Module 4 - Section 2 - Polarisation Of Waves Flashcards
What is a polarised wave ?
A polarised wave is a wave that only oscillates in one direction
Explain what polarising is ?
Basically if you take a rope and you swing it from left to right repeatedly then it makes a transverse wave that oscillates from left to right but if you take that same rope and swing it from up to down and do this repeatedly then you get a transverse wave that oscillates up and down - both are still transverse waves.
So if you want to get the waves on the rope through a vertical fence, the wave will only get through if the oscillations are vertical so if you swing it from up to down - the fence then filters out the oscillations in the other directions so that only the vertical oscillations can get through - this is called polarising the wave
Whats the plane in which the wave oscillates ( vibrates ) called ?
The plane in which a wave oscillates is called the plane of polarisation - e.g the rope wave was polarised in the vertical plane by the fence
What is plane polarisation ?
Plane polarisation is when you polarise a wave so that it only oscillates in one direction ( one plane )
What are ordinary light waves a mixture of ?
ordinary light waves are a mixture of different directions of oscillations —> and the things vibrating - oscillating are the electric and magnetic fields
What ones a polarising filter do ?
A polarising filter only transmits vibrations ( oscillations ) in one direction
Basically it blocks out all the other vibrations in the other directions and polarities the wave into one direction of oscillation
What happens if you have 2 polarising filters at right angles to each other ?
Then no light will get through
What types of waves can be polarised ?
Polarisation can only happen for transverse waves - the fact that you can polarise light is one proof that it is a transverse wave
How can you investigate polarising light using two polarising filters ?
You can observe polarisation by shining unpolarised white light through 2 polarising filters
1) Align the transmission axes of two polarising filters so that they are both vertical - then shine unpolarised light on the first filter and keep the position of the first filter fixed and rotate the second filter
2) light that passes through the first filter will always be vertically polarised
3) When the transmission axes of the two filters are aligned, all the light that passes through the first filter will also go through the second filter
4) As you rotate the second filter, the amount of light that passes through the second filter varies
5) as the second filter gets rotated, less light will get through as the vertical component of the second filters transmission axis decreases - this means the intensity of light getting through the second filter will gradually decrease
6) When the transmission axis are at 45 degrees to each other, the intensity of the light going through the second filter will be half of the intensity of light going through the first filter - then when they are at 90 degrees to each other - no light will pass through - the intensity is 0
7) As you continue turning the second filter the intensity of light should again begin to increase once again
8) When the two axes realign after a 180 degree rotation, all the light will be able to pass through the second filter again
How can you polarise microwaves ?
You can use metal grilles - which are squares full of metal wires which are all aligned - to polarise them
Why wont polarising filters work on microwaves ?
Because the wavelength of microwaves are long
How can you investigate polarisation of microwaves using a microwave transmitter and a microwave receiver linked to a voltmeter ?
1) place a metal grille between a microwave transmitter that only transmits vertically polarised microwaves so you only need one metal grill and then use a microwave receiver that only receives vertically polarised microwaves
2) the intensity of the microwaves passing through the grille is at a maximum when the direction of the vibrations of the microwaves and the wires of the grille are at right angles to each other
3) as you rotate the grille, the intensity of polarised microwaves able to pass the grille decreases so the reading on the voltmeter decreases
4) When the wires of the metal grille are aligned with the direction of the polarised waves, no signal will be shown on the voltmeter
Why does the intensity of the microwaves drop to zero when the wires are aligned with the direction of the microwaves ?
Because the grille is absorbing their energy
- The vibrating electric fields of the microwave excites electrons in the metal grille ( meaning that it gives energy to the electrons )
- Then the energy of the incoming microwaves is absorbed and re-emitted in all directions and only a few of these re-emitted waves are vibrating direction of the microwave receiver - and also the microwave receiver only receives microwaves in one plane so even if the microwave is vibrating towards the receiver - the wave might not get picked up unless it is oscillating in the same plane as the receiver
- When the wires and vibrations of the wave are aligned - more electrons are excited than when the grille and the vibrations of waves are at right angles to each other - therefore causing the drop in intensity - and this is because unlike the other practical - when dealing with microwaves you get more intensity when the direction of the metal wires and the vibrations of the wave are crossed at right angles as less of the electrons are excited and therefore less waves are absorbed into the grille and re-emitted and therefore when its crossed at right angles more polarised waves are caught by the receiver its just that they have a smaller amplitude as they lost energy when travelling through the grille as the grille absorbed some of the waves energy and re-emitted it.
