Light as a Particle
Now that we have seen ample evidence that light behaves as a wave - in diffraction, interference and polarization effects - let's perhaps confuse things a bit by discussing several experiments where light seems to behave like a particle. Then we will discuss the modern picture of light as a photon.
At the beginning of the last century, there were several major experiments that could not be explained in terms of the wave theory of light. The first is known as the photoelectric effect and it was finally correctly explained by Albert Einstein in 1905; it was for this theory that he won the Nobel prize in 1921 and not for relativity, his best known work. In 1905 at the age of 26, Einstein published major papers in three areas: a quantum theory of light that explained the photoelectric effect, a statistical explanation of molecular motions, known as Brownian motion, and his most famous work, special relativity. Looking back on his work on light, in 1951 Einstein wrote in a letter "All these fifty years of conscious brooding have brought me no nearer to the answer to the question 'What are light quanta?' Nowadays every Tom, Dick and Harry thinks he knows it, but he is mistaken." Here we will try to give some ideas on this topic, but don't feel badly if a deep understanding still eludes you.
When light strikes a metal surface, under some conditions it can knock electrons off the surface which can then be collected to detect an electric current signal. This is known as the photoelectric effect. Classical physics dictates that light of any frequency with sufficient intensity (related to wave amplitude) should be able to give electrons sufficient energy to liberate them from the surface. However, when the experiment is done, it is found that only light with a sufficiently high frequency will liberate electrons. Further, light with a lower frequency, no matter how intense, will not release any electrons. Since in classical physics the frequency is not at all related to the energy of the wave but the amplitude is, this experimental finding is impossible to reconcile with classical physics.
Einstein proposed that light energy is packaged in basic units - which he termed photons - where the energy in the photon is given by Eg = hn, where h is Planck's constant as we saw in the quantization section. Because h is so small, visible light photons, with frequencies of about 1015 Hz have very small energies, of about 10-18 Joules, or a few electron volts (see the EM spectrum Table). When a single photon strikes the metal surface, it must have sufficient energy to liberate an electron and so there is a minimum frequency required. This simple idea agrees with the experimental results. No matter how many photons strike the surface, if their frequency is below the needed threshold then each individual photon has insufficient energy to liberate an electron. Above this threshold frequency, more intense light produces more liberated electrons. What's going on here? Simply that more intense light means more photons - and if each has sufficient energy, then more electrons will be liberated. Below the threshold frequency, no matter how intense the beam, no electrons are liberated because each photon is deficient in the needed energy. This explanation hinges on a "particle - particle" type interaction with each photon interacting with an electron.
The photoelectric effect is the basis for many different light detectors. For example, a device known as a photomultiplier tube can detect very low light levels and via the photoelectric effect, can produce an electric current that is proportional to the amount of incident light. Of course, the metal electrodes in these tubes must be designed for the particular range of incident light in order to be sure that the threshold is exceeded and that a signal is produced.
A second unexplained experiment in the early 1900's is Compton scattering, the scattering of high energy electromagnetic radiation (x-rays or gamma rays) from electrons. It turned out that this type of scattering could be completely explained by treating the electromagnetic radiation as coming in quanta - photons with energy given by the above equation - colliding with electrons just as billiard balls collide on a table.
Photons can have another quantum interaction in which a single high energy photon can spontaneously change into a pair of material particles, for example, an electron and another particle known as a positron. The electron and positron are complementary particles having the same mass and other properties, only differing in the sign of their electric charge. Since the photon has no electric charge, the final pair of particles must also have no net electric charge, and an electron and positron charges together add to give zero. This process is known as pair production. The opposite process also occurs - namely an electron and positron colliding and annihilating to produce only photons - known as pair annihilation.
How can we picture a photon? One way is to think of a localized, but dynamic, wave known as a wave packet shown below. The size of the wave packet is variable and depends on the interactions with the outside world. In some interactions, the photon wave packet will spread out and can appear to be essentially a sine curve, while in other cases it can collapse to a very small size and appear to be particle-like. This mixture of wave-like and particle-like abilities is known as wave-particle duality. It turns out that all elementary particles of nature, such as the electron and proton, have this property and not just the photon.
photon as a wave packet
Questions on Light as a Particle
