Saturday, September 14, 2013

Fireworks and the speed of sound

We were watching a fireworks display during our spring break vacation this year, and my son remarked that there is a delay in the sound that we hear from the fireworks that we see in the air.  Since the fireworks typically go off quite high in the sky, the delay can be a second or more. (This is the same reason why we see lightning before hearing the thunder and we can use the delay to estimate how far the lightning strike is.) Recall that the speed of sound is about 340 m/s and the speed of light is so high (about 300 Mm/s) that the delay is essentially the time it takes for the sound to reach our ears. I have never thought of that before, mainly because in a fireworks display many fireworks are going off at different times close to each other and this masks the delay phenomenon. After he mentioned it, I notice it now every time I watch fireworks (and there are many opportunities during the year) and makes me realize that the perfectly synchronized sound effects you hear in a movie when they have a fireworks display may not be accurate.

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