# Physics G492 AS Revision Notes

Below are some revision notes about the G492 paper for Physics A level. You may want to also look at Section C Revision Notes too. Feel free to skip to the parts most relevant to you.

### Displacement – Time Graphs

• The displacement is a distance is a specific direction.
• With these graphs, the gradient is the velocity.

### Velocity – Time Graphs

• A velocity is a speed is a specific direction.
• The gradient of these graphs is acceleration.
• The area under the line is the displacement.

### Superposition (and interference)

Superposition is when each wave is added together to make a combined wave. Waves are special from the way they have the ability to pass through each other. The resultant wave observed from the observer is a combination of the amplitude of the waves together at that point and time.
The frequency is the number of waves produced per second in Hertz, Hz. Huygen’s construction suggested that a wave front can be considered as a line of secondary sources of waves. Even though there wavelets (secondary waves) are not real, they give a way of constructing where the wave front will go and maybe even more. ### Reflection

The rules of reflection are that the angle of incidence, i, equals the angle of reflection, r. The rule of reflection is true at each reflected point. The trip time for every path to reach the focus, F, should be in phase and the same.

### Light as a Wave or a Particle?

We can measure light as a wave. To do this, we use the Young’s Two Slit Experiment which proves light to be a wave. The light is shined in two slits producing a pattern of light fringes and dark fringes on a nearby screen. The central fringe has a wave difference of 0, the first dark fringe has a path difference of  λ/2. The next bright fringe has a path difference of λ and so on.
We can measure the Two Slit Experiment through four variables:

• λ = wavelength of light.
• d = separation of the two slits.
• s = separation of the fringes (bright fringe-bright fringe or dark fringe-dark fringe).
• L = distance between slits and screen.
We can use the equation λ L = d s although it is just an approximation.

When talking about light we need to know about the spectrum. Below is a bullet point list of the light with most energy to lowest energy from UV – IR:
• Ultra-Violet – Highest Energy
• Violet
• Indigo
• Blue
• Green
• Yellow
• Orange
• Red
• Infra-Red – Lowest Energy

The lower the the energy the light has, the lower the frequency it has. This means it will have a longer wavelength.
When using a diffraction grating, we can use the equation nλ = d sinX (where n is the number of fringes it is from the centre):

This applies for the waves that are in phase where the path difference is λ or dsinX. When the phase difference is half λ, the waves interfere destructively causing a dark fringe to appear. When the waves are in phase and have a path difference of λ, they will interfere constructively producing a bright fringe.

Single Slit Experiment
• Wavelength = wavelength
• d = the width of the single slit
• Feta (X) = angle to the the first dark fringe
Double Slit Experiment
• Wavelength = wavelength
• d = separation of the double slits
• Feta (X) = angle to the first bright fringe.

## λ = d sin X

• As λ increases, the angle will increase.
• As d increases, the angle will decrease.
With waves, as the frequency increases, the speed of the phaser rotating increases. As the amplitude of the wave increases, the radius of the phaser increases.

Higher frequencies form dark spots quicker while lower frequencies form dark spots slower.

### Photoelectric Effect

hF = eV + hf0

Where hF is the total amount of energy put into the electron, eV is the energy of the electron and hf0 is the energy required to move the electron (and escape the metal). hf0 is also known as the work function because it is the energy required to fire the electron up. Therefore, eV (electron volts) equals the equation for kinetic energy: 1/2mv^2.
A frequency too low won’t move the electrons. The right frequency can move electrons. Increasing the intensity of the frequency will move more electrons from the surface of the metal. Increasing the frequency of the light but keeping the intensity the same will increase the light energy to the electron making the electrons escape from the surface of the metal quicker. The number of electrons escaping the surface will be the same. The stopping potential is the voltage needed to make the electrons stationary.

### Electron Diffraction

The wavelength of electrons , known as the de Brogile wavelength, is given by the equation:

Wavelength = Planck’s constant / mass of electron X velocity of electron (λ=h/p)

For which the mass of an electron is 9.1×10^-131kg. The electrons show particle and wave properties from the electron diffraction experiment. Electrons are fired in a vacuum from an electron gun (showing particle properties). They then go through a thin graphite screen causing them to diffract showing properties of waves.

For example, the diffraction grating of electrons at 3000V = 24 mm.
Energy of electrons at 3000V = 3000eV = 1.6×10^-19 x 3000 = 4.8×10^-16
What is the frequency of the electrons? E = hf. F = 7.2×1-^17.
What is their wavelength? v = fλ. λ = 4.4×10-11m.

Another equation could be used to work out the wave length of electrons:

## λ= sxd / L

Where λ is the wavelength, d is the spacing between light fringes, s is the spacing between the graphite and L is the length between the graphite and tube screen.

Some more equations:

• p = h/λ – This can be used for high voltages.
• p = square root of 2meV – This cannot be used for high voltages.
• h/λ = square root of 2meV – This clear that 1/λ is proportional to the square root of V.
So, if we increase V, electron wavelength will decrease.

λ = d sinX

If λ decreases, sinX will decrease.
If angle X decreases, the rings get smaller.
So, if we increase V, the rings should get smaller.

### Quantum Physics

• The trip time is the time any particle takes to on from one place to another.
• The path is the path the particle takes to on a trip from one place to another.
• The shorter the trip time, the smaller the phaser.
• The probability of a path is the resultant phaser value squared.
• If amplitude has decreases, it will not be as bright. 