The PHOTO-ELECTRIC EFFECT

IMPORTANCE

Physicists run into the problem of light interacting with charge. To explain the experimental data, a new physics is required.

APPLICATIONS

1. Started the "sound" revolution in Hollywood in the 1920's- movies no longer silent. Many older movie stars turn out to have bad voices and disappear. New stars appear. Boring.

2. The PHOTO-VOLTAIC (ref1 ref 2) industry using "solar cells" to directly create electricity from sunlight.

3. First detailed insights into how light interacts with matter - atoms, molecules - gives understanding to why UV light can cause skin cancer for example - why high frequencies are ionising. It also is the beginning of understanding why light can cause chemical reactions or changes - why we can see (rhodopsins change shape when light of sufficient energy arrives) - why chlorophyll can absorb light to create wood from CO₂ ............

HISTORY

• First really noted by Heinrich Hertz in 1886 as a side branch to his work on radio waves when he noted that sparking at his detector was enhanced when ultraviolet light fell on the metal balls from which sparks were produced. He did not do much to follow this up.
• Hallwach in 1888 pursued the problem by illuminating charged zinc plates. Negatively charged plates rapidly lost their charge under UV.
• JJ Thomson showed in 1899 that the charges emitted from an illuminated metal were identical to his previously "discovered" electrons.
• Further work by Lenard 1902 showed all sorts of difficulties using "commonsense" or classical approaches to explain the ejection of electrons by light. He started measuring energies using reversed voltages to stop electrons and by using variable intensities as well as using discrete colours.

"COMMONSENSE" PREDICTIONS arising from Maxwell's work.

1. Red light could be a "better" ejector of electrons than blue as the waves have a longer period so there is more time to interact with the electron although the blue end shakes them faster ( smaller period ) so could increase the energy.

2. More light ( higher intensity ) of a given colour should produce both more and higher energy ejected electrons.

3. Dim light should lead to time delays in ejection of electrons due to low energy of the dim light.

4. The energy of a wave will be shared with billions of electrons.

EXPERIMENTAL RESULTS - total contradiction of above

1. UV definitely leads to more energetic electrons than lower frequencies.

2. A "cut-off" or threshhold frequency below which no matter how long the light is applied, no emission.

3. Emission of electrons appears to be instantaneous no matter the intensity above the cut off. This makes it apparent that the light interacts with ONE electron rather than billions.

4. Higher intensity leads to MORE electrons of the same energy range as low intensity of the same frequency.

• ENTER EINSTEIN 1905 !! - ( Einstein produced 3 brilliant papers in 1905 as well as finished his thesis. Special Relativity, Photoelectric effect and methods of measuring Avogadro's Number. All started new areas of physics. )

Einstein took up Planck's theory and applied it to light radically. Instead of treating light as "particulate" in emission and absorption as Planck did, Einstein went the whole hog and conjectured that light IS particulate - PHOTONS with energy given by Planck's Law.

E = hf = hc / λ h = Planck's Constant = 6.63 x 10^-34 Js

Here light is treated completely as a "ball" or marble. When it hits a metal, it interacts ONLY with ONE electron and EITHER has enough energy to eject the electron - or doesn't.

Incoming energy (photon energy) becomes ( = ) energy to emit the electron + kinetic energy of the emitted electron

hf = emission energy ( "Work Function W" ) + 1/2 mv²

v = max speed

A frequency / kinetic energy graph should then be a straight line

E_k = hf - Work Function ( hf₀) ( y = mx + c )

Work Function = hf₀ f₀ = "cut off frequency or threshhold frequency" - the minimum frequency to achieve the effect.

• Robert Millikan of Oil Drop fame is HORRIFIED. Set about showing in a superb series of experiments what a load of rubbish Einstein's work was. He managed to cut the oxide layers off metals in vacuo and ended up showing Einstein was absolutely correct - he measured h = Planck's Constant to 5% accuracy and achieved the Nobel Prize for his work in 1923. There is a moral there somewhere.

(PS Einstein also received the Nobel Prize for the theory.)