How do polarising lenses work?

Polarising lenses are more effective at reducing glare than ordinary tinted lenses. Why is this so and what is the distinction?

The true explanation lies in quantum electrodynamics, however for our purposes classical physics will do nicely.

When light is emitted from a source it is transmitted as waves

Hold on, isn’t light made up of particles called photons? – The wave-particle duality is explained well by quantum mechanics but that would be a very large detour to take here! As previously stated, we will stick to classical physics here.

Those waves of light may vary in wavelength – thus determining their colour, but they may also vary in their orientation – that is the plane in which they oscillate. Depending on the light source, we may find that light is transmitted with a combination of different orientations. This is referred to as unpolarised light, and light transmitted from a source such as the sun is an example.

When unpolarised light hits a shiny non-mirrored surface, some of the light is absorbed and the rest is reflected. The interesting thing is that the reflected light has become polarised – all the waves are travelling in the same plane.

Not all with the same phase – phase refers to where the start and end points of the wave are. If they were all travelling with the same phase, the light would be referred to as highly coherent, that is - a laser

When a spectacle lens is referred to as polarised, it is designed to allow only light travelling in a certain plane to transmit through the lens. This polarising filter is essentially a grid with very small spacing so that only the light that is in-plane with the grid can pass through. This is the very reason why the lens is dark – it only allows a proportion of light through the lens.

Spectacle lens manufacturers now employ a clever trick. They set the axis of polarisation so that the polarised light from horizontal reflections cannot pass through the lens. It is the reflections from horizontal surfaces that are most troublesome to us. Examples include a wet road, or reflections from a lake. With polarised lenses, the road is virtually glare-free and anglers can see the fish underneath the water.

Some artificial light sources are already polarised, including LCD screens. If one were to view an LCD screen with polarised lenses at a certain angle, they would notice the screen goes black. It is for the same reason that heads up displays in cars become invisible with polarised lenses!

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