What's this about polarised sunglasses being bad for cycling (hexbear.net)

submitted 2 weeks ago by doublepepperoni@hexbear.net to c/urbanism@hexbear.net

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Finally got a pair of prescription sunglasses for summer, but apparently polarised lenses are potentially dangerous for cyclists according to Redditors due to light playing weird tricks on you. On my first day wearing them the only interesting thing I noticed was my phone screen shimmering like a foil trading card

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[-] SpookyVanguard64@hexbear.net 9 points 2 weeks ago* (last edited 2 weeks ago)

Some LED screens look dark at certain angles as well

For anyone curious as to why this happens, it's 'cause a lot of screens have a polarization filter on them as well, and the difference between the polarization angles of the two filters determines how much light from the first filter (i.e. the screen) makes it through the second filter (i.e. your sunglasses).

Longer (but still oversimplified) explanation:

Photons are emitted with random polarization angles relative to one another, and a polarization filter will selectively let photons through based on how closely they match the filter's polarization angle. If the difference between the polarization angle of the photon and filter is 0 degrees, the photon passes through without problem. If the difference is 90 degrees, the photon is completely blocked. And of course, there's a gradient on anything in between. The end result however is that all photons that pass through the filter will now have a polarization angle that matches the polarization angle of the filter.

So when you have light passing through two polarization filter (i.e. an LED screen and polarized sunglasses), the amount of light that makes it through the second filter is entirely dependent on it's polarization angle relative the the first filter. If the two filters are perfectly aligned, then 100% of the light coming through the first filter will make it through the second filter, since all of the light coming through the first filter will be polarized at the same angle as what the second filter will allow through. But if that second filter is then rotated 90 degrees, suddenly none of the light from the first filter will make it through the second, since all of the light that makes it through the first filter will be polarized at a 90 degree angle from what the second filter will allow through. And naturally, any angular difference that's between 0 and 90 degrees will allow between 100% and 0% of the light through.