# Wave Propagation: Mechanical Waves vs. Electromagnetic Waves

Instructor: Amanda Robb
In this lesson, we'll learn about key differences in the two main types of waves, mechanical and electromagnetic. By the end of the lesson you'll be able to define these two types of waves and compare how these waves move through space.

## What Are Waves?

Imagine going to the beach. You look out at the ocean and see buoys bobbing in the distance, roping off areas that are safe for swimming. A group of teenagers plays music from a boombox in the distance and you can faintly hear it over the wind rushing past your ears. The sun warms your skin in contrast to the cool water lapping against your feet.

This perfect day at the beach isn't just a great memory of summer, it's an excellent example of many type of waves. You probably can recognize the waves in the ocean right away, but there are more examples in this scenario as well. Sound is a type of wave, and both the noise from the boombox and the wind create sound waves. You might be surprised to learn that light from the Sun is a type of wave too.

Waves are simply the transfer of energy through space. There are two main types of waves, mechanical and electromagnetic. Today, we're going to review what each of the types of waves are, and how they move through space.

## Mechanical Waves

All types of sound, including music and spoken language, as well as the ocean waves in our scenario are examples of mechanical waves. Mechanical waves move through a medium, or a substance. Sound waves move through the air, and ripples in a pond move through the water.

However, the medium doesn't have to be a liquid or gas. Mechanical waves propagate through solids too. Picture standing near a train track. As the train gets closer, you can see the tracks rattle with the movement of the train. If you put your ear close, you can even hear the vibrations. This is a type of mechanical wave. The vibrations from the train transfer energy to the metal tracks. The energy causes the metal tracks to move, with the particles inside the metal bouncing off of one another, propagating the wave. Eventually the wave gets to you and you interpret the vibrations as sound.

Mechanical waves must move through a medium, such as the metal tracks, air or water. They cannot travel through a vacuum like space. This is why there is no sound in outer space. You must have particles to transfer energy, so without particles there can be no mechanical waves, and thus no sound.

## Electromagnetic Waves

Getting ready to study for your physics exam, you need to get some light by turning on your desk lamp. You're also going to need a snack, so you heat up some leftovers in the microwave. Lastly, you want to check your text messages once last time on your phone so you can put it away for the next hour. You might be surprised to learn that all of these activities, not just the lamp, involve electromagnetic waves.

Electromagnetic waves are light waves, but they don't just include the visible light we can see. Light waves come in all forms depending on how spread out the wave peaks are. Radio waves are used to propagate signals on AM radio. Microwaves are used to heat our food, and to propagate the waves we need for cell phone service. On the other side of the spectrum, X-rays are used in medicine, and energy intensive gamma rays are used to sterilize our food.

Electromagnetic waves propagate energy forward, but do not use a medium. This is why light can reach Earth from the Sun. If electromagnetic waves were mechanical, there would be no light, heat or life on Earth!

## Shapes

Wave propagate in different shapes, depending on if they are mechanical or electromagnetic. However, in all waves energy travels forward, and despite the shape of the wave the medium always returns to its starting point. Matter doesn't actually travel forward in a wave. Mechanical waves have three distinct shapes, while electromagnetic waves only have one.

#### Longitudinal Wave

The first shape is called a longitudinal wave. In a longitudinal wave, the particles move side to side. The particles move horizontally, compressing together. Here, the particles transfer energy forward in the wave. Then, the particles spread out, and the energy continues to move through the wave in this way.

To understand how a wave is propagated longitudinally, picture stretching a slinky out between your hands. Push one hand into the other, and you'll see the slinky move back and forth horizontally. This is a longitudinal wave.

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