# Is there a way to create a flickering frequency to be dependent on speed of the person looking at it?

Is there a way to make a screen or a road sign flash at different rates, depending on the velocity of the observer looking at it?

• I would like to achieve a state where two observers going at different speeds would see the screen flash at different rates at the same time.

• another thing i would like to check if possible is - is there a way to make the observer see a different image depending on his/her velocity? (without a radar).

-

You can use a reflector with gaps. Then the light from a car will alternate between reflecting and not reflecting at a rate dependent on their velocity towards the reflector. Please excuse my crude diagram:

As the car moves right to left, gaps in the reflector will cause it to appear to flash on an off.

-
but how will the car driver see the whole image on the screen behind the reflector? – Uri Abramson May 8 '13 at 9:31
@UriAbramson use lenticular printing: en.wikipedia.org/wiki/Lenticular_printing – Brandon Enright May 8 '13 at 15:45
This is brilliant. On reading the question my mind went "doppler effect". Never would have thought of this :) – Manishearth May 8 '13 at 15:51
cool! Thanks a lot!! now... how about the second bullet in my question? :) – Uri Abramson May 9 '13 at 10:39

Due to the speed of light, the "dopler shift" would be too small, at the usual speed of the observers.

-
This is true however if the flickering source is a reflector and the light source is the moving observer (via car headlights) then it's not so hard. – Brandon Enright May 7 '13 at 15:43
I suppose you could also achieve it via some mechanism. for instance a rotating device that would appear to "flicker" depending on how fast you pass it. It wouldn't work well for things far away, but near the point of closest approach you could set something up so that what you see depends on how fast you are going. – Jim May 7 '13 at 19:44
isn't there a filter we could setup to make the wave lengths longer and still achieve the dopler effect? and.. what on earth is a reflector? :) – Uri Abramson May 8 '13 at 14:28
The second bullet was added after my answer. I can't think of a way to do it. We're delving into engineering and geometry though so I don't think Physics.SE is a particularly good place for your question as it is currently written. – Brandon Enright May 9 '13 at 23:06