Week 3 Gravitational Lensing & Polarisation

Question 1

briefly describe the snellian approach to gravitational lensing

Answer 1

mass bends light so positions are calculated by finding bending angles

Question 2

briefly describe the fermattian approach to gravitational lensing

Answer 2

light suffers from a delay called shapiro delay so find a compromise minimum path between most direct and most bent

Question 3

what are the four key properties of Linear Polarisation

Answer 3

> E remains in the same plane as the wave propagates
plane containing E is at angle arctan[Ey/Ez] to x axis
max E is SQRT[Ex^2 + Ey^2]
no phase shift between x and y

Question 4

what are the two key properties of circular polarisation

Answer 4

> E remains same size but rotates around direction of propagation
phase shift of +-90 between x and y

Question 5

what is the elliptical polarisation equation

Answer 5

XExcos[kx-ωt+φ1] + YEycos[kx-ωt+φ2]

Question 6

what are the 3 key properties of the elliptical polarisation equation

Answer 6

> linear if φ1 = φ2
circular is φ2 - φ1 = +-90
E describes an ellipse with time at any point

Question 7

what is the condition for full polarisation by reflection

Answer 7

if the reflected ray is orthogonal to refracted ray then reflected ray is fully polarised

Question 8

what is the Brewster angle equation

Answer 8

tanθi = nr / ni

Question 9

what do the Fresnel coefficients do

Answer 9

they give the degree of polarisation for arbitrary angles

Question 10

what are the two Fresnel coefficients equations

Answer 10

|b|^2 = [sin(θi-θr)]^2 / [sin(θi+θr)]^2
|B|^2 = [tan(θi-θr)]^2 / [tan(θi+θr)]^2

Question 11

what is the degree of polarisation equation

Answer 11

|b|^2 - |B|^2 / |b|^2 + |B|^2

Question 12

briefly describe how polarising by scattering works

Answer 12

light passes through a scattering medium
depending on the position of the observer relative to the scattering medium the observer will see different amounts of polarisation