Quantum
Behavior
The
Quantum Universe
The Photoelectric
Effect (This page)
Other evidence that photons exchange energy in packets/quanta (E =
hf)
weblinks
A set of notes in Word format containing all the content from the four webpages (above) on quantum behavior is available here
The Photoelectric Effect
Einstein proposed that
light consisted of energy particles. This was related to a phenomenon
called the photoelectric effect. If you shine light with a
high enough frequency on a metal plate then electrons are emitted
from the plate.
| The electrons need energy to be released from the surface of a metal |
| This energy can comes from photons |
| Only photons with enough energy can give it to to electrons to release them |
| By increasing the frequency of photons you increase the amount of energy they have |
| The minimum frequency of electromagnetic radiation that will cause electrons to be emitted is called the threshold frequency |
| The corresponding threshold of energy that is needed to release an electron is called the work function (sometimes written as Ф) |
| If the photon has more energy than the work function then the extra energy is given to the electron as kinetic energy |
The diagram below shows
one way of showing the photo electric effect. In this case the
electrons move towards the positive plate and register a current on
the ammeter. Electrons caused by the emission of light are sometimes
called photoelectrons.
Einstein stated that
the amount of energy in these packets called photons could be
calculated with the following equation.
E
= hf E = Energy (in Joules) h = Plank's constant = 6.63 x 10-34 Js-1 f = frequency of wave in Hz |
The photoelectric effect can be demonstrated in the lab with less sophisticated equipment. This is based on an experiment by Heinreich Hertz.
A cleaned zinc plate is placed upon the top of a gold leaf electroscope. The electroscope is charged with a negative charge. (this will cause the leaf to rise and stay risen) A lamp emitting ultraviolet radiation is shone on the zinc plate on the electroscope (A mercury lamp will give off ultra violet radiation). When this is done the leaf falls back down. The only thing that can cause this to happen is if the electroscope has somehow lost negative charges.
Are you sure?
One way to see if this
is really happening is to charge the initial charge so that it is
positive. The leaf will still rise up as a result of an excess of
positive charges. When the lamp is shone on it then the leaf does
not drop back down.
Is it really the ultraviolet light that is doing this?
Ultraviolet light cannot pass through glass. If you place a piece of glass on top of the zinc plate and repeat the original experiment then the leaf does not fall. The ultraviolet light cannot pass through the glass and so cannot get to the electrons in order to give them enough energy to be freed.
What about infrared?
If you were to repeat
the original experiment with infrared light then there would be no
effect. Infrared has a lower frequency than ultraviolet. The energy
in the photons of Infra red is less than the work function of
the electrons in zinc and so they are not released.
What about other metals?
If the plate on the
electroscope was made of sodium then the experiment would be
different. The work function of sodium is less than that of zinc and
so both infrared and ultraviolet photons have enough energy to
release them.
Work function and velocity of escape
If the electrons get more than the work function then they will move away with a velocity that corresponds to the kinetic energy that it has from the 'leftover' energy from the photon.
The photoelectric equation is sometimes written as
E = hf = Ф + Emax
Where Emax is the maximum kinetic energy that the electron could have when emitted. This means that
Emax = ½ m v2
Where m = mass of an electron = 9.10 x 10-31 kg
v = maximum velocity of emitted electron