Polarisation.

Step 1 of 5

The first filter

Light is a wave of electric and magnetic fields, and its electric field always shakes across the direction the light travels: a transverse wave, like a wave on a rope. The direction the field shakes in is the light’s polarisation. In light from a lamp or the Sun that direction changes at random, very fast: the light is unpolarised. In the Scene, the yellow waves at different angles between the lamp and the first filter stand for that jumble.

A polarising filter passes only the part of the shaking along its transmission axis, and absorbs the rest. The light leaving it is polarised: it shakes in a single plane, along the axis. A sound wave, which shakes along the direction it travels, cannot be polarised.

Step 2 of 5

Malus’s law

The light reaching the second filter is already polarised, along the first filter’s axis. If the second filter’s axis is turned by an angle $\theta$⁠, only the part of the field along it gets through: the field times $\cos\theta$⁠. Brightness goes as the square of the field, so the share $\cos^2\theta$⁠ of the polarised light passes. This is Malus’s law:

$I = I_1 \cos^2\theta$$I =$$I_1 \cos^2\theta$⁠

$I_1$⁠ is the light that left the first filter. The chart beside the Scene shows $\cos^2\theta$⁠: the share of the polarised light that reaches the screen.

Step 3 of 5

A third filter

When the filters are crossed, at $90^\circ$⁠, $\cos^2 90^\circ = 0$⁠ and the screen is dark: the first filter left only an upright shaking, and the last passes only a sideways one. Predict what will happen when a third filter, turned to $45^\circ$⁠, slides in between them, then watch the Scene slide it in.

Step 4 of 5

Three quarters

The third filter is back up, and there are two filters again. This time the other way round: the amount of light is given, and you look for the angle. Work it out first, then turn Angle between filters.

Step 5 of 5

Glare

Light reflected from water, a wet road or a window is mostly polarised sideways. The light of a blue sky is partly polarised too. What do polarising sunglasses do with this?

Try it: with the filters crossed, at $90^\circ$⁠, imagine the light past the first filter is the glare, polarised one way, and the last filter is the sunglasses, at right angles to it.

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