Is light a particle or a wave?

Light manifests properties of both a particle and a wave, depending on the nature of the measuring apparatus used. Albert Einstein's photoelectric effect, explained in 1905, highlights the particle nature of light in terms of frequency and intensity of electron emissions. The wave nature is evident in large-scale applications.

Some of these large-scale applications of light involve interference, polarization and diffraction, which are fully described in classical physics by treating light as a wave. From these applications, reflection and refraction can be understood. Quantum mechanics takes into account both natures of light, and represents the most complete theory of light to date.