Infra Red Sunglasses?

[Tiberius]

Okay, so a while ago I needed to get a new pair of sunglasses, as the metal frames on my previous pair broke.

So I got a pair of plastic ones with mirrored lenses.

Now, the weird thing is that I noticed that sometimes when I looked at black objects, they'd have a kinda purply tinge. Looked kinda iridescant, but not, if that makes any sense. The same colour, but without the shimmeriness. And yet other black objects would not have this shimmery purply tinge. I was curious, but just assumed that it was a result of the tinting of the lenses. When I look without the sunglasses, it looks totally colourless. There's no sign of the colour tinge unless I am wearing the glasses.

Then, I got a toy version of night vision goggles, which works with an infra red light and a small infra red sensitive camera. And with that I've noticed that thoe black things that have the purply tinge through my sunglasses are also very reflective in infra red light. So the purply tinge I see with the sunglasses is definitely related to infra red light.

But how can it work? I admit that my knowledge of filters is not complete, but I find it unlikely that the sunglasses are altering the frequency of infra red light so that I can see it. And it's not like I have some kind of weird condition (superpower?) where the cells on my retina are sensitive to infra red light (or I'd be able to see it without the sunglasses).

So what could be going on?

 

[Holdfast]

Difficult to be sure from the way you're describing it, but if you're seeing an "oil on water" type coloured effect with some materials but not others when wearing the sunglasses, they've probably got a polarising filter on them. Try tilting your head one way and then the other while wearing them. If the "iridescence" changes, it's the polarising effect you're noticing.

Materials under stress (eg formed glass and plastic) show this up beautifully, if you catch the angle right under polarised light. I notice this while driving, as my car windows (though thankfully not the windscreen) have a polarising filter on them too. That means that there are two polarising filters (the windows, and my sunglasses), so you can see the stress patterns in the glass of the windows.

Any time you have two polarising filters at work at the same time, and clear(ish) objects under stress in between the filters, you'll see this effect. It's fun; sort of like a low-tech version of the stress analysis you sometimes hear Geordi doing in TNG with his visor.

Interestingly, your sunglasses themselves likely have stress points in them. If you view them in a mirror and angle yourself carefully, you display those stress patterns too.

 

[Jadzia]

Interestingly, your sunglasses themselves likely have stress points in them. If you view them in a mirror and angle yourself carefully, you display those stress patterns too.

And if you want even more fun, try introducing some magnets.

When light reflects in the presence of a magnetic field, it undergoes partial polarisation. This is how the Sony MiniDisc system works ~ by scanning the magnetised disc surface with a laser, and observing any change in polarisation of the reflected beam.

But how can it work? I admit that my knowledge of filters is not complete, but I find it unlikely that the sunglasses are altering the frequency of infra red light so that I can see it.

If you believe you are seeing infra-red, then try looking at the end of a television remote control, and pressing one of the buttons. The light from the LED is infra-red (940nm), and invisible to the naked eye.

Just be careful you don't switch yourself off in the process.

 

[iguana_tonante]

Yeah, polarization was my first thought, too. Infra-red light seems unlikely.

 

[Tiberius]

Definitely not polarisation. I'm a photographer and quite familiar with the effects of polarising filters. This is a very different effect, and the sunglasses aren't polarised anyway.

The reason I suspect that it's connected with infra red light is that every single thing that has this effect with the sunglasses is quite reflective in infra red light (determined with my night vision). It doesn't work on other objects. I can't see the light on a remote control however. But that could be because it only works on the infra red frequencies closer to the red end of the spectrum...