Ever watch a race from a distance? You see the puff of smoke from the starting gun and the runners start running. Then you hear the bang of the starting gun. It’s not a false start, but why does that happen? Same thing when you see the fireworks explode before you hear them. Searching for the reason can be a great starting place for parents looking for ways to enhance their child’s at home education, but in the spring you don’t have to wait for a race or the Fourth of July. Just wait for a thunderstorm.
The difference in time between the flash of lightning and the clap of thunder can teach you some great science about the behavior of sound and light that you can take as far as your child’s interest and ability allows. And on a practical level, it can give him or her a potentially life saving skill.
The speeds of light and sound can easily be looked up and compared. Your student will soon discover that light travels faster than sound (or anything else, for that matter). So whether it’s seeing the flash of lighting or the puff of smoke, when you see it is when it happened. Any delay for the light to travel to you is so small you’d never notice it.
Sound is no slowpoke. Mach 1 is pretty fast, but not as fast as light, not by a long shot. That’s why the sound lags behind the light and you see it before you hear it. For younger children you can leave it at that, the faster runner (the lightning) crosses the finish line first and then later comes the slower runner (the thunder).
For more inquisitive students, this can be taken much farther. You can look into the speed of sound, find out that it varies and discover why. You can ask questions about the speed of light, why it is said that nothing can go faster, and what would happen if you try. (Watch out, you’re getting into Einstein territory there!) You will find other differences, too; sound needs something to travel through, like air, but light travels best through a vacuum. Why is that? Your high school physics students will have learned a lot if they can explain that to you in a way that makes sense.
For some practical application, imagine you’re out camping and see the flash of lightning then hear the boom of thunder. You can tell by the time between the two things about how far away the storm is. Count the seconds. When the next flash and boom happen, count them again. If the time is shorter, that means the second flash was closer and the storm is probably approaching. If the time keeps getting longer between the lightning and thunder, that is a sign that the storm is heading away from you. Next time we have a spring thunderstorm, you can practice this with your student from safely inside your home. Notice that when there is no time in between, the flash and the crack happen at the same time, the storm is right on top of you. (Most people notice that right away without being told.)
For some practice with math, you can have your student apply the rule of thumb that if the lighting is a mile away it will take 5 seconds for the thunder to get to you. This is only a rough estimate since sound travels at different speeds depending on air temperature and moisture content, and it echoes off of mountains too. But to get some idea of how far away the storm is, count the number of seconds between the flash and the bang and divide it by 5. So a 10 second delay means the storm is 2 miles away, 30 seconds means 6 miles, etc. But there is a much better rule of thumb to use around a thunderstorm. If you can hear thunder it at all, you should take cover in a safe place like a sturdy building or hard topped vehicle (as opposed to a convertible or open tractor). Like the National Weather Service says, “When Thunder Roars, Go Indoors.”