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In the early 1800s, Thomas Young discovered that light was a wave. In his famous double-slit experiment, he passed light through a plate with two slits. He placed a screen behind the plate. If light were a particle the pattern on the screen should look like the plate's slits. When he observed the pattern on the screen, he saw a series of light and dark bands. The light and dark bands were an interference pattern. He immediately recognized that light was a wave!
By the end of the 1800s, many scientists were having difficulty with the wave nature of light. For example, Max Planck was studying blackbody radiation. Blackbody radiation is the light emitted from a heated metal. For example, the coil on an electric stove glows orange. In Planck's experiments, the theoretical predictions did not match the experimental results. Scientists called this difference the blackbody radiation problem. The blackbody radiation problem started a quest for a better theoretical explanation.
Einstien and the Photoelectric Effect
In 1905 Albert Einstein discovered a new model of light. He was trying to explain the photoelectric effect. He observed that shining blue light on potassium (K) produced an electric current. The electric current indicated that electrons were released from the potassium atoms. But, shining red light on potassium did not produce a current. If light were a wave, high-intensity red light should have produced a current. Yet, even at the highest intensities, the red light did not produce a current.
Einstein needed to explain the difference between the red light and the blue light. He proposed that light is a particle. If light were a particle it would have energy. The light particle's energy would be proportional to its frequency. Red light has a much lower frequency than blue light. So, red light has less energy. He imagined a light particle hitting an electron. With enough energy, the light would overcome the nucleus' attractive force. The light would free the electron, producing an electric current. The associated energy is the ionization energy.
To help you understand the photoelectric effect, imagine a game of billiards. The light particle is the cue ball. If the cue ball grazes another ball, then the other ball will not move. If the cue ball collides with the other ball, the other ball will shoot across the billiards table.
In 1914, Einstein's theory was experimentally confirmed by Robert Millikan. Now we had a problem. We had experimental confirmation that light was a wave. But, we also had experimental confirmation that light was a particle. How can we reconcile the wave and particle nature of light?
The Wave-Particle Duality
Louis De Broglie explained light's wave and particle nature. He also realized that wave-particle nature applied to all matter, not only light. In 1924 he developed the wave-particle duality. The wave-particle duality states that all matter possesses a particle and a wave nature. How did he realize it? He combined Einstien's relativity equation with Planck's quantum theory. Anything with mass had an associated wavelength. Why can't we see that? The wavelengths are unbelievably tiny! For example, a person's wavelength is meters. So, the length is minuscule.
In 1927, Davidson and Germer conducted experiments that confirmed the wave-particle duality. Davidson and Germer redid Young's double-slit experiment but used electrons instead of light. Until their experiment, we had assumed that protons, neutrons, and electrons were particles. But, like light, electrons exhibited an interference pattern! The wave-particle duality is the start of quantum mechanics.
