Look at the last device which uses p-type and n-type materials to conduct - the MOSFET. It will only switch on if there is a sufficient voltage across the gate of the transistor.

By putting some n-type and p-type materials together you can make all sorts of wonderful devices. Diodes, LED's, solar cells and LDR's all rely upon the technology of semiconductors. We explained how each of these devices work in terms of electrons and holes. You need to be able to distinguish between reverse bias and forward bias as well as photovoltaic mode and photoconductive mode.

Started semiconductors today. Semiconductors are, as the name suggests, materials that 'sort of' conduct electricity. We can add impurities to an element - the process is called doping - which allows the electrons to move. This can be done by adding an element with an extra electron (n-type) or by adding an element with one less electron (p-type).

Did more work on the Bohr model today. Energy is taken in by the electron to jump up one level or more and energy is given out in the form of a photon as the electron moves back down to its original level. The energy required to move from one level to the next can be displayed in an energy diagram. The bigger the gap between the lines, the more energy it takes to move between the levels. These energies are always shown as negative with the ionisation level (where the electron leaves the atom) being at 0 Joules.

If light and other electromagnetic radiations are made of photons then each photon must have an energy associated with it. This energy is E = hf. Thinking back to the photoelectric effect, we know that some materials will eject electrons when you shine radiation on them. The minimum energy required for this to happen is called the work function of the material. If a photon has an energy equal to this work function then it will cause an electron to be ejected to the surface. If the photon has an energy greater than the work function then the remaining energy will converted into kinetic energy of the photoelectron produced.

Photoemission is the release of electrons from a material. Whether an electron is released or not depends upon the frequency of the radiation hitting the material not the irradiance. This leads us to the idea that light and other types of electromagnetic radiation are made up of tiny particles called photons. The energy that these photons have is directly proportional to their frequency as stated in the the equation E = hf.

Irradiance is the power of radiation per unit area. The closer you are to a source of radiation, the greater the irradiance. As you move away from the source the irradiance will decrease, in other words there is less radiation per unit area. If you continue to move away from a radiation source then the irradiance will reduce as the square of the distance from the source. This means that if you double the distance from the source of radiation then the irradiance will be four times less.

As we already know, a diffraction grating can be used to produce interference in the same way a double slit can. We looked at how we can use our knowledge of path difference with a bit of trigonometry to give us an equation for use with diffraction gratings. This gave us the equation nλ = d sinθ where n is the order of the maxima e.g. 1,2,3 etc. This equation can also be applied to the path difference for a minima but I haven't seen this asked for some time.

To calculate the slit separation, d,  you must know firstly how many lines there are per metre and then divide 1 by that value. The angle of deviation, θ, is the angle that the wave/ray is deviated from the normal line.

Remember also that wavelength of red light = 700 nm and wavelength of violet light = 400 nm.

As we saw today, path difference is what causes interference. The path difference is the difference in distance travelled by the waves. This difference in distance will mean that the waves become either in phase or out of phase. Waves that arrive in phase will interfere constructively and waves that arrive out of phase will interfere destructively.

We introduced two equations for working with path difference, one for maxima and one for minima. You need to be able to select the correct one for the given situation. Remember of course that the path difference equation for a minima isn't actually very useful! It might be best to use p.d. = (n - 1/2)λ.

This will not be given in the data booklet though.