Polarisation and Malus's law – I = I₀cos²θ, transverse waves through slits, reflection and sunglasses
PhysicsOscillations & WavesAges 16–17
Loading…
Sign in to playA virtual practical on polarisation: light from a lamp passes through a polariser and an analyser; turn the analyser and read the light meter to test Malus's law I = I₀cos²θ, with a results table and graphs of I against θ and against cos²θ. You can insert a third filter in the middle or switch to microwaves with a metal grille. The Waves at slits screen shows why only transverse waves can be polarised; the Reflection screen calculates Fresnel reflection, the Brewster angle and the effect of polarising sunglasses.
Lesson: Polarisation: light as a transverse wave, polarising filters, Malus's law, polarisation by reflection and the Brewster angle
What it shows
Light is a transverse electromagnetic wave: its electric field oscillates at right angles to the direction of travel. Ordinary light is unpolarised, with the field in all directions across the beam. A polarising filter lets through only the component along its transmission axis, so it halves the intensity of unpolarised light. A second filter at angle θ passes the component E₀cosθ, and because intensity is proportional to amplitude squared, I = I₀cos²θ (Malus's law). Longitudinal waves cannot be polarised. Reflected light is partly polarised and fully polarised at the Brewster angle, tan θB = n.
How to use
On the Malus's law tab, keep Polariser P fixed, set Analyser A with the slider or by dragging sideways on the picture, and press Record at several angles. Switch Graph to I against cos²θ and read I₀ from the gradient. Try Add a middle filter between crossed filters, Room lights on, or Source: Microwaves. On the Waves at slits tab, change Wave, Shake direction and Slit 1. On the Reflection and sunglasses tab, change Incidence angle i and Polarising sunglasses.
Parameters you can change
- Screen Malus's law, Transverse and longitudinal waves through slits, Polarisation by reflection, sunglasses
- Source Light (lamp), Microwaves
- Polariser (or transmitter) axis P 0–180 °
- Analyser (or receiver) axis A 0–360 °
- Middle filter (or grille) in place
- Transmission axis of the middle filter M 0–180 °
- Room lights on
- Wave (Waves at slits screen) Transverse wave on a rope, Transverse, shaken in all directions, Longitudinal wave on a spring
- Shake direction 0–180 °
- Slit 1 direction 0–180 °
- Second slit in place
- Slit 2 direction 0–180 °
- Reflecting surface Water (n = 1.33), Glass (n = 1.50)
- Angle of incidence i 0–89 °
- Polarising sunglasses Not worn, Transmission axis vertical, Turned 90° (axis horizontal)
Questions to explore
- What reading do you expect at θ = 45°, and why is the graph of I against cos²θ a straight line?
- Why does a third filter at 45° let light through two crossed polarisers?
- Why can a polarising filter not block a longitudinal wave such as sound?
