# Making Waves: Properties of Electromagnetic Waves

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### The Idea

Many of the things physics deals with are waves. This includes sound, light, and vibrations in matter. It is helpful to use vibrating objects, such as we do in this project, to help visualize more abstract waves, such as electromagnetic waves, which include light.

### What You Need

• coiled spring "snakey"
• string
• stopwatch
• tape measure
• short, thin, metal pole or wooden dowel (10 cm in length and 2 mm in diameter should work well)

### Method

For the following, be careful when working with stretched springs. Be careful not to let the spring go accidentally, which could cause the spring to whip around and possibly hit someone.

### Longitudinal wave with a slinky

1. Stretch the slinky to about double or triple its original length. This requires two people.
2. Measure the distance between the two ends of the slinky.
3. From one of the ends, pull back on the slinky in the direction that the slinky is stretched by a few inches and release.
4. Observe the pulse moving from one end of the slinky to the other. Time how long it takes to go the measured distance from one end to the other.
5. Calculate the velocity of the pulse by dividing the distance the wave travels by the time it takes. (Use consistent units, meaning if you measure the distance in meters, the velocity will be in meters per second. If you measure the distance in inches, the velocity will be in inches per second.)
6. Increase the tension and calculate the velocity.
7. Decrease the tension and calculate the velocity.

### Transverse wave with a coiled spring

1. Stretch the snakey (coil spring) to about double or triple its original length.
2. Measure the distance between the two ends of the spring.
3. From one of the ends, displace the coil along the floor perpendicular to its length by a few inches and release.
4. Measure the velocity of the pulse with increased and decreased tension as in the previous section.

### Reflection from fixed and unconstrained ends

1. Working with the coiled spring, release a transverse pulse down the spring.
2. Both ends of the spring should be held tight.
3. Observe what happens to the pulse when it reaches the end held tightly in place.
4. Now, insert the dowel or a metal rod through one of the ends of the coil. The dowel should pass through one or a few coils in such a way that the coil is able to slide freely on the dowel.
5. Again release a transverse pulse down the spring and observe what happens when the pulse reaches the end.

### Waves crossing into different media

1. Connect the slinky and the coiled spring together with a string.
2. Observe what happens to a pulse sent from the slinky side.
3. Now see what happens to a pulse sent from the coiled spring side.
4. In which section (slinky or coiled spring) does the pulse move fastest? If your springs are long enough, time it and calculate the velocity. With everything else equal, a less-tense spring gives you a little more time to make the measurement.

### Superposition—constructive and destructive interference

1. Place a coil on the floor. Hold the coil from both ends using two people—one on either side. Stretch it fairly tight.
2. Each person holding the coil simultaneously should release a pulse the same size from the same side of the coil. Observe what happens.
3. People holding the coil should now release a pulse the same size from the opposite sides of the coil. How does this compare with the pulse released from the same side?

### Standing waves and nodes

1. Place a coil on the floor. Hold the coil from both ends using two people—one on either side. Apply moderate tension.
2. One person should hold the end of the coil stationary.
3. The other person should begin shaking the coil, slowly at first.
4. Observe what happens, for a given tension, as you increase the frequency of the vibrations. A node is a point on the wave that does not move while the wave vibrates. See Figure 61-1.
5. Quantify this by counting the number of nodes as a function of the frequency of the vibration. The frequency can be determined by the number of seconds it takes for ten vibrations (back and forth) divided by ten.
6. Observe what happens as you increase the tension in the coil.

### Longitudinal wave

The velocity of a longitudinal wave increases as the tension increases, but is not dependent on the amplitude.

### Transverse wave

Similarly, the velocity of a traveling wave increases as tension increases.

### Reflection

When a wave comes to the end of the spring that is rigidly held, the wave reflects by flipping over to the opposite side. If the spring is free to move, the wave reflects, but begins its return on the same side it came from.

### Different media

When a wave goes from one spring to another, the wave is partially reflected and partially transmitted into the second spring.

### Superposition

Pulses coming from the same side of the coil add together to form a larger combined pulse. This is called constructive interference.

Pulses coming from the opposite side of the coil negate each other, resulting in a smaller pulse at the point or even no pulse. This is called destructive interference.

After combining, the original pulses continue moving through the spring.

### Standing waves

The greater the tension and the more rapidly the spring is shaken, the greater the number of nodes formed.

### Why It Works

Waves exhibit characteristic properties that include: traveling waves, standing waves, reflection, moving between different media, superposition, and interference.

### Other Things to Try

1. Standing waves can also be shown by vibrating (or rotating) a string held under tension by two vertical supports. Waves can be generated by a motor or a speaker, as picture in Figure 61-2.
2. A fun way to do some of these investigations is to use glow-in-the-dark rope available from PASCO.
3. A significant, but often overlooked, point is the connection between the frequency of the traveling waves and standing waves in a coil.

They should be the same, and the previously techniques developed are a good way to verify this. A standing wave is really a mixture of many standing waves traveling back and forth along the coil. The overall standing wave pattern is generated by the traveling waves interfering constructively and destructively. The relationship between standing waves and traveling waves is used when measuring the speed of sound by determining the wavelength of a resonant standing wave. This works because the speed of both is the same.

### The Point

The properties of waves, including movement in a medium and reflection, can be observed in springs.