3. Photoelasticity Flashcards
Photoelasticity relies on temporary birefringence…….. …… But what is it?
The property of a LOADED material having 2 refractive indices orthogonal to each other (involving right angles)
- vectors parallel to principal planes
- VELOCITY proportional to principle strains
Unload the material and the effect dissappears
The polariscope (pictured) is used to observe the effects of this birefringence. How does it work (with reference to polarisation states and what is actually produced)?
A backlight is employed. The light enters the first polariser, and then passes through the material as linearly polarised light.
It is birefracted and the two waves are out of phase, so that when they exit the material, the light is elliptically polarised.
The analyser, is a second polariser, which combines the two waves to produce interference fringes.
The resulting phase difference of retardation can then be observed.
Name 2 indicators of isochromatic fringes
One wavelength (colour). The difference between prinicipal stresses remain constant.
The interference fringes produced by the analyser are isochromatic unless what?
They are isochromatic unless the direction of the principal stresses are constant and coincide with the polariser axes, which would result in isoclinic fringes.
There are 4 types of polarisation:
Explain linear/plane polarisation
Linearly/plane (light vector fixed in space)
Described well by simple harmonic vibration (sin wave with direction of vibration in y-axis)
4 types of polarisation:
Explain circular polarisation
vector rotating with constant magnitude
4 types of polarisation:
Explain elliptical polarisation
vector rotating with sinusoidal magnitude
think of it either as in the picture, or as one resulting vector: the vector is propogating but it is changing it’s magnitude (according to simple harmonic motion aka sinusoidally) as it travels forward in an eliptical helix
4 types of polarisation:
Heres the trick one….
Unpolarised/randomly (no preferred vector orientation) Described well by a light source
Describe the electric vector of light for a harmonic plane wave moving in the z-direction in terms of A, v and lambda.
How does the analyser produce interference states
It combines the resulting vectors of the light waves, meaning the light is in different polarisation states, which in turn cause the interferance states.
The stress-optic law uses the phenomena to calculate stress. Derive it.
Using the coordinates in the picture, find the light intensity by resolving the vector on the exit.
Amplitude is asin2thetasin(alpha/2)
Light intensity is amplitude sqaured.
See picture for full calculation.
Recall the scenarios that produce either isochromatic or isoclinic fringes (as in picture).
Resolving the light intensity when i=0 for either scenario, show which part of the equation (for i) pertains to each scenario.
Isoclinic fringes show stresses of axes that coincided with the polariscope. Therefore Theta=0
Therefore (sin^2)*2Theta is the significant part
For i=0 when theta has a value (Isochromatic), alpha (phase shift) must equal 0. Therefore alpha=N2pi where N is an integer.
Therefore (sin^2)*(alpha/2) is the significant part for isochromatic fringes.
What do isochromatic fringes depend on?
The wavelength of light
What is the danger with isoclinic fringes, and what can be done with apparatus to remove this effect (normally we are only interestined in isochromatic)?
They can obscure the isochromatic fringes.
A circular polariscope can be used, which employs quarter wave plate between the polarisers and the material.
