OP - Interaction of Light and Matter - Scattering and Polarisation - Week 2

Question 1

What happens when changes occur to an electric field?

Answer 1

Changes occur to the corresponding magnetic field.

Question 2

What happens when light hits an atom?

Answer 2

The electric field induces changes to the electron cloud, causing them to oscillate, resulting in a change to the magnetic field.
Light is then radiated off in all directions (scattered), or dissapated as heat.

Question 3

What is polarised light?

Answer 3

Light whose electric field is orientated in a specific plane.

Question 4

Must polarised light always have a planar electric field?

Answer 4

No, it can be radially polarised. Not always a perfect circle, can be eliptical.

Question 5

Are all photons polarised?

Answer 5

Yes, in unpolarised light, it is really just a mix of randomly polarised light.

Question 6

Define refractive index.

Answer 6

How much light is attenuated in a transparent medium, resulting in it being x times slower, x is refractive index.

Question 7

Can refracted light be polarised?

Answer 7

Yes.

Question 8

Which scatters more easily, shorter or longer wavelengths?

Answer 8

Shorter.

Question 9

How is light polarised?

Answer 9

Light passes through a parallel grid like structure, in which only one prientation can pass, resulting in polarised light. The rest get absorbed.

Question 10

Consider a perfect polaroid. How much unpolarised light will it absorb? What orientation of a second filter is needed to get extinction?

Answer 10

50%

Second filter orthogonal to the first results in extinction.

Question 11

How do sunglasses work? What phenomenon does this rely on?

Answer 11

They absorb horizontally polarised light.

It relies on the fact that light is partially polarised by reflection.

Question 12

Explain polarisation by reflection, and Brewsters angle.

Answer 12

Brewsters angle is when the reflected beam is 90 degrees to the refracted beam. Results in partially polarised reflected light.

Question 13

Define Rayleigh scattering vs Mie scattering.

Answer 13

Rayleigh - for when the size is very small

Mie - for when particles are large

Question 14

Is light polarised by Rayleigh scattering?

Answer 14

Light scattered in the forward and backward direction are completely unpolarised.
Becomes increasingly more polarised as the angle increases.
Orthogonally, light is completely linearly polarised.

Question 15

Describe the relationship between particle size and backscattering.

Answer 15

Backscattering increases with particle size.

Question 16

Describe the formula for changing lightspeed when changing mediums of different refractive indices.

Answer 16

v = c / n
v = speed of propogation
c = lightspeed
n = refractive index

Question 17

Define Snells law.

Answer 17

n sin (i) = n' sin (i')
i = incidence angle
i' = angle of refraction
n = refractive index

Question 18

Why does the sun appear red at sunsets/sunrises?

Answer 18

Due to the longer distance (direct path 30x longer), more low wavelength light is scattered out, resulting in redder light.