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photo effect

When light shines onto a metal, it can knock electrons out of the metal’s atoms. This is the photoelectric effect, and its properties – how does the number and energy of the electrons depend on the frequency and intensity of the light? – can only be explained if one accepts that light is no mere electromagnetic wave, but somehow made up of some kind of light particles. With this postulate, Einstein, in 1905, paved the way for the later development of quantum mechanics.

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light

Light in the strict sense of the word is electromagnetic radiation the human eye can detect, with wave-lengths between 400 and 700 nano metres. In relativity theory and in astronomy, the word is often used in a more general sense, encompassing all kinds of electromagnetic radiation. For instance, astronomers might talk about "infrared light" or "gamma light"; in this context, light in the stricter sense is referred to as "visible light". Within classical physics, the properties of light are governed by Maxwell's equations; in quantum physics, it turns out that light is a stream of energy packets called light quanta or photons. In the context of relativistic physics, light is of great interest, and for a number of reasons. First of all, the speed of light plays a central role in both special and general relativity. Also, there are a number of interesting effects in general relativity which are associated with the propagation of light, namely deflection, the Shapiro effect and the gravitational redshift.

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photo effect

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