Answer:
Young's double slit experiment is the classic proof of the wave nature of light. In the experiment light from two sources interfere. Particles are thought to be unable to interfere, but wave interference is well established by observation.
The photoelectric effect is the experiment that challenged the prevailing view that light was a wave (established by Young as above). There are several bits of detail to the experiment that confirm that light exhibits particle behaviour. The main thing to appreciate is that the threshold frequency and instantaneous emission of electrons support a discrete nature to light.
