Irradiance of Light

1. I = P A box V bulb Solar cell Irradiance of Light P – power in watts (W) A – area in m2 I – irradiance in W/m2 or Wm-2 Experiment to find out how irradiance varies with distance. • Why use a box over the experiment ? Give another way of doing this. • Why use a solar cell and not an LDR?

2. I I d 1/d2 d 2d I1 d22 I2d12 I  1/d2 NB Laser light doesn't spread out Example The irradiance of a bulb is 0.3 W/m2 at a distance of 50 cm. What will the irradiance be at 1 m? Tutorial Questions 29 to 33 SAQ to page 3

3. Photoelectric Effect – evidence for light being particles. 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’tmatter howirradiated 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 x10-34Js • 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 = hf0. 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 – hf0 I = Nhf N – number of photons I = irradiance Tutorial Questions 34 to 39

6. Bohr Model - only certain orbits allowed Ionisation level Excited states Ground state 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) Click here Emission – electrons falls to lower energy level giving out energy in the form of photons Emission Line Spectra EB Photon, hf = E EA 10 lines possible Large E high f small E low f

8. Continuous Emission Spectra – from lots of different atoms and molecules so a whole range of frequencies Low energy Low f High energy High 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 Click here mirror Half silvered mirror 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 Tutorial Questions 46 to 51

10. Word List Irradiance photoelectric photon energy level ground state excited state Ionisation level emission spectra absorption spectra Line spectra continuous spectra threshold frequency photoelectron LASER spontaneous emission coherent Stimulated emission monochromatic