MAX PLANCK PHOTOELECTRIC EFFECT © John Parkinson

THE PHOTOELECTRIC EFFECT THIS IS THE EMISSION OF ELECTRONS FROM MATTER WHEN MATTER IS ILLUMINATED BY CERTAIN TYPES OF ELECTROMAGNETIC RADIATION. IT WAS FIRST OBSERVED BY HEINRICH HERTZ IN 1887 THE EFFECT OCCURS WHEN METALS ARE ILLUMINATED BY UV LIGHT AND CAN OCCUR WITH THE ALKALI METALS FOR VISIBLE LIGHT.

The electromagnetic radiation releases electrons from the metal cathode. These electrons are attracted to the anode and complete a circuit allowing a current to flow Radiation mA Anode +ve vacuum Cathode -ve electrons

The setup An adjustable voltage is applied. Voltage can be forward or reverse biased (which slows down the electrons) Photoelectrons return to cathode through an ammeter which records the current

Photoelectric Effect Schematic When light strikes E, photoelectrons are emitted Electrons collected at C and passing through the ammeter are a current in the circuit C is maintained at a positive potential by the power supply

Photoelectric Current/Voltage Graph The current increases with intensity, but reaches a saturation level for large ΔV’s No current flows for voltages less than or equal to –ΔVs, the stopping potential The stopping potential is independent of the radiation intensity

If the polarity is reversed, the pd across the tube can be increased until even the most energetic electrons fail to cross the tube to A. The milliammeter then reads zero. Radiation mA A C electrons electrons V The p.d. across the tube measures the maximum kinetic energy of the ejected electrons in electron volts.

Quantum Theory of the Photoelectric Effect In 1905 Einstein developed Planck’s idea, that energy was quantised in quanta or photons, in order to explain the photoelectric effect. Electromagnetic radiation is emitted in bursts of energy – photons. The energy of a photon is given by E = hf, where f is the frequency of the radiation and h is Planck’s constant. [h = 6.6 x 10-34 Js] But velocity of light = frequency times wavelength Substituting into E = hf

BLUE !!! λ frequency the visible spectrum violet light light 400 nm red light light 700 nm uv light < 400 nm λ Blue photon Red photon Which photon has the most energy ????? BLUE !!!

The electron can then either Quantum Theory of the Photoelectric Effect The photons are sufficiently localized, so that the whole quantum of energy [ hf ] can be absorbed by a single electron at one time. The electron can then either share its excess energy with other electrons and the ion lattice or it can use the excess energy to fly out of the metal. Because of the interaction of this electron with other atoms, it requires a certain minimum energy to escape from the surface. The minimum energy required to escape depends on the metal and is called the work function, Φ.

For electron emission, the photon's energy has to be greater than the work function . The maximum kinetic energy the released electron can have is given by: EK = photon energy – e x the work function. EK = hf - eΦ For every metal there is a threshold frequency, f0, where hf0 = eΦ , that gives the photon enough energy to produce photoemission. The number of photoelectrons emerging from the metal surface per unit time is proportional to the number of photons striking the surface that in turn depends on the intensity of the incident radiation It follows that the photo electric current is proportional to the intensity of the radiation provided the frequency of radiation is above threshold frequency.

Maximum EK emitted electrons / J metal A Maximum EK emitted electrons / J metal B Threshold frequency f0 Frequency f / Hz EK = hf - eΦ Work function, Φ Gradient of each graph = Planck’s constant, h.

f / Hz 1014 0 5 10 15 Max Ek / eV 1 2 Potassium Magnesium Aluminium

Summary For incident radiation of a given frequency, the number of electrons emitted per second is proportional to the intensity of the radiation. For any metal there is a minimum threshold frequency, f0, of the incident radiation, below which no emission of electrons takes place, no matter what the intensity of the incident radiation is or for how long it falls on the surface. Electrons emerge with a range of velocities from zero up to a maximum. The maximum kinetic energy, Ek, is found to depend linearly on the frequency of the radiation and to be independent of its intensity. Electron emission takes place immediately after the light shines on the metal with no detectable time delay .