Put out the lights on the age of reason

[venta]

Help me, LJ, you (and your knowledge of physics) are my only hope.

Why do LEDs bounce up and down in car mirrors?

Comments

[zotz]

I've seen similar illusions, and I've seen it mentioned in print, but I don't know what it's called.

[venta]

I notice you stop short of affirming that I am not insane :)

[zotz]

There is surely no room for doubt concerning your sanity.

It sounds like it should be related to the illusory movement described here, which even mentions car mirrors, but annoyingly doesn't give it a name. All the examples I can think of do involve vibration, but I don't know why it would be more obvious with some images, such as LEDs, than with others.

[venta]

That's interesting. Although, as you say, it doesn't really explain LEDs vs. everything else, I didn't know you could produce stroboscopic conditions just by vibration. I may develop an interest in eating carrots, now...

[huskyteer]

I've noticed something similar with LED cats-eyes; I find it really distracting. I've always assumed it was because they're very small and very bright.

[venta]

Actually, now you mention it, so have I. When they first installed lit cats-eyes on the A4074 between Reading and Oxford a few years ago, I noticed that they were very distracting in the wing mirror.

I'm not sure whether they were actually bouncing about, or just unexpectly present.

[bateleur]

Well, I don't know what the actual answer is, but here's a completely mad theory: is it possible that you're seeing interference patterns due to the fact that all of these sources use multiple small LEDs and the light from LEDs is usually a pure, single frequency? The vibration could be moving your direction of vision rapidly enough relative to a fine-grained interference pattern that it produces a sense of flickering even though you would not normally be able to perceive the interference.

[venta]

I'd say it very much doesn't produce a sense of flickering - it's definitely a sense of movement without flickering.

But I'd have bought the argument if only you'd ended with more plausible handwaving!

[bateleur]

Just as well I didn't then!

(According to zandev I have a particular talent for persuading people of things that aren't true. Normally in the context of mathematics, but it's possible it works for physics too.)

[venta]

I can't judge your skills in that direction, because I'm extraordinarily easily persuadable of almost anything!

[ar_gemlad]

I blame polarisation. I have nothing else to add, because I haven't had enough coffee for science, and I've had too much coffee for batshit handwaving.

[venta]

Polarity has a lot to answer for.

Surely lots of coffee makes for excellent batshit hand-waving?

[ar_gemlad]

I shall have to drink more coffee. For science.

[lanfykins]

Here comes the war...

...or to put it another way, I have rear-view mirrors? :)

[venta]

Your kudo is temporarily suspended until you assure me you're joking about the mirrors ;)

[lanfykins]

I am joking about the mirrors. I actually rely on them quite significantly.

And now, I'm going to be glued to em watching for the phenomenon you mention...

[venta]

Phew, I shall release the kudo :)

Please do also remember to look forwards. I hear it's qutie important, too.

(Best phenomenon-spotting time is, I think, on the motorway at dusk.)