36.5 - Photoelectric effect

Photoelectric effect: The ejection of electrons

from a material due to light striking it. Aspects

of this effect were used by Einstein to

demonstrate the quantization of light.

What caused Einstein to believe that light was quantized? In the year 1905 he used a

quantum model of light to explain the results of an experiment that could not be

explained using classical electromagnetic theory. In fact, Einstein won the 1921 Nobel

Prize in Physics “for his services to Theoretical Physics, and especially for his discovery

of the law of the photoelectric effect.”

In 1887, Heinrich Hertz had shown that shining light on metal could cause electrons to

be ejected from the metal. You can think of this process as analogous to evaporation.

When light shines on water, it can cause some of the water molecules to escape as a

gas. When light shines on metal, some of the electrons can escape the metal.

We illustrate this phenomenon in Concept 1, with electrons escaping from the metal

electrode on the right. The “ejected” electrons could be readily explained by the

classical model of light as a wave composed of electric and magnetic fields. These

waves transported energy, and it made sense that some of the electrons in the metal

could absorb enough of this energy to escape the attractive force binding them to atoms

in the metal.

In Concept 1, you also see parts of an apparatus used to conduct experiments whose

results were not so readily explained using classical theory. It consists of two electrodes

enclosed in a vacuum. The left electrode has a lower electric potential than the right

electrode, which means it will repel electrons. You may also think of the apparatus in

this way: an electric field is established between the electrodes that points from the right

to the left. This field “pushes” the electrons to the right, back toward the electrode they

have escaped from.

Shining light on the right-hand electrode causes electrons to be ejected from it and to

move to the left. The faster they are moving, the more negative the electric potential of

the left electrode has to be to keep them from reaching it. By adjusting the electric

potential on the left electrode so that the electrons “just fail” to reach it, an experimenter

can determine the maximum kinetic energy of the electrons.

An expected result of this experiment would be that, the more intense the light shining

on the right electrode, the more energy its electrons would absorb, and the faster they

would move when ejected from the metal. It would be like chopping wood with an ax. As

you chop the wood, the harder you strike, the faster you expect some of the chips to fly

off.

However, to the surprise of the experimenters, this proved not to be the case. There is

no correlation between the intensity of the light and the maximum kinetic energy of the

electrons. Rather, when the intensity of the light is increased, more electrons are

emitted. Instead of the kinetic energy of the escaping electrons increasing, only their

number increases. The incorrect “classical” expectation is shown in Concept 2, and the

actual observed behavior in the experiment is shown in Concept 3 (refresh your

browser to restart these animations). In terms of the ax analogy: It is as if hitting the log

harder results not in more energetic chips, but in more chips of the same energy flying

off.

Another surprising result was that when the frequency of the light was below a certain

value, known as the cutoff frequency, the light could be of great intensity, but no

electrons at all would be ejected. Classical physics cannot explain these phenomena.

To use the ax example one more time: If you strike a log very infrequently, but with

great force (energy), you would expect chips to fly off. However, if the photoelectric

effect applied to wood chopping, then when you chop at a slow rate, no chips will fly off,

no matter how hard you strike the log. Very odd!

Quantum theory, however, explains both these results quite neatly. Considering light as

packets of energy means that with more intense light more packets are striking the

metal each second. The energy of each packet does not change with intensity; the

number of packets does. More photons (each having the same energy) striking the right

electrode each second increases the rate at which electrons are ejected from the metal,

but not their maximum kinetic energy.

This can also be likened to knocking over bottles with bullets from a rifle. In classical

theory, more “intensity” means switching to a bigger, more powerful rifle, so that each

Light and electrons

Shining light on metal causes it to emit

electrons

Incorrect expectation

Speed/energy of an electron

corresponds to light’s intensity??

What was actually observed

Number of electrons increases with

light’s intensity

Relationship of frequency and

electrons

No electrons emitted

·For colors below a certain frequency

·No matter how intense the light

(^666) Copyright 2007 Kinetic Books Co. Chapter 36