Malux

Meet Malux, a striped filter who decides how much light gets through just by turning.

Light is a wave of electric field that wiggles in some direction. A polarising filter only lets through the part of that wiggle lined up with its own axis. If light that is already polarised meets a second filter, the share that passes depends on the angle between the light's direction of wiggle and the filter's axis.

Malus's law says the transmitted intensity is I = I₀cos²θ. At θ = 0 everything gets through. At θ = 60°, cos²θ = 0.25, so only a quarter of the light passes. At 90° the filters are crossed and almost nothing gets through.

Ordinary sunlight is unpolarised, so the very first filter it meets always passes about half of it, whatever the angle.

Polarised sunglasses use this to cut glare. Light reflecting off water or a road is partly polarised, and the lenses are turned so that this light is blocked.