While wandering around downtown a few years ago, I snapped a shot of a car in motion by panning my camera to match the car’s speed. I thought it was a pretty boring shot until I noticed a pulsing pattern on all the lights in the background.
My friend John and I put on our science caps and figured out what was going on.
Our power grid operates at 60Hz (cycles per second).
- Open the full size image and count the number of pulses on the lights. For the lazy, each has 9.
- Note the shutter speed. Again, for the lazy, the shutter was open for 1/13 of a second.
- Multiply the 9 by 13 to get the number of pulses there would be in a second given that we know that 9 occurred in 1/13th of a second.
We arrive at ~120.
Didn’t we just say that our grid runs at 60? Did we mess something up?
Our power grid runs on an alternating current (much to the disappointment of Tesla edit: Edison). This means that current in our wires has two peaks as it switches between positive and negative voltages once every cycle (-110V and +110V).
Since bulbs shine brightest at voltage peaks, there are two bright spots per cycle. This is why we have double the number of pulses that we might have assumed.
120 / 2 pulses/cycle = 60 cycles per second or 60 Hz.
Which matches what we know about our grid.
Bonus: speed of the car
Eyeballing the image, I’d guess that the streak just to the right of the car is almost 1/3 the width of the Civic’s length which happens to be 176 inches.
We know that the exposure ran for 1/13 of a second so the car is travelling 686 inches =~ 17m/s which is ~60km/h.
Another bonus: car lights
As pointed out by scienceduck, the car’s lights aren’t pulsing. This is because instead of being hooked up to to the grid, they’re running off the car’s direct current battery which does not cycle the same way as AC.
Taking it up another level
Maybe there’s some lessons about the red shift waiting to be teased out… ;)