How Do 3D Glasses and Polarised Sunglasses Work?

3D glasses and polarised sunglasses both work by filtering light: letting some of it through and blocking the rest. Old-style red-cyan 3D glasses filter by colour. Each lens passes one colour and blocks its opposite, so each eye sees only the image printed in its colour, and the brain fuses the two into depth. Modern cinema 3D glasses and polarised sunglasses filter by the orientation of the light wave instead. Light can vibrate in any direction; a polarising filter passes only one direction. Cinema glasses give each eye a different direction. Sunglasses block the direction that glare happens to have.

• All of them are filters: they let some light through and block the rest.
• Red-cyan 3D glasses filter by colour so each eye sees a different image.
• Cinema 3D glasses and polarised sunglasses filter by the direction the light wave vibrates.
• Test polarisation by rotating polarised sunglasses in front of a phone screen.

Here is each one properly.

By colour subtraction. A 3D image of this kind is two pictures of the same scene, taken from slightly different positions, printed on top of each other: one in red, one in cyan. The red lens passes red light and blocks cyan, so the eye behind it sees only the cyan-printed picture as dark lines on a bright ground, and the red-printed one disappears into the red glow. The cyan lens does the reverse. Each eye now sees one picture, from one viewpoint, and your brain does what it always does with two viewpoints: it builds depth. Red and cyan are chosen because they are complementary, which is exactly the relationship that makes cyan one of the three subtractive primaries. Cyan is white minus red. The three colours that cannot be mixed article explains why.

The cost is colour. Because each eye is seeing through a strong filter, the fused image looks washed out and strangely tinted, which is why this method gave way to polarisation.

Light is a wave, and the wave can vibrate in any direction at right angles to the way it is travelling: up and down, side to side, or any angle between. Ordinary light from the sun or a bulb is a jumble of all directions. Polarised light is light in which the vibration has been sorted into a single direction. Some surfaces do this naturally: light reflecting off water, glass, wet roads and car bonnets at a shallow angle comes off mostly polarised horizontally. That is glare. A polarising filter is a material with microscopic aligned chains that pass vibration in one direction and absorb the rest.

By blocking horizontal vibration. Glare from a road or a lake is mostly horizontally polarised, so a lens whose filter passes only vertical light removes the glare and keeps most of the rest of the scene, which is coming at you unpolarised and so loses only half its brightness. That is why polarised sunglasses make a wet road look darker and a lake suddenly transparent: the reflected sky has been taken off its surface. It is also why they do odd things to some screens. A phone or laptop display emits polarised light, and if its direction happens to line up with the direction your lens blocks, the screen goes dark. Tilt the phone ninety degrees and it comes back.

By giving each eye its own polarisation. The projector shows two images at once, or alternates them very fast, polarised in two different ways. The left lens passes one and blocks the other; the right lens does the reverse. Each eye receives its own viewpoint in full colour, and the brain builds depth exactly as it does with red-cyan glasses, but without the colour loss. Most cinemas use circular polarisation, in which the vibration direction rotates as the wave travels, clockwise for one eye and anticlockwise for the other, because that keeps working when you tilt your head. Linear polarisation, the simple up-down versus side-to-side kind, breaks down as soon as you lean.

With sunglasses and a screen. Look at a phone or laptop screen through polarised sunglasses and slowly rotate the glasses. At one angle the screen looks normal; ninety degrees away it goes dark or shows strange colours. That is your lens crossing the screen's polarisation. Two pairs of polarised sunglasses held one behind the other and rotated will go from clear to black in the same way. And on a clear day, look at a patch of blue sky about ninety degrees from the sun through one lens and rotate it: the sky darkens and brightens, because scattered skylight is partly polarised too.

They are both filters, doing the same fundamental job in two different dimensions. Red-cyan glasses sort light by wavelength. Polarised lenses sort it by vibration direction. A CMY Cube is the wavelength version made hand-held: its cyan face is exactly the cyan lens of a pair of old 3D glasses, removing red from whatever passes through, and its magenta and yellow faces remove green and blue respectively. Look at a red-cyan 3D picture through the cyan face of the cube and one of the two images vanishes, precisely as it would through the glasses. Then turn the cube and watch the overlaps make colours the glasses never could. The CMY Pack gives you all three filters as separate cubes to play with.

Because filtering is how almost all colour and a good deal of imaging works. Your eye filters by wavelength with three kinds of cone. A camera sensor filters by wavelength with a mosaic of tiny red, green and blue filters. Photographers use polarisers to darken skies and cut reflections. LCD screens are two polarisers with a layer between them that twists the light on command, which is how every pixel turns on and off. Once you have understood a pair of 3D glasses, you have understood the screen you are reading this on.

Red-cyan 3D glasses filter by colour, so each eye sees a different image and the brain builds depth, at the cost of washed-out colour. Cinema glasses and polarised sunglasses filter by the direction the light wave vibrates: cinema glasses give each eye a different direction, sunglasses block the horizontal direction that glare has. Test it by rotating polarised sunglasses in front of a phone. A colour cube is the same idea, sorting light by wavelength in the palm of your hand.

Hold three filters at once, and see light through a different lens.