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Optics > experiment 6 - polarisation > Flashcards

experiment 6 - polarisation Flashcards

1
Q

what are the equipment needed for this experiment ?

A
  • computer or mobile phone display
  • desk lamp
  • white sheet of paper
  • sheet polarisers
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2
Q

how can light be represented ?

A
  • particle

- wave motion

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3
Q

what does transverse electromagnetic wave mean ?

A
  • as photon travels in direction of propagation then the electric field vector changes from 0 to maximum to 0 again to negative
  • it keeps on oscillating sinusoidally along the direction of propagation
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4
Q

where is the energy of photon transferred between ?

A
  • energy of photon is transferred between electric and magnetic field
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5
Q

what does the shaded area indicate ?

A
  • indicates the plane of polarisation for photon that travels in this direction
  • plane polarisation can rotated with respect to x-axis
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6
Q

what is the speed of light ?

A
  • the wave length(m) x frequency (s)

- m/s

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7
Q

how many photons does it represent ?

A
  • 1 photon
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8
Q

what is the direction of propagation ?

A
  • direction of propagation is along x-axis
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9
Q

how does the electric field vary along the direction of propagation of the light ?

A

electric field varies sinusoidally along direction of propagation of light

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10
Q

is the light from our lamp or sun polarised or un polarised ?

A
  • unpolarised
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11
Q

what is the only light that is polarised ?

A

the only light that tends to be polarised is light that comes from some lasers

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12
Q

what does the plane of polarisation represent ?

A
  • plane of polarisation represents the plane in which the electric field oscillates sinusoidally along the direction of the wave
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13
Q

what are polarising material ?

A
  • they are a material which absorbs all photons falling on its surface except for those with their plane polarisation in the vertical plane
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14
Q

how do polarisers work ?

A
  • some elongated crystals
    (quinine iodosulfate) are anisotropic ( one axis of crystal is much longer than other axis ) when photons pass through crystals they are absorbed in one plane but allowed to pass in the orthogonal plane
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15
Q

why are photons from lamp or sun unpolarised ?

A

because this plane of polarisation rotates around the direction of polarisation

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16
Q

how can we produce polarised light ?

A
  • can be produced by reflection

- can be produced by using polarising material

17
Q

what happens when you look at a piece of paper under your desk lamp with only one eye and judge its brightness ? cover your viewing eye with piece of paper ? rotate polariser through 90deg and look at brightness of paper ?

A
  • paper looks less bright because light that passes through the polariser is plane polarised
  • when you rotate polariser through 90deg the brightness of paper remains unchanged - the remaining polariser still pass through
18
Q

what happens when you look at your computer screen with on eye and judge its brightness ? place one polariser in front of the viewing eye and rotate until you see maximum brightness ?

A
  • its gets darker than lighter and then there is one angle where its completely lightest
  • can get it to be completely dark
  • brightness changes from maximum to zero
  • darkest at 90 deg
19
Q

what can you conclude about the light emitted by your phone or computer screen ?

A
  • light emitted by screen is completely polarised this is the reason why rotating the polariser in front of eye extinguishes light completely at a certain angle of rotation
  • when light is distinguished completely that means that the pervious direction of your polariser is perpendicular to the direction of light that comes form screen so no passes through
    cos 90 = 0
  • when you exceed certain angle the light passes through
20
Q

explain the diagram to the left that examines Malus’ law ?

A
  • we have light coming from a mobile phone or an LCD display and it is polarised in particular direction
  • now hold a polariser in front of computer display which allows light in a certain direction to pass
  • light your eye sees is polarised along the direction of polariser
21
Q

what happens if preferred direction of polariser is in the same line as the plane polarised light from computer ?

A
  • all light will pass through
  • angle will be 0
  • cos 0 =1
22
Q

why is the light passing through polariser from the LCD display attenuated?

A
  • this is because there is angle between the preferred direction in polariser and direction of polarised light
  • so light coming through is plane polarised but attenuated as less light has passed through
  • transmitted light is proportional to cos^2 θ
23
Q

how many degrees do you have to rotate the polariser to get 0 light transmitted ?

A

90

24
Q

what happens as you rotate the polariser ?

A

cos 0 = 1
cos90=0
cos 180 = -1

25
Q

what happens when you take a white piece of paper and bring it close to the screen so that the light is mostly received from screen? use polariser to view white paper and rotate polariser ?

A
  • brightness of the paper remains unchanged

- this is because paper scatters the light in all angle and depolarisers it

26
Q

when does an optical boundary reflect and transmit light ?

A

when the incident light is unpolarised , the reflected light is completely plane polarised when the refracted and the reflected rays are normal at 90 deg to each other

27
Q

when is the reflected light fully plane polarised ?

A
  • the reflected light is only fully plane polarised with the plane of polarisation ( perpendicular to the surface ) when the reflected and refracted light are perpendicular to each other
  • this happens only for one angle of incidence known as brewster angle
28
Q

what are the two components of unpolarised light ?

A
  • one plane is perpendicular to the surface of the paper

- one plane is perpendicular to the direction of propagation

29
Q

what happens when using a glossy surface to the reflect light of lamp and looking through a polariser?

A
  • when rotating the polariser there will be a change in brightness
  • you can increase or decrease the amount of darkening you see if you adjust the angle you view the light from reflected surface
  • this is because light illuminating the surface comes from a broad source
30
Q

what happens if angle of incidence is large ?

A
  • most light reflected close to 100%
31
Q

why polarising sunglasses can help your vision

when looking at glossy objects or still water surfaces?

A

Polarised lenses work by preventing light glare from hitting you directly in the eye. Because glare is typically horizontal light, polarised lenses block this light and only allow vertical light. With the horizontal light blocked by polarised lenses, this helps eliminate glare from shining directly into your eyes

Optics (34 decks)

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