Waves: An Introduction

How many "waves" can you think of? This morning you may have been waving bye-bye to your little brother or sister. Or what about that wave of nausea that came over you when you saw the grade on the test for which you had forgotten to study? We all sing about those "…amber waves of grain…" don’t we? There probably are very few days that pass that you don’t use the word wave to describe some motion. Now try and remember the last time you talked about hearing waves of sound or seeing waves of light. That’s more difficult, isn’t it? You don’t usually talk about those glorious waves of light that enabled you to find your way to the bathroom during the night. And I’m sure you don’t tell your cousin that her singing sends out sound waves of undetermined frequencies and amplitudes. And yet if we do want to describe what sound and light are we have to explain them in terms of waves.

Your adventure at the Yerkes Summer Institute this year is all about waves. You will be exploring questions like: What are waves? What properties do we use to describe waves? What are the characteristics of waves that are common to all waves? How do waves transmit energy from one place to another? How do astronomers and cosmologists use an understanding of waves to discover more about our universe? As you work through the sessions during the institute, you will need to keep a journal. In this journal each day, you’ll record thoughts and ideas that bring you closer to the answers for some of these questions. You will also write down other questions that you may want to ask about waves.

What Is A Wave?

Have you heard something like this? "He is always making waves!" What does that mean? What characteristic would describe that person? Someone who always dives into a pool to cause a big splash to drench everyone else? Of course not, you know when that phrase is used, it means someone has caused some kind of a disturbance. As we start to construct our idea of wave, motion and disturbance will be good places to start. So far we think we know that a wave has to have something to do with motion and that it causes some kind of disturbance in the usual state of things.

Think of a water wave crashing onto shore. Before that wave appears, picture the undisturbed water. We can see a smooth glass-like surface suddenly transformed into rolling waters. But as the waters calm once again, all of that charging water is hardly any farther up on the beach than it was before. The water really hasn’t moved forward. The waves moved, you know that for sure, you saw them. And it is true. Some of the waves that have reached California’s shores have originated over 7000 miles away and traveled that distance at speed of 40 mph and higher. Why isn’t California sitting under a whole bunch of water? What’s the deal? What is a wave?

The scenario described above gives us a little more information. Obviously there was motion and most certainly there was a disturbance of the water. However, did the water itself move? Is California sitting under water after that wave hits? So, here it is —

WAVE CLUE # 1
A wave is a disturbance that travels from one place to another through water without moving the water.

Picture the school bully standing on a giant cork in the middle of Lake Michigan. He seems to be on a yo-yo bobbing up and down. Keeping an eye on the bully, you notice that whenever the waves roll by he begins to bob. When the waves stop, he stops. A disturbance is carrying something through the water that is causing the waves to form which then causes the Bully to bob. As we stated above, the blob of water that he is floating upon is not moving because he is as far away from you as he was before. That something is moving. What is the something that the travelling disturbance is carrying through the water?

What is happening to the water when that something moves through it? How did it look before the disturbance? How does it look after the disturbance? Was there a change? There definitely is. Remember earlier we visualized the tranquil waters changing into high raging waters ? What causes things to change? Ask yourself these questions…

Figured out the answer? Here it comes…

ENERGY!

Yes, there it is, the Big E. Energy is the ability to cause change. An easy way to understand what energy does is to remember this sentence, "Energy makes things move, change, grow, or glow." (Wong, 1995, p. 7)

If you’ve gotten the point, you’re ready for …

WAVE CLUE # 2
The travelling disturbance in a wave is carrying energy.

Okay, the water is changing as the wave moves through it and that is our evidence of the wave carrying energy. But where does this energy come from? During the Waves and Water Waves labs, you will actually have the chance to "make waves". But let’s try to think about a wave now and try to figure out from where this energy comes.

Imagine that you have a rope. Tie one end of the rope to something stable, maybe a doorknob or a table leg. Hold the opposite end of the rope (the end not tied) in your hand and use your wrist to jerk your hand up and down several times. What happens? You create a vibration, a movement that follows the same path over and over. Anything that is vibrating is moving. Anything that moving has energy. AndVoila! A wave motion moves from your end of the rope to other end.

Remember this whole idea of energy? Before you made your wrist jerk there was all of this potential energy just waiting for a chance to do its thing. Once you made your wrist jerk the potential energy became kinetic energy. Kinetic energy is the energy of motion. Your wrist’s vibrations gave off some energy to nearby particles so they started vibrating. Then they gave off some energy to the particles next to them and they started vibrating. Then they gave off…yada, yada, yada. This movement of energy from a vibrating source (your wrist) outward is guess what?

Yes,

a wave!
You know that it is now time for…

WAVE CLUE # 3
The source of energy contributing to movement is a vibration.

So far, we have talked about water waves. Of course, there are other types of waves. Earlier we mentioned those incredible sound waves that your cousin makes when she does her version of singing. What about those old cowboy movies? The good guy gets down on all fours, puts his ear to the ground, and figures out from the vibrations how far away the bad guys are. This must mean that water isn’t the only thing through which waves travel.

