Light and sound
what a sound wave is;
how a sound is produced.
to explain the speed of sound in air;
to explain why nothing can be faster than the light.
Speed of sound
We already know that the sound is a wave that can travel only through a medium comprised of particles. The speed of sound travel depends on the density of these particles. In air, just above the ground, sound travels with the speed of 340 m/s (1224 km/h). Therefore, it needs approximately 3 seconds to travel a kilometre. As the altitude increases, air becomes increasingly thinner and the speed of sound decreases – for instance, at the altitude of 10 km it travels with the speed of only 1080 km/h. In water, on the other hand, sound travels at the speed of approximately half a kilometre per second!
What happens when there is no medium? In such case we are dealing with a place exposed to vacuumvacuum. There are no particles that could transfer vibrations between one another and carry the sound wave. Sound cannot travel in these conditions. This occurs in space – you cannot hear any sound in space.
Faster or slower than the light?
Why is it that during a storm we first see a flash and only later hear thunder? Light in a vacuum travels with the astounding speed of 300,000 km/s! Moreover, this is the greatest speed in nature and no body is able to achieve it. When light enters the atmosphere or another medium, it slightly slows down as in contrast to sound it travels at its fastest speed in vacuum. In air, light travels slightly slower – with the speed of 291 thous. km/s. Nevertheless, its speed still remains many times greater than the speed of sound.
a flashlight,
red foil that permeates light,
a whistle (a gong can also be used),
experiment is to be conducted in pairs, in an open field.
Tape the transparent red foil over the flashlight. The red colour is clearly visible even from large distances in bright sunlight.
Give the whistle and flashlight to your partner. Move 20 steps away from each other.
When you’re standing appropriate distance from each other, your partner should point the flashlight in your direction and turn it on, blowing the whistle at the exact same moment. Assess whether you first heard the sound or saw the light.
Move approxiamtely 300 steps from each other. Your partner should once again blow the whistle and turn on the flashlight. Which stimuli reached you first: light or sound?
As your partners moves away from you, the difference in the speed of sound and light becomes more noticeable.
How far away is the storm?
During a lightning strike, the air gets very hot. For this reason, it becomes a source of light as well as sound. The difference between the speed of sound and light means that we first see the lightning flash and only then hear the thunder that accompanies a discharge.
By using the difference in the speed with which sound and light travel, we are able to approximate the distance between our location and the storm. When you see a lightning flash, start counting the seconds until you hear the sound of thunder. Keeping in mind that sound needs approximately 3 seconds to travel a kilometre, we’re able to approximate how far away is the storm. For instance, if the thunder reaches 12 seconds after the flash, the storm is raging 4 km away (12:3=4).
Explain why it’s necessary to divide the time between the lightning flash and thunder by 3, rather than by 2 or 5, in order to approximate our distance from the storm in kilometres.
Sound travels at different speeds depending on the medium Order media according to the speed at which sounds travels through them. Start with the one through which sound travels at the slowest speed and continue in ascending order.
- vacuum
- water
- air
- glass
Specify whether the statement is true or false.
| True | False | |
| Sound cannot travel in vacuum. | □ | □ |
| The speed of light in the vacuum is greater than in nature. | □ | □ |
| It is not possible to move faster than sound. | □ | □ |
| In order to calculate the distance between the observer and the site where lightning struck, it is necessary to divide the time elapsed between the lightning flash and the crackle of thunder by 3. | □ | □ |
Summary
The speed of the sound depends on the medium through which it travels.
The speed of the sound in air at sea level is 340 m/s
Sound doesn’t travel through vacuum.
Light travels faster than any body in the world.
Keywords
light, vacuum, storm
Glossary
próżnia – przestrzeń pozbawiona drobin, w której nie rozchodzą się fale dźwiękowe