Why do we see lightning first?
A plain-English answer for grown-ups of kids aged 6–9, taken from the Yufena lesson that explores this question.
This lesson is free to try. No card, no sign-up.
The answer, for parents
Short answer: Light is super fast and sound is much slower, so we see the flash first and hear the boom later.
A storm cloud makes a giant spark called lightning. The spark makes light and sound at the same moment. The sound is called thunder. Light is super fast. In one second, it could go around the Earth seven times. So the flash reaches your eyes almost at once.
Sound is much slower. It takes about three seconds to travel one kilometre. So the boom arrives later. The farther away the storm, the longer you wait. Thunder rumbles for a long time. The bolt is very long, so its sound arrives bit by bit. Sound also bounces off hills and buildings. This bouncing sound is an echo.
In the lesson, your child puts this cause-and-effect chain in order: Lightning makes light and sound together → Light reaches us almost at once → Sound travels much more slowly → We see the flash first → The boom arrives later.
What if the flash and boom come together? The lightning is very close. Go indoors right away. When thunder roars, go indoors.
Can we see lightning from far away? Yes. At night you can see a flash from very far. But the sound fades, so you may not hear thunder at all.
Try it with your child
Let's count a storm. Stay safe inside, away from the window. When you see a flash, start counting seconds. One, two, three... Stop when you hear the boom. Now divide your number by three. That is how many kilometres away the lightning was. Five seconds is not even two kilometres!
Safety: Safety first: if you hear thunder, go indoors. Stay in until 30 minutes after the last thunder. Safety first: thunder means the storm is close enough to be dangerous. Go indoors.
Explore it together in the free lesson
Guess, find clues, build the chain and try the experiment. Read it together.