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?

I noticed ages ago that, when driving down the motorway, the LED variable-speed-limit signs bounce around crazily when seen in the rear-view mirror. Obviously in a mirror you're looking at the other carriageway's signs, and the road, cars, streetlights, gantry etc are all more or less stationary but the LED signs? Well, they're joggling around like wild things.

Trundling back on Monday from a trip to Scotland, I spotted that some cars (mostly newish Mercs, I think) now have a strip of white LEDs under their normal headlights. When viewed through the rear mirror the car and the headlights look perfectly sensible, while the LEDs are - you'll have guessed - madly dancing around. I don't know if our car has an unusually vibration-prone mirror, but in general the image in the mirror seems stable.

I haven't noticed this driving at non-motorway speeds - though I'm also not sure if I've had the requisite LEDs-behind-me in any other situation.

I just asked Physics!colleague about it, and his first theory ran thusly: LEDs have quite a narrow field of view (compared to other lights), so if the object in the mirror is offset (as a sign on the other carriageway would be) you might be on the edge of the field of view, and the effect is that the light pops in and out of vision. This feels inherently wrong to me: I'd expect the sign to look more flickery than it does. Also, a Merc driving in the lane behind me is not offset.

His second theory (bolstered by a quick google) was that there is no such effect, I am insane, and ChrisC (who claims also to have observed it) is just humouring me.

So... has anyone else seen this happen? Anyone know why it happens?

Comments

[bateleur.livejournal.com]

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.

</implausible_handwaving>

[venta.livejournal.com]

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.livejournal.com]

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.livejournal.com]

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