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The Speed Difference Between Light and Sound and Why It Matters
A cloud hangs in the sky. You see a flash of light. After a moment, you hear thunder.
One thing is happening in the sky. Why aren't we receiving the light and sound at the same time?
I'll tell you why.
The light and sound waves are moving at two different speeds.
What does that mean for the thunder and lightning?
🌩️ Something Happens in the Sky
Lightning is an electrical phenomenon resulting from activity.
It can strike within a cloud, between clouds, or between a cloud and the ground.
The lightning bolt superheats the air along a narrow channel.
The heated air pushes outward rapidly, creating a shockwave in the form of thunder.
Lightning and thunder are two sides of the same event. One creates the brilliant flash of light. The other creates the deep sound.
Lightning and thunder are both produced by the same strike. The two signals travel at very different speeds.
💡 Light Travels Really Fast
Light has a speed, and it is very fast. 300,000 kilometres per second in a perfect vacuum. A little slower in air. It's not much slower, but it is slower. The flash of light will reach you almost instantly.
A strike of lightning will appear to you long before the sound reaches your ears. If a storm is going to hit you, you will probably see the lightning before you hear the thunder.
🔊 Sound Has a Speed Too
The speed of sound depends on the medium it is travelling through. In air near room temperature, sound travels at 343 metres per second.
By comparison, the speed of light is about 875,000 times faster than sound.
That means that the thunder is about 875,000 times slower than the lightning. This is why you can see lightning strike before you can hear the thunder.
The greater the distance between you and the lightning, the longer it takes for the thunder to reach you.
⏱️ Can We Predict How Far Away a Strike Is Based on When We See it and When We Hear it?
Yes, a certain method can help you to get an estimate of how many kilometres away the lightning was when it struck.
Here is how it works:
• First, watch for a lightning strike.
• When you see the strike, start counting.
• When you hear thunder, stop counting
• Divide the number of seconds by 3 to get the rough distance in kilometres.
Let's say you see a lightning strike. It takes 9 seconds for you to hear the thunder.
9 ÷ 3 ≈ 3 kilometres.
Keep in mind that this isn't an exact science. The temperature of the air, the path length of the lightning bolt, and the speed of sound can combine to give you an inaccurate result. This method only gives you an estimate of the distance.
Also remember that this method only gives you the distance. It does not tell you how far you are from the storm. It does not tell you whether you are safe or not.
🌧️ Why Did I See the Lightning, But Not Hear the Thunder?
There are a number of reasons why you might see lightning, but not hear the thunder.
Lightning could have been far enough away that you couldn't hear the thunder.
The wind could have been so strong that it carried the sound away from you.
The lightning could have been illuminating the clouds on the distant side of the storm.
Lightning could also have been within the clouds, and much of the light was scattered by the clouds.
The lack of thunder doesn't necessarily mean that the lightning is safe or that the storm is far enough away for you to go outside.
🧠 The Brain Gets Light and Sound From the Eyes and Ears
This experiment isn't just something for you to do outside on a stormy day. It gives you insight into your brain.
Your brain uses your eyes to detect light. It uses your ears to detect changes in air pressure that we call sound.
It takes time for the signals to reach your brain. The signals combine in your brain, giving you an understanding of what is happening around you.
When it rains, the brain detects lightning strikes. As we mentioned earlier, you can use the time between a flash of lightning and a strike of thunder to estimate your distance from the storm.
You have used your brain to perform an amazing experiment to detect something that is happening in the world around you.
⚠️ A Safety Reminder
Lightning is beautiful when it strikes the clouds. It can also be deadly when it strikes you.
When you hear thunder, you are close enough for the lightning to get you. So, get inside. Seek shelter in a sturdy building or a hard-sided vehicle with the windows rolled up.
Avoid open fields, isolated trees, exposed hilltops, and bodies of water. Wait at least 30 minutes after the last thunder to go back outside. Don't go outside to see the storm. Your life is more important than seeing the lightning.
❓ FAQ
Does lightning actually happen before thunder?
They are the two sides of the same event.
What causes thunder to rumble and not make a loud boom?
The lightning is going through a long channel. The sound waves bend and reflect off the channel, bouncing around and creating a rumbling sound that lasts several seconds.
Can lightning strike without thunder?
Lightning will always produce thunder. You may not hear the thunder because the sound is too far away to be heard.
Does a longer amount of time between the lightning and thunder mean that the storm is moving away?
No. It mostly tells you how far away the lightning was when it struck.
🔑 KEY TAKEAWAYS
Lightning is an electrical discharge.
Thunder is the sound created by the rapid heating of the air along the channel of the lightning bolt.
Light travels much faster than sound.
You can estimate how far away the lightning was based on how long it takes for you to hear the thunder after seeing it.
The lightning and thunder can be affected by weather conditions and how far it is.
If you heard thunder, then you should think about your safety in regards to the lightning.
🌿 Something to Think About
Lightning can strike in many ways.
The world will always surprise and awe you.
A flash of lightning and the sound of thunder may only be one aspect of the world that you experience. It will give you an appreciation for the way the world works.
Light moves at a much higher speed than sound moves at. That is why you will see the flash of lightning before you hear the thunder, and it takes time for the sound of the thunder to reach you. You can use the time it takes for thunder to reach you to determine how far the lightning was when it struck.
The time it takes between flashes of lightning and strikes of thunder will differ depending on how the air is moving that particular day.
You can also estimate the distance of lightning from where it struck based off of how long it takes for you to hear the thunder.
One stormy day can teach you many things. You can see how fast things move in the world around you and learn something new and interesting each day.
🏁 CONCLUSIONS
The next time you see lightning strike the sky and hear thunder follow shortly after, you can appreciate the beauty and power of a storm. You saw the lightning strike and heard thunder follow in a matter of seconds. You saw something that was moving much faster than the sound of the thunder.
You now know something very interesting about the world and how it works. Always remember to take safety precautions in regards to lightning and thunder.
💭 Question to Think About
If you see lightning and hear thunder five seconds later, how far away do you think the lightning may have been?
📚 Further Reading
To read an in depth article about lightning, thunder, and storm safety, go to the National Weather Service's lightning safety web page or the National Severe Storms Laboratory's page on lightning and thunder.
