# Waves of Light and Sound Judith Lachance-Whitcomb, instructor Angela Stover, TA

Okay, Wave Experts, you are getting the idea that a wave is a wave is a wave…You’re pretty clear on what makes a wave, what a wave is, and what characteristics all waves share. You know that there is one kind of wave that must have a medium through which to travel (mechanical) and another kind that can travel through a medium or a vacuum (electromagnetic). In this lab you are going to have to put your understanding of the differences of the two types of waves to use.

As we mentioned above, one main difference is that mechanical waves can only move through a medium but electromagnetic (let’s refer to these as EM waves from now on) can move through a medium OR a vacuum. If one wave type needs matter in order to transfer energy and the other does not, there has to be a reason why. Let’s see if we can determine what’s happening.

Remember our discussion in the introduction about vibrations? We considered the source of energy transfer to be vibrations. It is pretty clear when we think about particulate matter vibrating. Think back to our hand shaking that rope or Slinky. We can imagine an object (our hand) giving off some of its energy (jerking) to a nearby particle () so that particle begins vibrating: then that particle vibrating and giving off some energy to another particle that begins vibrating, and so on.

But with EM waves, if they can travel through a vacuum, they can travel when there are little or no particles to vibrate. How is that? What’s going on?

To answer this question, we have to think of what an EM wave is. A hint, you want a hint? Okay, here is the hint:

There we go. My best hint yet, don’t you think? Yes indeed, a finer hint has never been given and…what’s that? You don’t get it? Humm…Well, let’s take this apart and see what we have.

First there is the word itself, electromagnetic. Obviously it is a combination of electric and magnetic. Since we use that name to describe it, that must mean that an EM wave is something that is both electric and magnetic. And that is right. The nature of EM waves is one that consists of both a magnetic field and an electric field. These fields aren’t made up of matter. They are regions that form when the push and pull (forces) of charged particles is exerted. The interesting thing about charged particles is that they can push or pull objects without touching them. How many of you have used the north pole of one magnet to create a magnetic field that pushed against the north pole of another object?

But what happens when you place opposite poles near each other? Yes, the magnets pull together.

These regions are created when magnetic poles (north and south) interact either as attraction or repulsion as you see in the diagram. Electric fields are generated by electric currents, the movement of tiny electric charges that create areas with north and south poles. Then the attraction and repulsion behavior of charged particles is similar to those of magnetic fields.

The electric field and the magnetic field in an EM wave are positioned at right angles to each other and to the direction of the motion of the wave. Let’s look at a diagram to help us understand this idea.

Electromagnetic Wave

As you can see, electromagnetic waves are transverse waves. And you know that in a transverse wave, the motion of the medium is at right angles to the direction of the wave, remember the bobbing Bully?

 If you look back at the diagram of the EM wave, you will observe the electric field (solid line) is at a right angle to the direction of the wave. The magnetic field (dashed line) is also at a right angle to the direction of the wave. Now observe that the electric and magnetic fields are at right angles to each other.

You may be wondering, "Okay, these fields are at right angles to each other and to the direction of the wave, but what starts the disturbance that we call a wave?" Electrons are tiny charged particles that are part of an atom. They can create electric and magnetic fields. Of course, we know that to get that energy flowing, there must be some movement. When a charged particle starts moving back and forth, the electric and magnetic fields that it created begins to vibrate. We can think of the source of all electromagnetic waves is charges that are changing speed and direction. Most of the forms of EM waves that we are familiar with (light, x-ray, radio waves, gamma rays etc.) have their beginning in atoms.

Okay, nice to know but what has it got to do with our lab? Much of what you will be doing in this lab will be thinking about that question. Consider yourselves the new Wave Detectives. First you will be detecting how some technology tools work. Then, since you will be spending much time during this institute discovering how waves are similar and different, you will be challenged to determine if those differences (or similarities) effect ability to use waves in gathering data.

So, to make a long story short, in this lab, you are going to "play" with both mechanical waves and electromagnetic waves. And, Lucky You get to do this with science labs that use a computer and another piece of hardware called an interface to aid in your sleuthing. This is a sensor technology. That is, neat tools that SENSE information. Part of your job will be to figure out how they "sense" information. Good Luck! Have fun!

In the resource section, you will be given information on how to use the Science Workshop® materials produced by PASCO®. During the experimentation portion of the labs, keep this material handy for a quick reference. And don’t forget to look for the detective. He’ll remind you to think about an important clue, gather evidence, etc. Keep a separate section in your journal for clues or questions that relate to the "Mystery."

During this portion of the lab you will use a motion sensor to see how closely you can follow a path that has been drawn for you by the computer. You will need you journals and the section marked MSSS01-1 to MSS01-3. Be sure that you respond to the prompt on page MSS01 —3 in your journals. (You may run out of time now, but make sure to do it during one of your study periods.)

 Think about waves. How do you think this sensor works? Since we are using it during a "Wave Institute", I wonder if it could have anything to do with waves?

Now let’s see how closely you can follow a new path that has been created for you by your partner. You will need pages MSS02-1 to MSS02-3 for this one. But you may begin with #4, since our equipment is already set up.

 Let’s see…the motion sensor has two digital plugs. I wonder why? It has to have something to do with the signals going out and coming in. Hmmm…I think I’m on the right track…waves…uh-huh.

We’ll now put our heads and clues together and see if we can figure out what is going on here. We’ll decide:

 That sounds like a song I know… Ahh, yes. Music…sound…. Of course, I’ve got it!

### 3: Let There Be Light

Okay we have that under control. Now Let’s take a look at some data I’ve collected for you. It is on the computer. This is data gathered with the light sensor. The sensor recorded the intensity of the sun for a 24 hour period.

Using the small flashlights and the light sensor, explore how you can gather information about distance and intensity. Before you begin, plan with your partner what you will do. Decide on your experimental design plan and record it in each of your journals. Also, record what you found out through your explorations.

### 4: I Can Do Anything Better Than You

Now it’s time for us to really get our experiment going. Here is the Driving Question for this lab: Is there a difference in the range that a mechanical wave has compared to an EM wave?

We will work together to prepare our data table and conduct our experiment during the remainder of the lab.