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Commuting today, I noticed that the car behind me had LED headlights that appeared to "bobble" within their lenses--in other words the reflection of the car and the reflection of the headlight lamps moved in opposite directions.

The illusion appeared only in my rear view mirror and not in my wing mirrors. When I held the rear view so it could not shake the illusion disappeared.

Since it had to do with the vibration of the rear view mirror I could only think of two things: some sort of path-length difference effect that is not the same for the car's image as for the LEDs' images, or some sort of frequency difference between the vibration of the rear view mirror and the LED lamps. (I always think of LEDs as being DC but I don't know how they are implemented in cars.)

Has anyone else noticed this illusion? Does anyone else have an explanation?

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  • $\begingroup$ LED lamps are not DC driven, but are driven by switching current sources, for efficiency reasons. The switching frequency, however, is usually of a few hundred kilohertz, which makes it improbable to observe the beat note, but maybe there was some particular condition. $\endgroup$ – Massimo Ortolano Apr 19 '17 at 20:21
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Your eyes respond differently to bright lights and to dim lights - basically there is some "low pass filtering" in the eye to remove noise from a dim image.

When you see a (dim) car with a (bright) light, and the entire image moves (maybe because of motion of the mirror with which you view it), then your eye sees the bright source move "immediately" while the dimmer source "lags".

It's really more biology than physics, but I am pretty sure that is the mechanism behind your observation.

This is a variation of the Pulfrich effect

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I did figure it out. Floris's explanation is close but not quite.

I used a window. I looked outside at a bright yellow fire hydrant against a dark background and I moved my head slightly.

What I saw was a reflection from one of the window surfaces of the fire hydrant, displaced slightly, that moved opposite to the direction the fire hydrant appeared to move when I moved my head. Of course everything else visible was also reflected from a window surface and moved in the same way, but the image of the background was so dim it was difficult to notice.

My rear view mirror of course has more than one reflecting surface.

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