Light is a form of energy that travels in the form of electromagnetic waves. Waves can be broadly classified into two. Electromagnetic waves and mechanical waves. Waves that travel through the vacuum of outer space are termed electromagnetic waves. They do not require a medium for transferring energy from one place to another. But, a mechanical wave can transfer energy only through a material medium. The light waves are made up of electrical and magnetic vectors. These vectors are perpendicular to each other and also to the direction of transmission. This electro-magnetic theory was put forward by physicist Maxwell. It was a complete one; still it was not sufficient to explain certain features of physics.
Imagine that an iron rod is put in a furnace. When it becomes red hot, it emits red light. But the frequency of red light is determined not by the iron rod (substance), but by the temperature of the substance. Studies were conducted on the basis of electro –magnetic theory to explain the relationship between temperature and frequency. The studies could not provide a proper explanation.
In 1900, Max Karl Planck, a German scientist suggested that light emitted in such cases is not a steady stream. Instead, light is emitted in bundles or packets. This packet of light is called quantum. The theory suggested by Plank is known as Quantum Theory.
Quantum theory was used by Albert Einstein in 1905 to put forward his theory of photoelectric effect. The light quantum was named photon by him. Both features, waves and quanta of light, are used by physicists to explain various facts.
A recent version of quantum theory put forward by John G Cramer in 1986, suggests quantum interactions in terms of retarded waves and advanced waves. This has provided better insight into the studies in the field.