# Wave Propagation

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• 0:00 What is Wave Propagation?
• 0:55 Wave Speed & Medium
• 2:40 Transverse vs. Longitudinal
• 3:30 Lesson Summary

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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After watching this video, you will be able to explain what wave propagation is, and describe the factors that affect the speed of wave propagation. A short quiz will follow.

## What is Wave Propagation?

There are lots of types of waves, but they all move. Sound waves move from an object making a sound to your ear - if they didn't, you wouldn't hear anything. Light waves move from the Sun, bounce off an object and go into your eye - if they didn't, you wouldn't see anything. So, it isn't hard to prove that waves move. But what is a wave?

A wave is a vibration in space and time that carries energy. Ocean waves, for example, are able to move large amounts of sand and rock from place to place. They can only do that because of the large amount of energy stored inside them.

Wave propagation is the physics term for the movement of waves. Today we're going to discuss the various ways that waves of different types can move and how those are different from each other.

## Wave Speed & Medium

If waves move, it makes sense that they should have a speed. If you literally measured how fast a peak of an ocean wave got from point A to point B, you would be measuring the wave speed. But what decides how fast a wave moves?

It turns out that some waves need a medium - a material - to travel through, and others don't. The only type of wave that doesn't is electromagnetic waves like light, infrared, microwaves, ultraviolet and radio waves. Since they don't have to move through a material, that material doesn't slow them down. They're the fastest moving waves, and they propagate at the speed of light, which is 3 * 10^8 meters per second.

Waves that do need to travel through a medium go more slowly. The exact wave speed depends on the material they travel through, the presence of any tension in the material and the temperature of the material. If these factors change, so does the wave speed. Denser materials, like Jello, tend to have lower wave-speeds, because it takes longer for the vibrations to pass from particle to particle. But often denser materials are also more rigid. More rigid materials, like cookies, have a higher wave speed. So, it's hard to predict what the wave speed will be using density alone. Stretched out materials (materials with a tension), like salt-water taffy, tend to have higher wave speeds, because particles are more spread out. Last of all, high temperatures lead to higher wave speeds, because particles move faster when they're heated up, causing the wave to pass through the material, like hot fudge, more quickly.

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