Polarisation Flashcards
what are plane polarised waves
transverse waves where the oscillations occur in a single plane
if you were to attach a rope to a sturdy point and grab on to the other end, what movements of the hand would create a wave oscillating in a horizontal and vertcial plane
- moving your hand up and down would create vertical oscillations
- which would make it vertically plane polarised
- moving your hand to the left and right would create horizontal oscillations
- which would make i horizontally plane polarised
what would a polarising filter with a vertical slit do to vertically and horizontally oscillating waves
- it would allow waves oscillating in the vertical plane to be transmitted through it
- whereas ones with a horizontal plane wouldnt be able to pass through
why cant longitudinal waves be polarised
- because the particles that carry the energy dont oscillate in planes perpendicular to the direction of the wave
- the energy travels through compressions and rarefractions which are parallel to the direction
would a longitudinal wave like sound be able to pass through a polarising filter like the one with the vertical slit and why
- it would
- because it doesnt consist of oscillations horizontally or vertically
- the particles (probs air) that would carry the energy of the sound would simply pass through the slit regardless
if you have a transmitter and a receiver facing each other, what would need to be done in order for the strongest signal to be picked up from the receiver if waves were shot at it
- the transmitter and receiver would both need to be standing up or lying on their sides
- this is because the receiving areial (where the waves come from) must be aligned in the same plane as the transmitter
- in order for their signal to be received well
if you had a device on the receiver which would measure the strength of the signal from the transmitter, what would it show if the receiver along were to be turned so that it was laying on its side and why
- the strength of the received wave would fall until it reached 0
- because they are now at right angles to each other
- meaning that the wave coming from the transmitter is on a different plane from what the receiver’s antenna can pick up
what does this tell us about how receivers work
the receiver will only pick up the component of the signal in the plane of its antenna
what two electromagnetic waves could be used for this
- radio waves
- microwaves
are most sources of light polarised or unpolarised
unpolarised
why are most sources of light unpolarised
because light emitted from bodies like the sun consists of variations of electric and magnetic fields in all planes
how is light artificially polarised
- by using a polarising filter
- which would only allow light waves in one specific plane to pass through them
what are polarising filters usually made of
- transparent polymers
- with the molecular chains aligned in one direction
what would happen if two of the same light filters were to be held together and rotated until the transmitting planes were at right angles
- a source of light viewed through them would decrease in intensity
- eventually resulting in it disspearing
what would happen to light that was falling on the surface of an object at an angle
it would be reflected and refracted
how can these two things happen at the same time, starting with relfection
- reflection would occur if light was in the plane of the reflecting surface
- it is said to be plane polarised
- although reflection occurring in general still depends on the angle of incidence and the materials refractive index
what would the angle for glass be at which light in the plane of its surface would undergo complete plane polarisation
56 degrees
if you had two filters where one allowed light in the vertical plane and the other in the horizontal plane, how would light consisting of horizontal and vertical planes travel through both of them at once (through the vertical plane one first then the horizontal)
- when the light hits the first filter, the light in the vertical plane would pass through whereas the light in the horizontal plane wouldnt
- this leaves only light in the vertical plane to be travelling to the horizontal filter
- when it hits this, it cant pass through as they are in different planes
- meaning that no light passes through it
what is a natural example of the polarization of light by scattering
the sky
why does the sky look blue
- because the short-wave blue region of the visible spectrum is scattered more than the red
- meaning the scattered light reaches our eyes
- making it look like the whole sky is blue
what can some complex molecules do to light
they can rotate the plane of polarisation to transmitted light
in sugar solutions, what does the angle of rotation of this light depend on
the concentration of the solution
what does LCD stand for
liquid crystal display
how do LCD screens work
- optically active crystals are aligned by an electric field between a polarising filter and a reflecting surface
- the plane of polarisation is rotated by the aligned crystals
- this makes it so that when it has been reflected back to the filter its plane of polarisation is at right angles to that of the filter and no light passes out
- the element of the seven-segment display therefore looks black
long chain polymers can also rotate the plane of polarisation. what does the degree of rotation for them depend on
- the strain on the molecules
- the wavelength of light
how is this knowledge useful in industrial design
- models of load-bearing components are made from perspex and are put under stress
- they are then viewed through crossed sheets of polaroid
- multi-coloured interference stress patterns can be detected and are analysed to detect potential regions of weakness
