Microwave radar systems use the Doppler effect to detect both the direction and speed of moving objects by measuring shifts in the frequency of reflected waves. Understanding how wave compression and expansion relate to motion is fundamental to how these systems work in real-world applications.
Another thing we could use are microwaves. In fact, we can use microwaves to understand the speed that something is traveling at.
Remember, microwaves is just those frequencies that are in between here. So it's a range of frequencies of waves. We're not going to get deep into waves and how they respond - it's certainly something I'd love to do, it's just not applicable to this lesson. We're going to look at how microwaves can be used to determine a speed or determine motion. But to understand that, we have to understand Doppler.
Let's say we have a drop, a drop in water, and that's going to create a wave that's going to go out. Or maybe this is some sort of electronic signal that's generating a wave. The first wave is created, and it starts right here at the circle, but then works its way out. Then at some point in time we're going to generate another wave. And so we generate another wave, but as that one expands and goes out, the first wave keeps expanding. So now this is the first wave and this is the second wave. And now we generate a third wave, and so on and so forth. And then when we generate the fourth wave, this one expands out. So now this is the fourth wave, this is the third wave, the second wave, and now this is the first wave. And they just ripple out from there. Once again, think of a drop of water in the pond.
Now think, though, what happens when something moves. If something was right here and dropped, we dropped something and it expanded out, and so now it's at this location, but by the time it sent out this next wave, it has moved. And so has the wave. And then when it moves again and makes the third wave, then it's going to be closer. Maybe it moves into this and sends out the third wave.
What we can see is that those waves that are being sent out are actually shifted. We have them tighter together over on this side, but they're more spread out on this side. This is the effect that we have with the waves: they're closer together on one side than the other side. And that's because the source has moved in position whenever it releases these waves.
Now, sound is the same thing. So think about sound. When you're hearing sound, there are lower frequencies, which is when the waves are spread out further, and there are higher frequencies, which is when they're closer together.
So think about when a train whistle or a car horn goes by you, and what happens, exs out as a high pitch, and then as soon as it passes you it gets lower. That's the Doppler effect that's happening, and that's because what you're hearing as it's approaching you, those waves are actually grouped up tighter together, and as it goes away from you, then those waves are farther apart.
In fact, if you're going faster than the waves are going, then what happens is they're all grouped up all at once. Well, this is a sonic boom. This is when you go faster than the speed of sound. The speed of sound is how fast these waves go out, and if we can go faster than that, they group all together, and that's when you get a sudden boom. And that's when planes go really fast, you get that sonic boom.
Now let's see how Doppler can measure speed. We're going to flip the scenario here. Now we have a place that's standing still, and this place that's standing still is going to be emitting a signal, emitting waves that are going to be very consistent. They're not going to change. And there's also a sensor, so that's going to pick up anything that's reflected. These waves will reflect off of objects.
So now let's look at an object that's standing still. Here we have an object that is standing still, it's not moving. What's going to happen is these waves are going to hit it at a certain frequency, and then because it's standing still, it's just going to bounce off at that same frequency. It's not moving at all, so it's going to go back at the same frequency. And then what happens is this gets picked up and says, oh yeah, that's the same frequency that went out, and so that car is standing still.
But now we have another car that is moving forward, moving towards this radar detector here, this microwave detector here. So what's going to happen on this car is it's going to hit the first wave right here. By the time the second wave is coming, then it's already moved forward, and so the second wave is going to be closer. Same thing with the third wave, and same thing with the fourth wave. All the waves being bounced back are actually going to be closer. They're going to hit this sensor right here at a higher frequency than was sent out. And so now it determines, oh, that car is moving towards me. And not only can it tell that it's moving towards this object, but it also can tell at what speed, because it depends on how close these frequencies are right here.
Now let's look at something that's moving away. So we've got a car that's moving away. What's going to happen is these frequencies are going to hit the car. The first frequency hits the car and gets reflected. By the time the next wave hits the car, the car has already moved forward, so it's actually farther apart. And then the next one is going to be farther yet, and the next one is going to be farther yet. So those are going to come in, and this realizes, oh, well, now the frequency is further apart, so I can tell this car is moving away. Not only that, but it can tell the speed at which it's moving away based off of the distance, how much it's traveled between each of these waves hitting it.
So that's how a radar detector works. That's how we can use microwaves to detect the speed of an object.
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