Generation of Polarised Light

Question 1

What is the principle of the pile of plates polariser?

Answer 1

After numerous brewster angle transmissions, mainly the parallel polarisation remains

Question 2

What is the principle of a brewster window for a laser

Answer 2

Light polarised in a particular state and incident on a parallel plate at brewster angle will be transmitted without loss in that state

Question 3

How does a polariod filter produce linearly polarised light

Answer 3

using a modified form of dichroism, a type of selective absorption of different polarisations in long chain molecules

Question 4

What causes dichroism and what is it

Answer 4

Caused by anisotropy which is different properties in different directions of space

Question 5

How is the component of polarised input perpendicular to the optic axis absorbed by dichroic crystals

Answer 5

strongly

Question 6

How is the parallel component (to the optic axis) of input into a dichroic crystal absorbed

Answer 6

it is mostly transmitted

Question 7

why are dichroic crystals highly coloured

Answer 7

because absorption is wavelength dependent

Question 8

Describe the principles of the wire grid polariser

Answer 8

light can excite electrons to move along the wires which then emit light which cancels out the input light.
This cannot happen perpendicular to the wires

Question 9

What is the scattering intensity of sunlight scattered by air molecules?

Answer 9

𝐼(𝜆) ∝ 1/𝜆^4

Question 10

Why do clouds look white

Answer 10

high concentration of water droplets which scatter light.
due to high density, scattering is more and all wavelengths eventually get scattered giving white appearance

Question 11

How is sunlight polarised?

Answer 11

scattering off air molecules produces LPL in plane perpendicular to incident light.
scatterers radiate perpendicular to their line of oscillation so at 90 degrees from beam direction, scattered light is linearly polarised

Question 12

What pattern do molecules scatter in?

Answer 12

dipole angular distribution - no light emitted along direction of oscillation
no light is scattered along the input field direction

Question 13

What can cause light to depolarise?

Answer 13

passing through a random material, eg human tissue, atmosphere (due to warm and cool regions having lower/higher n), clouds

Question 14

What happens to the light intensity after passing unpolarised light through a linear polariser

Answer 14

I= Io/2

Question 15

What is the intensity of light which has passed between crossed polarisers?

Answer 15

0

Question 16

How do you create an optical modulator?

Answer 16

place a device which can alter polarisation in a controlled way between two crossed polarisers.
This allows us to modulate information onto the light beam

Question 17

What causes stress birefringence in normally isotropic substances

Answer 17

under compression or tension, the material takes on properties of a uniaxial crystal with the effective optic axis in the direction of the stress

Question 18

what happens if there is a non uniform stress?

Answer 18

the retardation of incident light will vary

Question 19

What does it mean for a substance ti be optically anisotropic

Answer 19

optical properties are not the same in all directions within a given sample

Question 20

What is the definition of a dichroic crystal

Answer 20

it strongly absorbs one polarisation state while transmitting the orthogonal one. If y and z directions are identical, the x axis defines the direction of the optic axis

Question 21

What is the consequence of the x and y polarisations experiencing different refractive index curves?

Answer 21

We see a double image transmitted - double refraction

Question 22

What happens if a transparent dichroic material is rotated above an image?

Answer 22

the image due to the ordinary rays stays fixed
the image due to the extraordinary rays rotates in a circle centred on the first

Question 23

What is the definition of a priciple section

Answer 23

The principle plane, through the optic axis, normal to opposite surfaces of cleavage form

Question 24

Describe what happens to the o-ray along the principal section

Answer 24

it is polarised orthogonal to the principal section
so it moves straight across the crystal, direction unchanged

Question 25

Describe what happens to the e-ray as it travels along the principal section

Answer 25

it is polarised parallel to the pricipal section
the wave propagates at an angle to the original direction and emerges parallel but displaced sideways
the ray direction is not normal to the wavefront

Question 26

What does a point source of natural light embedded in a uniaxial crystal give rise to?

Answer 26

spherical o-wavelets and ellipsoidal e-wavelets

Question 27

In a uniaxial crystal, what is the speed of the o-wave

Answer 27

v⊥ in all directions

Question 28

In a uniaxial crystal what is the speed of the e-wave

Answer 28

v⊥ only in optic axis direction, v|| everywhere else

Question 29

what is no

Answer 29

c/v⊥

Question 30

what is ne

Answer 30

c/v||

Question 31

what is the formula for birefringence

Answer 31

Δn = (ne – no)

Question 32

how do you define positive and negative uniaxial crystals?

Answer 32

the sign of Δn

Question 33

If the uniaxial crystal is cut with the optic axis parallel to the front face, what would happen to the o-ray and e-ray entering normal to the face

Answer 33

the ray would not diverge
o and e ray would travel same direction at different speeds and emerge with a phase difference

Question 34

If the uniaxial crystal is cut so that the optic axis is perpendicular to the plate faces, what would happen to the o-ray and the e-ray

Answer 34

they would travel at the same speed, no birefringence effects