Photoelectric Effect

We are going to explain shortly what the photoelectric effect is.  First we have to remember that all metals emit electrons.  When a metal is exposed to a coherent Light of any wavelength, the metal will emit electrons. Let’s suppose that we expose a metal to a coherent violet Light.  It will be verified through measurements that the metal will emit electrons that have a certain velocity and therefore a certain kinetic energy.

By the energy conservation principle, we know that the energy carried by the electron must come from another source.  But where does an electron get this energy from?  There energy source is Light.  When Light falls over the metal, the metal emits electrons.  This is the called photoelectric effect.

Now, let us see what happens if we increase the Light intensity but keep the color (and therefore wavelength) constant.  Should we expect the electrons to leave with a greater energy?  This may be what we expect, but suprisingly all the electrons leave with the same velocity and energy as they did with the lower intensity.  Instead, the number of electrons which bounce off the metal surface will increase.

And what happens if we increase the wavelength?  Experimentally, we demonstrate that when a metal is exposed to a coherent red Light (which has a larger wavelength than that of the violet color), the energy and velocity that the electrons leave with is half of the energy produced by the violet color.

The explanation of this phenomenon can be done by the quantum theory of Light.  Let’s imagine that Light is composed of an infinite number of quantums.  When Light strikes the metal, each quantum is hit with an electron and bounces off the metal. If we increase the intensity of the Light, the quantity of quantums increases, but it doesn’t increase the energy that the electron is pulled out with.  This is the reason why the electrons leave with the same velocity and energy.  On the other hand, if we increase the wavelength, the quantums arrive with less energy, and therefore, the electrons leave with less velocity.