THE PHOTOELECTRIC EFFECT

The Photoelectric Effect

Light hits a metal plate, ejecting electrons

Once ejected, electrons are attracted to a positively charged electrode.

Electrode is connected to a sensitive meter.

When light shines on the metal plate, meter deflects (gets a current reading)

When light is turned off, meter reads zero.

Put a negatively charged screen in the way. Ejected electrons will be repelled back down to the metal. No current will be read on the meter.

Low Intensity Light = No Current in Meter

HIGH Intensity Light = Still No Current in Meter

Brighter light does not result in more energetic electrons punching through the screen.

Low Intensity UV Light = Current in Meter!

Electrons make it through the screen

Planck’s Formula Planck deduced that the energy of a wave

of light is tied to its frequency. E = hfwhere h = 6.6x10-34 J s

UV light = high frequency = high energy. Red light = low frequency = low energy. Red light cannot give the electrons enough

energy to get through the screen, but UV can.

Einstein’s Explanation of the Photoelectric Effect

Light is a wave, but it is sent in packets Each packet has a small quantity of

energy given by its frequency (E = hf) One packet knocks one electron, if it has

the energy Higher intensity does NOT equal more

energetic electrons

Einstein (cont’d) Low frequency light produces low energy

electrons. These electrons cannot push through the negative screen.

Sending more packets of low frequency light will not help. It will just produce more low energy electrons, none of which can get past the screen.

Einstein (cont’d) High frequency light produces high

frequency electrons. Even a dim beam of high frequency light

will get a current reading in the meter. Increasing the intensity of the beam in

this case will mean an increased current reading in the meter.

The Work Function Applying the law of conservation of energy:

Ebefore = Eafter If we treat this like a perfectly elastic collision

Elight = Eelectron Turns out this is not quite right. It takes some

energy to eject the electron from the atom, so the electron ends up with less energy than the light. Every element has a different hold on its electrons. The work done removing the electron is called the work function, W.Elight = W + Eelectron