Answer1

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	WHAT'S YOUR ENGINEERING QUOTIENT?
Problem 1—You are out walking your dog at sunset. You notice that there is a light sensor at the top of the streetlight next to you. This sensor is used to activate the lights along the street. The street lamps are around 100m (100 yards) apart. You notice that as the lights come on, the nearest light comes on first and the farthest light just a fraction later. If you assume that you can distinguish between two different events about 1/60 second apart, how fast did the sensor signal propagate between the lights? There is only one problem — yesterday you were walking the dog at the other end of the street when the lights turned on. Then the nearest light again came on first, and the light with the sensor on top — that turned on last! Answer: This has nothing to do with the speed of electricity down the sensor wire. It has more to do with the apparent 3D effect when you look at the TV using just one lens of a pair of sunglasses. The far light is less bright than the nearer light. This results in a delay of a few milliseconds between when the brain considers the far light to have turned on relative to the brighter closer light. According to Jearl Walker in The Flying Circus of Physics (ISBN 0-471-02984-X), there is an even more interesting effect if you are walking your dog on a hill over looking a series of roads and intersections. I have not seen this effect, but apparently the group of lights at the intersections appear to come on first as they are collectively brighter than the individual street lights. Contributor: Michael Smith 5-01 — NEXT Q&A For questions or comments about Test Your EQ, e-mail eq@circuitcellar.com.