Young’s double slit experiment showed the dual behaviour of light (and matter) as a wave and as a particle.
Although currently it is known that light behaves as a wave and as a particle. It should be noted that many years ago it was thought light had only a corpuscular behaviour. In fact, the first to propose the corpuscular theory of light was Issac Newton, while Christian Huygens (who was contemporaneous with him) proposed the wave theory.
However, when Thomas Young performed the double slit experiment with photons (light) the result was: Light is able to behave also as waves.
