2 THE PHOTOELECTRIC EFFECT THIS IS THE EMISSION OF ELECTRONS FROM MATTER WHEN MATTER IS ILLUMINATED BY CERTAIN TYPES OF ELECTROMAGNETIC RADIATION. THE EFFECT OCCURS WHEN METALS ARE ILLUMINATED BY UV LIGHT AND CAN OCCUR WITH THE ALKALI METALS FOR VISIBLE LIGHT. IT WAS FIRST OBSERVED BY HEINRICH HERTZ IN 1887
3 Radiation mA Anode +ve Cathode -ve electrons 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 vacuum
4 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
5 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
6 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 –ΔV s, the stopping potential –The stopping potential is independent of the radiation intensity
7 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. mA A C Radiation electrons The p.d. across the tube measures the maximum kinetic energy of the ejected electrons in electron volts. V
8 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
9 the visible spectrum λ frequency violet light light 400 nm red light light 700 nm uv light < 400 nm Blue photon Red photon Which photon has the most energy ????? BLUE !!!
10 Quantum Theory of the Photoelectric Effect Because of the interaction of this electron with other atoms, it requires a certain minimum energy to escape from the surface. 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. The minimum energy required to escape depends on the metal and is called the work function, Φ.
11 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: E K = hf - eΦ For every metal there is a threshold frequency, f 0, where hf 0 = eΦ, that gives the photon enough energy to produce photoemission. It follows that the photo electric current is proportional to the intensity of the radiation provided the frequency of radiation is above threshold frequency. 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 E K = photon energy – e x the work function.
12 Maximum E K emitted electrons / J Frequency f / Hz metal A Work function, Φ Threshold frequency f 0 metal B E K = hf - eΦ Gradient of each graph = Planck’s constant, h.
13 f / Hz 10 14 0 5 10 15 Max E k / eV 1 2 PotassiumMagnesiumAluminium
14 Summary For any metal there is a minimum threshold frequency, f 0, 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, E k, is found to depend linearly on the frequency of the radiation and to be independent of its intensity. For incident radiation of a given frequency, the number of electrons emitted per second is proportional to the intensity of the radiation. Electron emission takes place immediately after the light shines on the metal with no detectable time delay.