Final answer:
Albert Einstein received the Nobel Prize in Physics in 1921 for his explanation of the photoelectric effect, which introduced the concept of photons and helped lay the groundwork for quantum mechanics.
Step-by-step explanation:
The individual who won the Nobel Prize in Physics for explaining the photoelectric effect is Albert Einstein. Einstein's hypothesis was pivotal in understanding that radiant energy arrives at a metal surface in quantized packets known as photons. The energy of these photons must be greater than a certain threshold to eject an electron from the surface of a metal. This concept could not be explained by classical physics and was instrumental in the development of quantum mechanics. Einstein's contributions to theoretical physics went beyond the photoelectric effect, but it was this discovery that earned him the Nobel Prize in 1921.