1. Irradiance of Light I = P A P – power in watts (W) A – area in m 2 I – irradiance in W/m 2 or Wm -2 Experiment to find out how irradiance varies with distance. bulb Solar cell V box 1.Why use a box over the experiment ? Give another way of doing this. 2.Why use a solar cell and not an LDR?

2. I 1/d 2 I  1/d 2 d 2d I 1 d 2 2 I 2 d 1 2 Example The irradiance of a bulb is 0.3 W/m 2 at a distance of 50 cm. What will the irradiance be at 1 m? NB Laser light doesn't spread out I d Tutorial Questions 29 to 33 SAQ to page 3

3. Photoelectric Effect – evidence for light being particles. Metal surfacelightchargedischarges Zincuv-ve+ve ZincWhite-ve+ve Steeluv-ve+ve SteelWhite-ve+ve Only negative charges can escape the surface of a metal. Different metal surface need different amounts of energy to release an electron. The work function. It doesn’t matter how irradiated the metals is, there will be no electrons released from the surface unless the light is above a minimum frequency, the threshold frequency, fo. Click here

4. Einstein suggested: Electromagnetic radiation is made up of small bundles called photons. Each photon has a certain amount of energy which depends on its frequency, E = hf h – Plank’s constant = 6.63 x10 -34 Js A bright light produces more photons each second than a dim light. When a photon hits an electron all its energy is given to that electron. The minimum amount of energy a photon needs to release one electron from the surface of a metal, the work function = hf 0. If the photon has more energy than this it will give it to the electron in the form of kinetic energy. Kinetic energy = energy of photon – work function Ek = hf – hf 0 I = Nhf N – number of photons I = irradiance Tutorial Questions 34 to 39

6. Bohr Model - only certain orbits allowed Ground state Ionisation level Excited states Energy Level Diagrams Ground state – the lowest energy level an electron can be in. Excited states – when energy is put into an atom electrons can jump up to these higher energy states. Ionisation level – an electron getting more than this energy will leave the atom which then becomes an ion.

7. Spectra (plural of spectrum) Emission – electrons falls to lower energy level giving out energy in the form of photons Photon, hf = E EAEA EBEB Emission Line Spectra 10 lines possible small E low fLarge E high f Click here

8. Continuous Emission Spectra – from lots of different atoms and molecules so a whole range of frequencies High energy High f Low energy Low f Absorption Line Spectra – electrons jump to higher energy levels and absorb photons of the correct energy (frequency) Tutorial Questions 40 to 45

9. LASER – Light Amplification by the Stimulated Emission of Radiation Excited lasing material Photons stimulates electrons to fall to lower energy state giving out photons of the same frequency and in phase ie radiation is coherent and monochromatic mirror Half silvered mirror Click here Tutorial Questions 46 to 51

10. Word List Irradiance photoelectric photon energy levelground stateexcited state Ionisation levelemission spectraabsorption spectra Line spectracontinuous spectrathreshold frequency photoelectron LASERspontaneous emissioncoherent Stimulated emissionmonochromatic