Einstein+and+the+Photoelectric+Effect


 * Einstein and the Photoelectric Effect **

In 1905 while Einstein was only 26 and working as a clerk in the Swiss Patent office, he published 5 theoretical papers in physics journals. The most famous and indeed the most important was the paper on special relativity, in which he showed that another fundamental tenet of Newtonian physics, the notion that space and time were absolute, was an illusion. He started with 2 simple postulates: 1) The laws of nature remain the same no matter how fast you are moving, and so no one can claim to be truly at rest. All motion is relative. (Conflicts with everyday experience) 2) The speed of light through empty space is a fundamental constant of nature measured to have the same value no matter what speed the observer is travelling at. (If you were running alongside a beam of light at speed ‘c’, you would still see the beam of light travelling away from you at speed ‘c’, again conflicting with real-life experience of relative movement) With these two ideas we are led to the conclusion that both time and space are aspects of a grander four-dimensional spacetime. Einstein also proved that the speed of light is the maximum speed possible in the universe. Special relativity forces us to accept strange notions such as time slowing down when we travel very fast, or rather when we travel. However, time is a relative concept. If you were travelling near the speed of light and your friend was at rest, he would observe that you have grown old five years in the space of five minutes, while you would observe that he is five years younger than you are in the space of five minutes. If you took five minutes at rest to write a letter and you decided to travel at that speed, you would be observed by your friend to have taken 5 years. The photoelectric effect was another theory by Einstein. There are three puzzling features of the effect. Firstly you would think that if light has the ability to knock out particles of matter, then their energy would presumably depend on the brightness of the light. Surprisingly it was found that the ability of the light to kick out the electrons depended on its wavelength. This is a rather unexpected result if we think of light as a wave, because increasing its intensity and hence energy would imply increasing its amplitude. For example, water waves which are larger crash against the shore with a larger shore and would break away parts of the shore more easily. In the photoelectric effect, high intensities of light did not give rise to higher-energy ejected electrons, just more of them. The second feature is related to the first. According to the wave theory of light, the photoelectric effect should occur at any frequency of light as long as it was intense enough to provide the electrons with the necessary energy to escape. However, a cut-off frequency below which no electrons are emitted is observed, no matter how bright the light is. Finally, if exposed to a light wave, the wave theory suggests that electrons would need a finite time to absorb enough energy to break away from the surface, especially if the light is feeble. No time lag was detected and electrons were knocked out as soon as light was shone on the surface. Einstein’s proposal was that all light is made out of energy quanta, now known as photos. To make it easier to understand, increase the brightness of light would result in increase in number of photons, increasing the wavelength of light would increase the energy (multiples of the Planck constant) of each photon. This was the dual nature of light. Again, we would not experience such a dual nature in common experience.