The dual nature of light refers to the fact that light can act as both a wave and a particle. Albert Einstein's photoelectric effect experiment proves that light can behave as a particle while Thomas Young's double-slit experiment shows that it also behaves as a wave.
In 1905, Einstein proved that light could behave as a particle by showing that a beam of light could eject electrons from metal. This suggested that light consisted of photons that could eject electrons of similar frequency. Einstein's discovery altered the prevalent theory at the time, which held that light was only a wave.
Proof that light could act as a wave was demonstrated as early as 1801 by Young. In his experiment, Young shone light between two parallel slits, causing the light waves to interfere with each other and form a patter of dark and light bands. If light was primarily a particle, it would have formed two parallel lines.
Quantum mechanics explains the duality of light by describing it as a wave-packet. A wave-packet refers to waves that may interact either as spatially localized, acting as particle, or interacting like waves. This means light photons can either act as a particle or wave, depending on the circumstances.