Making Waves: Properties of Electromagnetic Waves
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"
- tape measure
- short, thin, metal pole or wooden dowel (10 cm in length and 2 mm in diameter should work well)
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
- Stretch the slinky to about double or triple its original length. This requires two people.
- Measure the distance between the two ends of the slinky.
- From one of the ends, pull back on the slinky in the direction that the slinky is stretched by a few inches and release.
- 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.
- 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.)
- Increase the tension and calculate the velocity.
- Decrease the tension and calculate the velocity.
Transverse wave with a coiled spring
- Stretch the snakey (coil spring) to about double or triple its original length.
- Measure the distance between the two ends of the spring.
- From one of the ends, displace the coil along the floor perpendicular to its length by a few inches and release.
- Measure the velocity of the pulse with increased and decreased tension as in the previous section.
Reflection from fixed and unconstrained ends
- Working with the coiled spring, release a transverse pulse down the spring.
- Both ends of the spring should be held tight.
- Observe what happens to the pulse when it reaches the end held tightly in place.
- 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.
- Again release a transverse pulse down the spring and observe what happens when the pulse reaches the end.
Waves crossing into different media
- Connect the slinky and the coiled spring together with a string.
- Observe what happens to a pulse sent from the slinky side.
- Now see what happens to a pulse sent from the coiled spring side.
- 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.