electromagnetic waves also interfere with each other in regular wave-like patterns. However, it has certain properties that cannot be explained by the wave-model alone. Energy seems to be delivered in packets, or "quanta," more similar to particles than waves. These particles have zero mass, however.
There is a key experiment that demonstrates the theory of photons as packets of energy. A negatively charged plate of zinc (which can repel a piece of gold leaf) has an excess of negatively charged electrons. When white light is shone on it, no matter how intense, it does not discharge the zinc. However, if UV light is shone on zinc, it starts to discharge and can no longer repel the gold leaf. Only UV light has enough energy to displace the electrons.
The explanation is that the energy of light depends on its frequency rather than its intensity. A wave theory would predict that if you increase the intensity of white light, its energy would increase, and it could eventually discharge electrons. But since the energy is delivered in packets, only one of these packets comes into contract with an electron,
Max Planck produced an equation for the energy of a photon:
E = h x f
where E is the energy of the photon, h is Planck's constant and f is the frequency of the electromagnetic radiation. The equation demonstrates the paradox in quantum physics: to calculate the energy of a particle, you need to account for its wave-like properties (frequency). UV has a higher frequency than white light. Einstein built on Planck's work. He produced his Photoelectric Equation that showed that the kinetic (movement) energy of an electron as it leaves the surface of a substance due to electromagnetic radiation is equal to the energy of the photon, less the minimum energy needed for the electron to escape the surface of the material.