What Is Coherent Light?

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Coherent light refers to light waves that are in phase with one another; waves that have the same amplitude. Learn about the monochromatic nature of coherent light with help from a science teacher in this free video on physical science.

Part of the Video Series: Physical & Life Science
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Video Transcript

Hi, I'm Steve Jones and I'm going to explain what coherent light is. Now one of the things that we know is that light is a wave motion. So if we represent a wave in this way and if I put, if that's wave 1 and I put wave 2 here and I put the 2 waves like this, then we can see that the 2 waves, notice they're the same size and peak is on peak, so wave 3 if I add the 2 waves together, I'm going to get a very big wave like this. This is called interference. Now to get interference therefore, I have to have two waves and one of the problems is that the waves come from individual atoms producing an effect and each atom in each source if we've got two sources, is going to be different and therefore to get two actually like this is almost impossible or at least it's not impossible but we'll get lots of pairs. As many would produce waves like this, which as you can see where the peak and the trough are together and in fact where you get nothing. So we will end up with a series of waves which are either reinforcing or canceling each other out but every wave will behave differently and so therefore on average, it will look just an even color. So if I did this experiment, here's my light source, I've got two sources here, I would expect to see if this was yellow, I would expect to see a yellow screen. I would not expect to see what I actually see which is a series of stripes on here causes by interference. And the reason is that we are creating here coherent light, coherent sources. The conditions first of all, monochromatic, one colored, one wavelength, so if it's yellow, it is exactly the same yellow, it's not just yellow, it's the same yellow, one wavelength. They are in faze, they start together at the same point. So from this source and from this source, I've got to have them starting exactly the same. And they've got to be the same in size. And the fact is, the only way we can do this is produce them from one source and that is what we do here. You have to remember that this, we're not talking about large distances here, if I were doing this, these fringes here, this distance here would be maybe a millimeter to one centimeter. So this is a very small scale. But we can do this if we use a laser for example, we can do this very easily with a laser here and two slits here and we can see this very big because these stripes get wider and wider the further away we get so if we can make them 30 meters away, we can see them a foot apart. So what we have are two identical sources in size, that's amplitude, in faze they start with the same part of the wave together and they're the same color. That is what we mean by coherent, from the same source or similar sources is the only way we can do this. This for example could be a sodium light, sodium yellow is a single color of sodium, well too close, but very, very, very monochromatic. Two little slits these may be a millimeter apart and we will see this through a microscope we will see these fringes. So that is what we mean by coherent light. Monochromatic, in faze, identical in amplitude.


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