Wave-Particle Duality

We have seen that experiments such as blackbody radiation, the photoelectric effect, and Compton scattering can be explained using the photon picture of light, but not with the wave picture. However, it is important to realize that experiments such as diffraction and interference all need the wave picture, as a photon (particle) picture fails in these cases. Both pictures are needed in different circumstances; one says that light exhibits a wave-particle duality:

Light has a dual nature; in some cases it behaves as a wave, and in other cases it behaves as a photon.

This wave-particle duality is the basis of the quantum theory of light, and has some profound physical and philosophical implications which are still being debated today.