Reading: WAVES
Waves are all around us. We experience waves every day, whether we know it or not. We can see and feel some types of waves, like ocean waves or the wind causing waves in a flag. These are called physical waves. However, there are other types of waves we can not see or feel, like sound waves and electromagnetic waves.
Sound waves, like ocean waves, need something to travel through. That is, the wave moves from one point to another as particles bump into each other. Energy is transferred from one particle to the other in much the same way as a cue ball in pool transfers energy to the ball it hits causing the second ball to move. We can hear each other talking or a foot tapping on the ground because the particles in the air bump into each other and transfer the sound to our ear. When the particles bump into and vibrate our ear drum, we “hear” the sound.
Sound waves are compression waves. An easy way to think of the way a compression wave transfers energy is by having two people hold opposite ends of a slinky and stretch it slightly. As one person pushes and pulls on their end of the slinky, energy is transferred through the slinky to the other end.
In space, there is no sound because there is no medium (air, water, etc.) for the sound waves to travel through. How then, do astronauts communicate with each other and with people on Earth if sound waves do not travel through the emptiness of space? How does light reach the earth from the sun? The answer is electromagnetic waves. Electromagnetic waves do not need a medium to travel through. That means they can travel through air, as well as, the emptiness of space because the energy is not transferred by particles bumping into each other.
Electromagnetic waves are transverse waves. An easy way to think of the way a transverse wave carries energy is by having two people hold opposite ends of a string. As one person moves their end of the string up and down, energy is transferred to the other end.
Regardless of the type of wave, all waves are characterized by two things: wavelength and frequency.
Frequency of a wave is the number of waves that pass a certain point in one second. For example, the number of sound waves that hit your ear in one second or the number of light waves that hit your eye in one second. Because all electromagnetic waves travel at the same speed, the shorter the wavelength, the more waves will pass a point in one second (the higher the frequency.) Since electromagnetic waves are really, just one way energy can be transferred, the more waves hitting a point in one second, the more energy is transferred to that point.
Summary:
· Sound waves move from your mouth to someone’s ear by bumping the molecules in the air, transferring energy each time, until they bump into your eardrum and cause it to vibrate.
· Light waves are beams of energy that can pass through space. If they hit molecules in the air they can be stopped, sent back, or slowed.
Review: Complete the following sentences
1. Long wavelengths have a LOW frequency.
2. Long wavelengths transfer LOW energy.
3. Waves that transfer a lot of energy have a HIGH frequency, meaning that the number of waves
passing a given point must be greater than that of a LOW frequency wave.
4. How are sound waves different from light waves?
Sound waves move through a medium (air) bumping from molecule to molecule until stopped by your ear. Light does not move that way.
5. Why doesn’t a sound wave travel in space?
There is no medium for it to move through
6. Explain why a light wave, or electromagnetic wave, travels much faster than a sound wave.
It is shorter – more waves per second - higher frequency
Solving for the Energy of a Wave
The energy that a wave carries is directly proportional to the frequency at which the crest of a moving wave passes a given point. So what is frequency? Frequency is the measurement of the number of times that a repeated event occurs per unit of time. For example, if you are standing still next to a train track (you represent a given point) for 10 seconds and 5 cars pass you what is the frequency? To find this you would take the 5 cars and divided by 10 seconds. Therefore, frequency equals 2 cars/second.
Frequency = # of occurrences = # of waves
time period time
Calculate the frequency for each wave and answer the following questions. For each calculation, write the formula, show the complete set up and express your answer with appropriate units.
Number of Waves Time Frequency
Wave #1 150 waves 15 sec 10 w/s
Wave #2 240 waves 30 sec 8 w/s
Wave #3 550 waves 50 sec 11 w/s
7. Which wave has the highest frequency? Wave 3
8. Which wave has the longest wavelength? Wave 2
9. Which wave has the highest energy? Wave 3