In the 17th century there was much debate about the nature of light. Newton advocated a particle nature of light, while others argued that light was a wave. Early in the 19th century, a scientist named Thomas Young carried out an experiment which is now known as the double-slit experiment. At that time Young’s conclusions nullified Newton’s theory on the nature of light.

It is worth describing Young’s double-slit experiment here, as it will provide a startling insight into the nature of matter in the next section. His experiment was simple, and is easily reproducible. Essentially, it consisted of three flat pieces of cardboard, a powerful flashlight, and a wall to shine the flashlight towards. Each piece of cardboard had slits in them. The first piece had a slit towards its left side, the second piece had a slit towards its right side, and the final piece had two slits, one on the left side and one on the right side. When the piece of cardboard with the slit on the left side was placed between the beam of light and the wall, the wall had a single “streak” of light towards the left as expected. Similarly, when the cardboard piece with the slit on the right was placed between the wall and the beam of light, a streak of light towards the right was observed. If light truly has a particulate nature then when the third piece of cardboard is placed between the beam of light and the wall then we should see two streaks of light. Instead, what is observed is an interference pattern, a wave phenomenon.

Any amount of cash you have can be represented by an integer number of pennies. This is the fundamental American currency denomination. In 1900, Max Planck postulated that there is a fundamental energy denomination for light. Light can carry energy only in integer multiples of this fundamental energy denomination. Planck also postulated that the energy carried by light is proportional to its frequency.

The phenomenon that gave us insight into the particulate nature of light is known as the photoelectric effect. By the beginning of the 20th century the phenomenon had been well established, however there was no universal concurrence in the physics community as to what the consequences of the observations were. Once again, Einstein stepped in to enlighten the physics community. He would eventually be awarded the Nobel Prize for his explanation of the photoelectric effect.

In first year chemistry, you learn that metals have loose valence electrons. This is why they are such good conductors for electricity. This is also the basis of the photoelectric effect. When light strikes a metal, it dislodges some of the valence electrons. This in itself is not too puzzling. What baffled physicists was the fact that no matter what the intensity of the light was, the dislodged electrons moved with the same speed. Intensity of light only affected the number of electrons that were dislodged.
However, if the frequency (color) of the light was changed, the speed that the electrons were ejected at changed. What was more bewildering was the fact that below a certain frequency, no electrons were ejected at all despite the blinding intensity of the light.

Einstein postulated that light was quantized in little packets called photons. This simple assumption explained all the observations of the photoelectric effect. Instead of the light-wave being dispersed evenly throughout the entire metal, implying that the intensity (total energy) of light should directly affect the energy of an ejected electron, a single photon only carried a fixed amount of energy, and this is why an electron’s speed was invariable with intensity since only a single photon will strike an electron. It also explained why electrons weren’t ejected below a certain frequency. The energy carried by the photon was simply not enough to dislodge the electron at these low frequencies. With increasing intensity, more photons were fired at the metal; therefore more photons struck and dislodged more electrons. Hence, the particulate nature of light was revealed.