Isaac Newton was able to assert the particle theory of light by describing phenomena like refraction, reflection and dispersion. However, this particle theory of light could not explain everything that people saw light do. Christiaan Huygens thought that light was actually a wave and came up with the concept of wavefronts and his idea was able to model diffraction. Later on, Thomas Young's famous two-slit experiment (demonstrated interference and diffraction) and James Clerk Maxwell's idea showing it's the electric and magnetic fields that wave in light, supported wave theory of light.
The wave theory seemed an appropriate description of light until scientists began looking at the photoelectric effect in the early 1900s. To observe this effect, two electrodes connected to a battery were placed in a glass ball (with vacuum). As light waves incident on these electrodes, the electrons were supposed to absorb energy. As the electrons absorbed enough energy, they were expected to eject from the atoms and travel as current between the electrodes. The results of this experiment showed four problems with the wave theory of light:
- No electrons were ejected if the frequency of light was below a certain cutoff frequency, no matter how bright the light was. According to wave theory, electrons would just wait until they absorbed enough energy to leave the metal.
- The maximum kinetic energy of electrons was independent of the incident light intensity with frequency above cutoff frequency. As per wave theory, greater intensity would mean more energy thereby increasing kinetic energy of electrons.
- The maximum kinetic energy increased with the frequency of incident light. As per wave theory kinetic energy should depend on incident intensity and not incident frequency.
- The electrons were ejected from the electrodes instantaneously even when the incident intensity was very low. Wave theory suggested electrons should have waited before being ejected until they absorb enough energy from incident light intensity.
Wave theory could not explain the above results from the photoelectric effect. In 1905, Albert Einstein came up with the idea of light having a dual nature, both particle and wave. He proposed light being a wave but with finite extent unlike the wave theory. He called this localized wave as a quanta and also later suggested that it's energy depended on the frequency of light. In 1923, Louis de Broglie postulated that matter, similar to photons, has both wave and particle properties. Using Einstein's idea about momentum and energy for photons, he suggested similar properties for electrons, protons, neutrons, atoms, etc. With light, it's the electric and magnetic fields that are waving. With particles, what is waving is not clear. Wave function for a particle is a probability distribution function (likelihood of finding a particle in a particular location) in quantum mechanics.
As time progressed, things got more interesting as single electrons were sent through Young's double-slit setup. More number of electrons were collected at some locations (in places like a bright fringe in case of light) and almost no electrons were collected at other locations (in places like a dark fringe in case of light). This result was equivalent to the interference pattern produced by light. Electrons also exhibited diffraction when passed through crystals and analyzing the angle and spacing of the atoms that did the scattering led to the first experimental confirmation of the wavelength of the electrons. Also, these results exactly matched de Broglie's hypothesis. Concentrated electron counts in some areas and almost zero elsewhere could only be explained by the wave theory.
In a similar fashion, single photons were passed through Young's double-slit. It was observed that a single photon exists at both slits at the same time which showed there might be more to the wave theory. With that, perhaps photons can communicate with each other even though they are separated by vacuum. This property is referred to as entanglement. Entangled photons are two or more photons with a special link. This phenomenon is so mysterious that Albert Einstein called it a spooky action. Recently, many experiments have shown that entangled photons can exchange information instantly even if the distance between them is in miles. Although bizarre, photon entanglement opens possibilities for applications in the realm of super-secure communication and computing.
