Doppler Effect of Real Time

Updated on May 25, 2008

Sound

Sound is a series of compression waves in the air. Think of it like waves on the surface of a pond. The volume of the noise is how high the waves are on the pond, the pitch is how far apart the waves are, and certain properties of the sound depend on how fast the waves increase or decrease in height.

So what is the doppler effect of sound? Imagine putting your hand in the water of this pond. The waves will pass your hand at a certain rate, say 2 per second. This is the frequency, much like the pitch of sound is to your ear. Now move your hand in the opposite direction as the waves. Suddenly you encounter three waves per second - the frequency has increased because of your movement. This would associate with a higher pitch to the ear. That is why when you are moving relative to a fire truck, the pitch of the truck will change as your distance shortens or lengthens. Your brain has developed to naturally understand if something is approaching or departing based on this effect.

You can simulate this effect by taking a simple casette recorder. Record a sound at say speed 24. Play it again at a different speed. If you play it faster than you recorded it at, it will be cramming those compression waves into a shorter time frame and you will hear a higher pitch. Play it slower and you will hear a lower pitch. You can even record the sound of a vehicle and then change the speed of play-back to make it sound as if the car were coming or going.

Doppler Effect on Light

So using the analogy above with the waves, we can pretty much figure that anything that has a frequency can have it sped up or slowed down by moving towards or away from it. Since light has properties of waves, you can increase the apparent frequency by moving towards or away from the source of light. When I say moving towards or away, I mean very rapidly. You will not see your friend turn blue just by walking towards him. You won't see him turn blue if you're moving towards him at forty miles per second. You need to be moving hundreds or thousands of miles per second to see that.

An odd discovery was made when looking at the stars through spectrometers. Spectrometers are devices that split light into the primary colors depending on their frequency (there's more light there than meets the eye). They were trying to determine what elements are burning in the star based off of what light is being emitted from the star. They saw that all the light was shifted a little from the expected values. Some light was to the right of where they thought it should be, some to the left. For each star it was all either left or it was all right. They figured out that it is due to our motion relative to that star they were looking at. If that star were moving closer to us, we would see that light slightly bluer than if it weren't moving. We'd see that star redder if it were moving away from us. Pretty cool, huh? That is what is meant by red-shift or blue-shift if you ever hear those terms.

Slow-Motion and Time-Lapse

You know how I said anything that has a frequency can have that altered by moving towards or away from it? The same can be said about cinematography, but with a slightly different angle. The frequency I'm talking about it the number of frames per second that a video plays at. When you go to the theater and you look at the screen, it will actually flash a single image on the screen momentarily and then flash another shortly therafter, then another and another. The blank spot in between is actually filled in by your brain to look like a continuously flowing image and can actually fill in the blanks for a range of frequencies. If you film a movie at 24 frames per second you will need a certain shutter speed than if you filmed it at 32 frames per second. And to make your movie look just like you filmed it, you will need to replay it at the same speed you shot it at.

The tricky part comes in when you want to play it in slow motion. You usually have to film it knowing in the back of your mind that it will be played slow motion. What you do is film it at say 96 frames per second for ten seconds. When you replay it, do so at 24 frames per second - it will take forty seconds to replay. Thus, everything will appear to be moving at one-fourth the speed.

Time-Lapse is the same thing but backwards. You film at say 6 frames per second for forty seconds and replay at 24 frames per second. It will take only ten seconds to replay what you filmed in forty, making everything look like it is moving four times faster.

You can always tell what images are pulsed images. You can tell this by waving an object between your eyes and the image and watching image. If it's a continuous image, the object will not pulse. If it is a pulsed image that your brain is filling in the gaps, the object will appear to pulse as you move it. If you could count that fast, you might even be able to derive how many frames per second the image is based on how many pulses the object makes per amount of time. Crazy, huh? Confusing? Pointless? Entertaining?

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• This Mike Guy

7 years ago

Nice.

• dutch84

10 years ago

this is so cool.

I don't think most people even understand the implications.

It's like when you speak, you make an indentation in the atmosphere or something...

So cool...

• dutch84

10 years ago

this is so cool.

I don't think most people even understand the implications.

It's like when you speak, you make an indentation in the atmosphere or something...

So cool...

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