Let’s see now: ocean waves went through water and that’s a liquid; sound waves went through air and that’s made up of gases; and bad guys horses sent vibrations through the ground and that is a solid. Liquid, gas, solid…hmmm these sound familiar. Wait, wait it’s coming to me… Oh, yes. These things make up the stuff we call matter. The substance that waves can travel through doesn’t have to just be water or other liquids. It can be other forms of matter. We call this matter a medium.

Think about this: how could you watch Moesha if it wasn’t for those fantastic radio waves that bring her into your home each week? Maybe you are thinking that those waves travel through air like the sound waves. Do they? See if you can figure that one out this week. Let’s take a more obvious example — light waves from the sun. You still think that they are transmitted by air because the atmosphere of our planet is made up of gases. Before those waves reach the atmosphere, what do they have to go through? Right, space! Space is relatively free of particles. It is essentially a vacuum.

This is the final clue. After this, you’re on your own. With all of these clues and the work you do in the labs this week, you’re going to have to be able to explain what a wave is. Ready? Here it is:

WAVE CLUE # 4
Some waves are transmitted through a medium. Other waves can travel through a medium and a vacuum.

What Are the Wave Groups?

There are two major wave groups. In the section above, we talked about waves that were transmitted through matter. All of these waves are called mechanical waves. Two of your group names represent mechanical waves.

But what about the third group? What kind of waves are they? Remember those multi-talented waves that can move through through a medium and a vacuum are called electromagnetic waves. Along the left side of this page, you can observe a representation of the electromagnetic spectrum and all of the types of waves that we find in that spectrum. You can also see some of the ways that we use those waves. On the right hand side, you see a numeric representation that gives the wavelength in addition to the frequency. In electromagnetic waves the disturbance moves through electric or magnetic fields instead of moving through particles (matter)as the mechanical fields do.

What Are the Wave Characteristics?

Okay, you know that there are many kinds of waves: lake and ocean waves, sound waves, light waves, x-rays, microwaves, and radio waves to name a few. The question that may have come to your mind by now is:

What does a water wave have in common with a microwave?

And, that is a great question. We will try to build an understanding now of those things that make a wave a wave. We call these characteristics and there are three characteristics that all waves, mechanical and electromagnetic, have in common These three characteristics are: amplitude, wavelength, and frequency. Right now those are just words. Let’s try to start building an understanding of those terms now.

AMPLITUDE
Recall that when a wave disturbs a medium, the particles in that medium are moved from their usual position. Think of a long line of flexible material that you push together from both ends,

What happens? The material is displaced from its usual position. The distance that it is moved from its usual place is shown in the diagram by the wavy line. In mechanical waves, this is what happens to the particles. In an electromagnetic wave, a similar thing happens to the electric or magnetic fields. The point of the maximum (greatest) movement from rest is the amplitude of a wave.

WAVELENGTH
Waves, as we saw with the Bobbing Bully, move energy along in a way that we have a series of "mountains and valleys". Those mountains and valleys are called the crest and trough of a wave. Although a wavelength can be measured from any point on a wave as long as it is measured to the same point on the next wave, it is usually measured from crest to crest or trough to trough. The most common units of measurement for wavelengths can be in meters, centimeters, or angstroms. Lambda () ), a Greek letter is the symbol for wavelength.

FREQUENCY
The number of cycles, that is the number of complete waves (crest to crest or trough to trough), in a unit of time is called the frequency. The unit of measurement that is used for frequency is called a hertz (Hz). If we have a frequency of 1 Hz that means that one complete cycle passed in one second. So, 1 Hz = 1 wave per second.

Remember, earlier we talked about the source of energy movement in a wave being a vibration? Well, the frequency of a wave is dependent on the source of the vibrations. In our example of the rope on the door, the faster you moved your hand the more waves you created in a minute; the slower you moved your hand, fewer waves were created in a minute.

Let’s summarize this section on wave characteristics with a diagram that illustrates all three characteristics. Can you explain it?

How Are Waves Classified?

If we review what we know about mechanical waves, the big idea is that they need a medium through which they move. Both a sound wave and a water wave need some medium. But a sound wave and a water wave are not the same type of wave. They each move in a very different ways through that medium. Depending on the motion of the medium compared to the movement of the wave, waves are classified as either transverse or longitudinal.

TRANSVERSE WAVES

It will help us to think about a Slinky® (You will actually get to do this in one of the labs.) Imagine holding the Slinky® between your hands. You begin to wiggle your right hand up and down. Up and down, up and down, up and down. You are constantly moving your hand up and down. No doubt about it the motion of your hand (the source of vibrations) is moving up and down (vertically). But take a look at the Slinky® as the wave moves across it:

The energy isn’t moving vertically - it is moving horizontally across the Slinky® (medium). The wave motion (your hand) is at right angles to the motion of the wave energy going through the medium (the Slinky®). When this occurs, it is a transverse wave.

LONGITUDINAL WAVES

Let’s use the Slinky® again. Can you make the Slinky® walk? Notice how this time the coils seem to gather together and then spring apart, they clump and then spread apart. Get some to hold one end of the Slinky® and you hold the other. Push your end of the Slinky® towards your partner. What do you see? Now the wave energy moves across the Slinky® in the same direction that you pushed.

A wave that has the motion of the medium parallel to the direction of the wave is a longitudinal wave.

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