They won’t appear again until after my own classes have sat their prelims. However, the recent solutions you are looking for are available on the SQA past paper site.

• in phase
• out of phase
• coherent
• path difference
• constructive interference
• destructive interference
• irradiance
• refraction
• critical angle

This is not a full list but you get the idea.

There are video examples further up this page that show you how to do calculations based on an interference pattern. There is also a long comment from me about analysing an interference pattern and selecting the correct value for n in your equation.

Make sure you know the differences between a diffraction grating spectrum and the spectrum from a prism. These are explained further up this page.

Know how to use the information provided in a ray diagram to calculate properties of a material, such as refractive index, speed of light in that material or the critical angle of the light in that material – remember that all of these properties depend on wavelength.

Angles are always measured from the normal. A diagram may not show the normal but you need to know to use it.

For radiation, know how to write a nuclear equation and identify any missing particle or nucleus. Remember that the back page of the blue SQA data booklet has a periodic table with atomic numbers and chemical symbols..

Shielding questions might not just be about calculating the thickness or an absorber. Make sure you also calculate any new equivalent dose or equivalent dose rate if it has been requested.

If you are not sure about this, there are decent BBC Bitesize notes about but they don’t cover everything!

This does not cover everything (hopefully you’ve noticed that I haven’t mentioned photoelectric effect, absorption/emission spectra or semiconductors, for example) but it should help you to plan the work you have to do over the weekend to familiarise yourself with unit 3.

Have you watched the Valdo video?

A hole is really a place where an electron should be, but it’s missing.

Let’s imagine we have a piece of silicon. Each atom in our piece of silicon has 4 outer electrons. Each silicon atom shares its electrons with neighbouring atom so that there are 8 electrons associated with each atom. Sharing electrons like this is what your Chemistry teacher would call a covalent bond, but enough about the Chemistry…..

In silicon, the outer electrons are not free to move about. This is why pure silicon is not a good conductor of electricity. There are 2 ways to turn the silicon into a better conductor;

1.
We can add some electrons to the silicon to make it a better conductor. This is done by adding some atoms of a group V element (such as phosphorous or arsenic) to the silicon. The phosphorous has 5 outer electrons and it shares 4 of these with the neighbouring silicon atoms, leaving the 5th electron free. This 5th electron can move about if a potential difference is placed across the material, allowing a current to flow. Since the particle carrying the current is negatively charged, we call this an n-type type of semiconductor.

2. (…and might sound a little weird!)
We can modify the silicon so it is short of electrons. We do this by swapping some silicon atoms for atoms of a group III element, such as boron. Boron has 3 outer electrons. The boron atom we have added can only bond with three neighbouring silicon atoms, leaving a gap where the 4th electron should be. These gaps are called holes and Valdo shows these as empty circles in his video. Now you can place a potential difference across the material and the holes (which are said to be positive as they are “short” of an electron) move towards the negative potential. Since the charge carrying particle is positive, we say that this is a p-type semiconductor.

The problem with this explanation is that the “hole” is just a gap and it doesn’t really move, it’s just handy for us to think of it that way. Let me explain.

Have you ever played with one of those puzzles where you slide the tiles round to put the numbers in order or recreate a picture? Try this one – the Einstein photo is my favourite . Notice how the blank piece looks like it is moving around? Well, it’s not. What happens is that a tile moves in to the blank area and leaves a new blank area in its place. It looks like the blank bit is moving but really its the tiles are moving from place to place.

The same thing happens with holes. Electrons are able to move into the hole, leaving a new hole where the electron used to be. It kind of looks like the hole has moved but it hasn’t. It’s just easier to think of a positive particle called a hole moving in the opposite direction to an electron when a voltage is applied to the semiconductor.

In a forward biased p-n junction, an electron and hole can meet in the depletion region. They recombine and release a photon. This is how a LED works. Alternatively, a photon entering the depletion region in a reverse biased p-n junction can create an electron and a hole, producing a current.