Mechanical waves and light | Waves | Middle school physics | Khan Academy

Let's talk about waves. So, let's imagine that you were to take a string and attach it at one end to a wall, and then on the other end, you were to wiggle it up and down. Well, then you would have made a wave. You would see a pattern that looks like this.

Now, what could be a good definition for a wave? Well, we could call it a traveling disturbance. Well, what does that mean? Well, we're disturbing the rope. If we didn't move it, if we just held it straight, it might look something like that, or it might just hang down a little bit. But clearly, we are now moving it up and down, and those movements are disturbing that rope. That disturbance can move along that rope.

Now, we see waves not just in ropes that are moving up and down. You have probably seen water waves. If you were to take a tank of water like this, and if you were to start pressing on one end of the water here, you would see these waveforms that start. We can also see that with sound and sound waves. You might not realize it, but the sound of my voice right now is actually just a traveling compression or disturbance in the air that is getting to your ear.

Then, little hairs in your ears can sense those changes in pressure from the air, and your mind perceives that as sound. Once again, this is a traveling disturbance. You have particles that have high pressure, and then they knock into the particles next to them that then knock into the particles next to them. So, if you were to be able to observe this in slow motion, you would see these high-pressure parts right over here could be traveling, say to the right.

Even though this might be a pressure wave that's traveling through the air, we can represent it in a way that looks a lot like our first rope that we were moving up and down. Areas where things are high in the sound example, that's high pressure, and you have areas where things are low in the sound example, that is low pressure.

Now, when we talk about waves, there are common properties. For example, we might want to know how much we are getting disturbed from what we would call the equilibrium. You could view that as maybe the middle state right over there. Well, if we're getting disturbed that much, we could call that the amplitude. That's how much we are going above or below that equilibrium. This would be the amplitude as well.

We could think about how far it is from the same points on the wave. So, if we go from one peak to another peak, well, we could call that the wavelength. You could just do it from any one point on the wave that's just like it on the wave again. So, that would be the same wavelength as our original wavelength right over there.

You might hear the term frequency of a wave, and one way to think about that is if you were to just observe our original rope and if you were to say how many times does it go all the way up, all the way down, and then back up, so it completes a full cycle. How many times can it do that in a second? If it does that five times in a second, then someone might say it has a frequency of five cycles per second.

Now, everything that we have just talked about, these are called mechanical waves. It's a special category, probably the ones that you will see most often. Now, mechanical waves need a medium to travel through. In the rope example, the medium was the rope. In the water example, it's the water. In the sound example, the medium is the air.

Now, there are things that can be described as waves that don't need a medium in particular, and this is kind of mind-boggling: is that light can be considered a wave. If we think about the different frequencies of light, our brain perceives that as different colors. If we think about the amplitude of light, our brain perceives that as the intensity of light, how bright it is.

And even more mind-blowing, visible light are just certain frequencies of what we would call electromagnetic waves. There's actually higher frequencies of electromagnetic waves that have all sorts of applications. You might have heard of ultraviolet light, or X-rays, or gamma rays. Similarly, their lower wavelengths of light, you might have heard things like infrared or radio waves. These are all just different frequencies of what's known as electromagnetic waves.