Polarisation
- Polarisation is when:
Particle oscillations occur in only one of direction perpendicular to the direction of wave propagation
- Polarisation can only occur in transverse waves
- This is because electromagnetic transverse waves are oscillating electric and magnetic fields in any plane perpendicular to the propagation direction
- When transverse waves are polarised, this means:
- Vibrations are restricted to one direction
- These vibrations are still perpendicular to the direction of propagation / energy transfer
- The difference between unpolarised and polarised waves is shown in the diagram below:
Diagram showing the displacement of unpolarised and polarised transverse waves
- Light from the sun or a lightbulb, or any that hasn't gone through a polarising filter is unpolarised
- Longitudinal waves (e.g. sound waves) cannot be polarised
- This is because they oscillate parallel to the direction of travel
- Waves can be polarised through a polariser such as a polarising filter (light) or metal grilles (microwaves)
- This only allows oscillations in a certain plane to be transmitted
- This is called a plane-polarised wave
Polarising Filters
Diagram showing an unpolarised and polarised wave travelling through polarising filters
- Diagram A shows:
- Only unpolarised waves can be polarised
- Diagram B shows:
- When a polarised wave passes through a filter with a transmission axis perpendicular to the wave, none of the wave will pass through
- Light can also be polarised through reflection, refraction and scattering
Metal Grilles
- A metal grille is similar to a polarising filter, and commonly used for polarising microwaves
- Microwaves are electromagnetic waves with a longer wavelength than visible light
- By convention, the direction of polarisation is only the direction of the electric field propagation (rather than magnetic)
- A metal grille is different from a polarising filter
- The free electrons moving in the metal bar can cancel out the electric field in the same direction as the grille and completely absorb it
- Thus the horizontal electric field passes if the grilles are positioned vertically and vice versa
- This is the opposite of what happens in a polarising filter
A metal grille is used to polarise a microwave
Investigating Light Intensity with Two Polarisers
- If an unpolarised light source is placed in front of two identical polarising filters, A and B, with their transmission axes parallel:
- Filter A will polarise the light in a certain axis
- All of the polarised light will pass through filter B unaffected
- In this case, the maximum intensity of light is transmitted
When both polarisers have the same transmission axis, the intensity of the transmitted light is at its maximum
- As the polarising filter B is rotated anticlockwise, the intensity of the light observed changes periodically depending on the angle B is rotated through
- When A and B have their transmission axes perpendicular to each other:
- Filter A will polarise the light in a certain axis
- This time none of the polarised light will pass through filter B
- In this case, the minimum intensity of light is transmitted
When one of the polarisers is rotated through 90°, the intensity of the transmitted light drops to zero
- The resulting graph of the light intensity with angle, as the second polariser is rotated through 360°, looks as follows:
Graph showing how the intensity of the transmitted beam varies with the angle between the transmission axes of the two polarisers
Worked example
Which statement below describes a situation in which polarisation should happen?
A. Radio waves pass through a metal grid
B. Surface water waves are diffracted
C. Sound waves are reflected
D. Ultrasound waves pass through a metal grid
ANSWER: A
- Radio waves are transverse waves - they can be polarised by a metal grid so only the waves that fit through the grid will be transmitted, therefore, A is correct
- B cannot be correct as waves are not polarised when diffracted, but are polarised only when reflected, refracted or scattered
- C & D cannot be correct as polarisation only occurs for transverse waves, therefore, C & D can be ruled out as sound and ultrasound are both longitudinal waves
Examiner Tip
You may be expected to describe the intensity, or even draw the graph of intensity v angle, for light with two polarisers.