The quantum theory is concerned with the behavior of matter and energy on the atomic and sub-atomic level. It is mainly rooted in the idea that everything is available in discrete amounts, which Max Planck came to be aware of. He found out that the frequency(of a wave of energy) and the energy it carries are proportional. In mathematics, when two proportional subjects are expressed, they are considered as being equal to each other with a constant factor. Thus the equation : E = hf, where E is the energy, f the frequency and h the constant which was discovered by Planck thus carrying his name.
Without getting into much technical detail, the theory showed that waves of certain frequencies must carry certain amounts of energy. Visible light, along with the entire electromagnetic spectrum, are energy-carrying waves. The spectrum features waves of varying frequencies and wavelengths. So one can say that ultraviolet waves carry more energy than visible light waves, and infrared waves carry more than radio waves.
This discovery was put into good use. At the beginning of the 20th century, a famous experiment was conducted. An electrical circuit, with a power supply, was opened by cutting the loop of wire to have two detached ends. Each end of the wire was tied with a small metal plate. Between those 2 ends was a tube, inside the tube was vacuum. The system was exposed to a light of variable frequency. When they shone bright blue light (high frequency), current passed. This meant that some electrons passed through this vacuum from one end of the wire to the other. The electrons actually took sufficient energy to escape from their atoms, and get attracted to the positive end of the wire to go into the circuit and produce a current. Only thing is, when bright red light was shone, no current passed at all. This meant that the electrons needed a minimum specific amount of energy to escape from their shells, which was not provided by red light, even with a source that's much brighter no electrons will pass. The equation shows that the only factor here is the frequency, but scientists were looking for a more physical explanation.
Albert Einstein came to know of the results of that experiment, and had a very convincing explanation, which he received the noble prize for in 1905. Einstein said that photons-the virtual building units of light-each carry a specific amount of energy. An electron can be struck with only one photon at a time, and absorb its energy. The reason why a highly intense red light could not produce any current in the experiment is due to the fact that each single photon has the same amount of energy in that of a faint red light. However a blue light's photons will have much more energy than that of the red light photon's, thus giving current. If we use ultraviolet radiation, we will have more current, because as the frequency increases so does the energy carried. Two or more photons can never strike the same electron and combine their energies to set it free.
This opened doors for many advancements in quantum theory, quantum mechanics and quantum electrodynamics, even a bold approach to describe gravity in terms of quantum laws. The theory of gravity, as known from Newton, came to be corrected in Einstein's theory of relativity when it suggested a model of how gravity works, something Newton never came across. He was able to describe its effects with the greatest accuracy, but never really told us the reason it's there. We'll be discussing Einstein's theories next.
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