Polarisation and Malus’s Law
Lesson notes
Overview
A polariser passes only the field component aligned with it. This topic is rated intermediate and is number 58 of 227 in Physics. Builds on the basics with relationships, methods and trade-offs.
What you are seeing
The panel above shows a graph of the function. Each slider changes one parameter, so you can see exactly how the curve bends, shifts or stretches. The animation is drawn live from the model, so everything you see responds to the sliders. Press Space to pause, use Restart to replay from the start, and change the speed to slow down quick events.
In depth
Light is a transverse wave, and in polarised light the electric field oscillates in one direction. A polarising filter passes only the field component along its axis, so for polarised light of intensity I₀ at angle θ to the axis, the transmitted intensity is I = I₀ cos²θ, Malus’s law. Two crossed filters block all light, but a third filter between them at 45° lets some through. Polarised sunglasses reduce glare, and LCD screens depend on polarisation.
Reading the relationship
The expression "I = I₀ cos²θ" is a compact summary of the idea. It uses the symbols I, I₀, θ; name what each one stands for before you start. Notice that it contains a squared term, so doubling that quantity changes the result by a factor of four, and a trigonometric function, so the result repeats as the angle goes round. Then use the sliders to see how changing one quantity changes the others.
A guided exploration
1. Set every slider to its middle value and note the shape of the curve. 2. Move one slider at a time and describe whether the curve shifts, stretches or turns. 3. Find the value where the behaviour changes sharply, then connect it to the formula.
Try this
Predict the outcome first, then test extreme slider values to find where the pattern changes or breaks.
Common pitfalls
Check units and directions first. Many mistakes come from mixing up a scalar with a vector, or from using a formula outside the conditions where it holds.
How it connects
It builds on “Double-Slit Experiment”, the topic before it in Physics. Once you are comfortable, continue with “Mixing Coloured Light”.
Why it matters
Physics explains how the world moves, heats, shines and interacts, and the same principles power technology around you.
Where you might use it
- Revising “Polarisation and Malus’s Law” before a class, exam or certification
- Designing vehicles, bridges and devices where motion, forces or waves must be predicted
- Understanding how sensors, lasers and communication links actually work
- Explaining everyday phenomena such as sound, light, heat and orbits
Frequently asked questions
What is Polarisation and Malus’s Law?
A polariser passes only the field component aligned with it. It is an intermediate topic in Physics.
How do I use the Polarisation and Malus’s Law simulation?
The panel at the top shows a graph of the function. Each slider changes one parameter, so you can see exactly how the curve bends, shifts or stretches. Move the sliders one at a time, then press Space to pause the animation and look closer.
What does the formula mean?
The expression I = I₀ cos²θ packs the idea into symbols. Name each symbol first, then change one quantity with the sliders and see which others respond.
Who is this topic for?
Builds on the basics with relationships, methods and trade-offs. If the quiz feels hard, reread the lesson notes and try the previous topic first.
What should I learn before this?
Start with “Double-Slit Experiment”, the previous topic in Physics. Each topic in a subject builds on the ones before it.
How do I track my progress?
Press “Mark as learned” on this page. Progress is stored only in this browser and is never sent anywhere. The Browse page can show topics you have learned or still need to learn.