What is light? A wave? Or a stream of particles? In 1801, Thomas Young performed the famous double slit experiment. The resulting fringe pattern provided irrefutable evidence that light undergo interference. Furthermore, in 1865, James Maxwell demonstrated (through the famous Maxwell’s Equations) that an electromagnetic wave would travel at , which is the known speed of light! Surely, light has got to be an electromagnetic wave, right?

Near the end of the 19^th century, it was discovered that electrons are emitted from a piece of metal when ultraviolet light is shone on it. Electrons emitted in this manner were given the name photoelectrons. And this phenomenon came to be known as the photoelectric effect.

Can we explain this phenomenon? Of course. A piece of metal is full of electrons imprisoned by the positively charged metal lattice. From the energy perspective, the electrical attraction sets up an energy barrier that normally confines the electrons inside the metal. These electrons must have absorbed energy from the ultraviolet light to overcome the energy barrier and escape from the metal as photoelectrons. Easy-peasy. Mystery solved, right?

Not so fast. As they say, the devil is in the details.

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