Polarisation

Question 1

what are plane polarised waves

Answer 1

transverse waves where the oscillations occur in a single plane

Question 2

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

Answer 2

• 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

Question 3

what would a polarising filter with a vertical slit do to vertically and horizontally oscillating waves

Answer 3

• 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

Question 4

why cant longitudinal waves be polarised

Answer 4

• 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

Question 5

would a longitudinal wave like sound be able to pass through a polarising filter like the one with the vertical slit and why

Answer 5

• 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

Question 6

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

Answer 6

• 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

Question 7

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

Answer 7

• 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

Question 8

what does this tell us about how receivers work

Answer 8

the receiver will only pick up the component of the signal in the plane of its antenna

Question 9

what two electromagnetic waves could be used for this

Answer 9

• radio waves

- microwaves

Question 10

are most sources of light polarised or unpolarised

Answer 10

unpolarised

Question 11

why are most sources of light unpolarised

Answer 11

because light emitted from bodies like the sun consists of variations of electric and magnetic fields in all planes

Question 12

how is light artificially polarised

Answer 12

• by using a polarising filter

- which would only allow light waves in one specific plane to pass through them

Question 13

what are polarising filters usually made of

Answer 13

• transparent polymers

- with the molecular chains aligned in one direction

Question 14

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

Answer 14

• a source of light viewed through them would decrease in intensity
• eventually resulting in it disspearing

Question 15

what would happen to light that was falling on the surface of an object at an angle

Answer 15

it would be reflected and refracted

Question 16

how can these two things happen at the same time, starting with relfection

Answer 16

• 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

Question 17

what would the angle for glass be at which light in the plane of its surface would undergo complete plane polarisation

Answer 17

56 degrees

Question 18

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)

Answer 18

• 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

Question 19

what is a natural example of the polarization of light by scattering

Answer 19

the sky

Question 20

why does the sky look blue

Answer 20

• 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

Question 21

what can some complex molecules do to light

Answer 21

they can rotate the plane of polarisation to transmitted light

Question 22

in sugar solutions, what does the angle of rotation of this light depend on

Answer 22

the concentration of the solution

Question 23

what does LCD stand for

Answer 23

liquid crystal display

Question 24

how do LCD screens work

Answer 24

• 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

Question 25

long chain polymers can also rotate the plane of polarisation. what does the degree of rotation for them depend on

Answer 25

• the strain on the molecules

- the wavelength of light

Question 26

how is this knowledge useful in industrial design

Answer 26

• 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