Everything Eistein - 4 Photoelectric Effect

'On a heuristic viewpoint concerning the production and transformation of light', was the title of the paper published in March of 1905. The first of the five papers that Einstein would publish that year. This paper explained what is called 'photoelectric effect.'

It was experimentally determined that when light was incident on a metal surface it emitted electrons. A simple explanation would be that light is a form of electromagnetic energy and if a light wave hit an electron it could transfer enough energy to knock out the electron. But the problem was the details of the photoelectric experiment came out differently depending on how you looked at light - a wave or a particle.


Now let us consider light as a wave. The energy of a light wave would depend on its amplitude. Other factors like frequency(red light, ultra violet light) should not make any difference to the outcome of the experiment or in other words red light, blue light of the same intensity or amplitude should knock out the same number of electrons. The number of electrons knocked out would depend on the intensity(energy) of the wave and not the frequency(color). So if you have faint light of any color there will be no electrons.

Now let us consider light as a particle. According to Planck's theory of radiation, energy can come in discrete packages called 'quanta'. Einstein applied this to light energy in order to describe the photoelectric effect. So, energy of a photon depends on its frequency.

E=hf ( where h , Plancks constant,is a very small number 6.626E-34 Js.) and f is the frequency

So higher frequency photons come with higher energy and can knock out electrons at a faster rate. The higher the frequency the maximum kinetic energy imparted to the electron is also higher, so they come out fast. If you keep the frequency(energy) same, but increase the intensity(no of photons) more electrons would come out, but they would not come out any faster, since the photons still carry the same amount of energy except they are more in number now.

If the frequency is low, the photons will carry lesser energy, so no electrons will come out, no matter how many photons are there. If you use high frequency photons, you will still knock out some electrons even if the intensity is low. Increase the intensity, you only in increase the number of photons. If you increase the frequency, you increase the amount of energy carried by the photons.

When Planck first came up with his theory at the turn of the century, he was not convinced with the idea of 'quanta' of energy. Even though he had arrived at the equation mathematically, he was skeptical about its practical use. But Einstein came along and convinced not only Planck but the entire physics world that light indeed comes in discrete packets called photons.

In 1913-14 RA Millkan did a series of experiments to verify photoelectric effect. The experimental data made sense only when light was treated as a particle and not as a wave. It was also determined that each metal has its own threshold frequency below which no electrons wil be emitted. Einstein received the Nobel Prize for his work in theoretical physics and his explanation of photoelectric effect in the year 1921.