 Now we're going to look at wavelength and wave number. Remember, a wave is a traveling disturbance. And an oscillating wave is a traveling disturbance that has a sinusoidal pattern. As a matter of fact, the wave equation we're using in physics has that sign as the function inherent in that equation. Now let's take a look at the simulation. And again, we're using the PHET one that we've introduced in several of the other videos. So in this case, I'm going to go ahead and make a few changes here and start it oscillating. And so this is my oscillating wave. It's got a sinusoidal pattern, and that pattern travels along the string as I'm going. And I can freeze this at any moment in time to get a picture of what's going on with that wave. So going back to our equations here, we can now define our wavelength. That wavelength is a distance for one wave cycle. And it's got a symbol of lambda. So this is the Greek letter lambda. Looks a little bit like an upside down y, but it's a lowercase Greek lambda. And it's got units of meters. It is, after all, a distance for one wave cycle. And so that means it could have any other distance type unit as well, but our standard metric unit is going to be the meter. Now graphically, again, this is a picture of a frozen wave. In reality, that wave is traveling through space. We can define a couple of quantities on here, like the amplitude. But what we really care about right now is the wavelength. And we often define that as being from one peak to the next peak. But don't forget that you can define it from one troth, or the bottom minimum of each curve, to the next minimum. Or you can even define it in the middle. If you define it in the middle, make sure you have both an up and a down cycle in order to get a full cycle of that oscillation. And we can then label these with our lambda rather than having to label them with the word wavelength. Now wave number, and this is also called the angular wave number, is related to this wavelength we've just been talking about. And the equation is this one shown right here, where k is our wave number. And it's equal to 2 pi over lambda, which is our wavelength. Now this equation is very similar to how we define the angular frequency. 2 pi over the period, where the period is the time for one cycle, and the wavelength is the distance for one cycle. So what about our units? For wave number, I'm going to have radians per meter. And the meter comes from my wavelength, lambda. And the radian comes from the 2 pi. So this is really 2 pi radians per wavelength in my equation up here. So that brings us back to our standard wave equation we're using in physics. And in this case, k, again, is my wave number. And x is my horizontal position along the wave. So that's a little bit of a more introduction to wavelength and wave number.