Fri. Feb 23rd, 2024
Understand the speeds of light and sound.

Did you know that the speeds of sound and light can provide us with a lot of useful information?

First of all, some important numbers:

  • Speed ​​of sound at 20o C: 343 m/s1
  • Speed ​​of light in a vacuum: 300,000,000 m/s (299,297,456.2 km/s)2

sound delay

Boy jumping into water at sunsetWe can use the differences in measured velocities to help us understand the behavior of objects.

For example, on a warm summer day, you may be at a park where someone is safely jumping into the water.

If you are canoeing near them, you will see them fall into the water and hear the splashing sound almost immediately.

But if you are canoeing further away (say about 500m away) and you are looking at the diver, you will see him fall into the water, but you won’t hear the splash for almost 1.5 seconds.

Why is this?

The answer has to do with the differences between the speed of light and the speed of sound. The speed of light is so fast that a difference of just 500 m (between you and the diver) is almost negligible.

However, that distance is about 1.5 times the distance sound travels in one second. So while the sight of the diver hitting the water reaches your eyes in an instant, the sound of the splash takes 1.5 seconds.

The approaching storm

Storm approaching a beach.The trick of knowing the differences between the speeds of light and sound can be an extremely important tool for those who practice hiking, canoeing or enjoy other outdoor activities in our parks.

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For example, if you are canoeing and hear or see lightning, you should probably head toward shore. But the question could be: at what distance or at what speed is that lightning bolt approaching me?

Well, we know that sound takes about 343 m/s to reach us and that light is almost instantaneous. The common rule of thumb is that for every five seconds between seeing the lightning and hearing the thunder, you are approximately one mile or 1.7 km away.

So if you see lightning, start counting the time until you hear thunder. If it takes about five seconds, the beam is approximately 1.7 km away. If it takes about ten seconds, then the lightning bolt is twice as far away or 3.4 km (2 miles).

The trick is to continue monitoring the situation. If the distance between the lightning and the thunder becomes shorter, then the lightning is heading towards you and you may need to act immediately to get to safety.

Yellow kayak on the shore of a lake with a big storm overhead

If, on the other hand, the time between seeing the lightning and hearing the thunder stays the same or increases, you may be fine, since the lightning may not be getting closer. Obviously, it is important to always take the necessary measures to ensure the safety of you and your group.

Distances to the moon and planets.

A similar trick can be used to determine distances to the Moon and beyond. If you’ve ever listened to the Apollo astronauts’ communication with NASA controllers, you’ll notice that there is a delay in their communications.

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In this case, the speed of sound has no influence, since there is no air in space through which sound can travel. However, at just over 380,000 kilometers away, the distance to the Moon is large enough to affect the time it takes for a light signal to travel from Earth to the Moon.

Moon at sunset over a reflective lake with forest silhouettesRadio transmissions, which travel at the speed of light (300,000 km/s), take about 1.33 s to reach the Moon and another 1.33 s to return. So the delay heard between the astronauts and ground control is a result of this delay.

The precise distance to the Moon is known to the nearest millimeter through the use of LIDAR and lunar reflectors left by the Apollo astronauts on the Moon.

LIDAR (short for Light Detection and Ranging) uses the principle of bouncing a beam of light off a target and measuring the time it takes for the bounced signal to return. Knowing the speed of light allows the observer to calculate the distance to the target very accurately.

Using this approach has allowed astronomers to measure the distance to the Sun and planets, not by placing reflectors on these objects, but by sending a beam outward and measuring the return time of the beam’s reflection to Earth. This information has allowed us to predict celestial events more accurately and further verify scientific theories.

A simple knowledge of the speeds of sound and light can help you learn a lot about your environment!

Do you want to know more about our galaxy? Check out our Eyes in the Skies series!