The photoelectric effect

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When visible light, X rays, gamma rays, or other forms of electromagnetic radiation are shined on certain kinds of matter, electrons are ejected. That phenomenon is known as the photoelectric effect. The photoelectric effect was discovered by German physicist Heinrich Hertz(1857–1894) in 1887. You can imagine the effect as follows: Suppose that a metal plate is attached by two wires to a galvanometer. (A galvanometer is an instrument for measuring the flow of electric current.) If light of the correct color is shined on the metal plate, the galvanometer may register a current. That reading indicates that electrons have been ejected from the metal plate. Those electrons then flow through the external wires and the galvanometer, providing the observed reading.

The photoelectric effect is important in history because it caused scientists to think about light and other forms of electromagnetic radiation in a different way. The peculiar thing about the photoelectric effect is the relationship between the intensity of the light shined on a piece of metal and the amount of electric current produced.

       To scientists, it seemed reasonable that you could make a stronger current flow if you shined a brighter light on the metal. More (or brighter) light should produce more electric current—or so everyone thought. But that isn't the case. For example, shining a very weak red light and a very strong red light on a piece of metal produces the same results. What does make a difference, though, is the color of the light used.

One way that scientists express the color of light is by specifying its frequency. The frequency of light and other forms of electromagnetic radiation is the number of times per second that light (or radiation) waves pass a given point. What scientists discovered was that light of some frequencies can produce an electric current, while light of other frequencies cannot.

Einstein's explanation. This strange observation was explained in 1905 by German-born American physicist Albert Einstein (1879–1955). Einstein hypothesized that light travels in the form of tiny packets of energy, now called photons. The amount of energy in each photon is equal to the frequency of light (ν) multiplied by a constant known as Planck's constant (â„), or νâ„.

Einstein further suggested that electrons can be ejected from a material if they absorb exactly one photon of light, not a half photon, or a third photon, or some other fractional amount. Green light might not be effective in causing the photoelectric effect with some metals, Einstein said, because a photon of green light might not have exactly the right energy to eject an electron. But a photon of red light might have just the right amount of energy.

Einstein's explanation of the photoelectric effect was very important because it provided scientists with an alternative method of describing light. For centuries, researchers had thought of light as a form of energy that travels in waves. And that explanation works for many phenomena. But it does not work for phenomena such as the photoelectric effect and certain other properties of light.

Today, scientists have two different but complementary ways of describing light. In some cases, they say, it behaves like a wave. But in other cases, it behaves like a stream of particles—a stream of photons.

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