Doppler Shift of a Yellow Light?

Question

If Doppler shift changes the frequency of light waves, how fast do I have to drive towards the intersection to make the light change from yellow back to green?

I'm driving a 6000 lb Ford Expedition at 45 mph when I see the light, 400 yards away, change to yellow.

I toss that in, because it seems like my tires are going to need to have a pretty big coefficient of friction if I'm going to be able to accelerate to a fast enough speed by time I reach the light.

Answer 1

Ha

Interesting speculation

1. The light will not change color no matter how fast you go. Only in your perception it will, so unless the law enforcement is traveling within your frame of reference you would be found guilty anyway.
2. True, if you are moving towards the light its frequency will increase towards the blue and will appear green to you, but the acceleration would kill you.
3. If you could travel with speed of light why would you care about traffic lights?

Good question anyway.

Answer 2

light coming toward us shifts to red, while light leaving us shifts to blue. you'd have to drive backwards at the yellow light until it shifts to green, then drive forward. but that will in turn make the light red again one you get close enough. you're forever stuck at the light. just wait it out, dude.

Answer 3

It depends on what your interpretation of yellow and green are. Many traffic lights now use LED lamps - so

For example:

This Green LED is centered at 518nM
http://www.rohm.com/products/databook/led/pdf/slr343.pdf

This yellow one at 591nM
http://www.rohm.com/products/databook/led/pdf/sli-343.pdf


So the speedup would be such that 591/c = 518/(c-speed)

i.e. speed = c*(591-518)/591 = ~12.3% of the speed of light.

Answer 4

you got it backwards, digdug. Light shifts towards blue when you accelerate towards it.

145,000 mph.

If you hear some deep-toned cyclical sounds behind you... that's the police siren.