# What Is Interference of Light?

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Interference of light can arise in the form of constructive interference or destructive interference. Find out why interference of light isn't normally visible 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 tell you a little bit about what the interference of light is. We know that light is wave motion so if I've got two sources of light here, call it S one and S two, each will produce waves. This source needs to be monochromatic and also the same amplitude and in phase, and it is this that makes the property, property, we'll talk about that later. But, we have two sources of waves, we know if we have two waves, call them wave one and wave two, there are two ways they can react, either with constructive interference, that is when the peak of one wave hits the peak of another, it just makes a bigger wave and a bigger trough. So, this is constructive interference, you'll get a much bigger wave. But, also the peak of a wave can hit the trough of another wave which means that we get no wave, so we get zero wave, destructive interference. Now, this is the basis of interference, this is, would work with water waves, but we're dealing here with light. We have to remember the wave length of light is very short, millions of wave lengths in a centimeter, so it is not an effect that we're going to see normally, we would see it, we would need a microscope to observe these fringes as we call them. What is happening here, the light from S one and S two if they go the same distance to the center as they will, we will get them in phase, we'll get constructive interference and a bright band. Now, as we go further down, S one has to travel a bit further than S two, in fact we can travel so far down that S one has to go a whole wave length further than S two to get to here, and in that case, again we're going to get a bright band, because we'll get the two peaks coming together. It doesn't matter the fact that one has traveled farther than the other. And, this will occur again time after time, so we will get a series of bright bands equally spaced, going out from the center, the center one is going to be the brightest, and they will get gradually dimmer as they go further out. But, in between the bright bands we're going to get a dark band because the dark band is where S one is traveling half a wave length further than S two. If it travels a half a wave length further, it looks like this, in which case they cancel out, so it's going to be dark, and therefore, we'll get dark bands and bright bands, it'll look like a series of vertical stripes. Now, practically to look at this is a different matter. If I want to look at this practically, I would have a source, call it here, a source, I would have a single slit, very narrow single slit about this distance from the source and then something like twenty centimeters from that have another pair of slits and the separation of these slits should be something like maybe half a millimeter, point five millimeters. The closer they are, the better. And, then I simply have here a microscope and I look through, this source should be a sodium source that is a yellow light, do it in the dark. The first thing you have to make sure of though is that this slit is in line with these, because this looks like a piece of metal with a slit in it. This look like and you often do it with microscope slides, blacken them and then rule two lines so these will be vertical. Make sure these are in line, cause their at an angle, you won't get anything, so make sure their in line, so just rotate them around. And, in this you should see a series of bright and dark bands, if that's too hard, try the easy way, and the easy way is to get a laser. If you get a laser here, a red laser, it has got monochromatic light, it is very focused, just shine is through your double slit and on the wall at this side will appear a series of spots, showing the interference fringes. So, when you ask me the question, what is the interference of light. Well, this is what it is and this is how you see it, you won't normally see it, you won't normally see it in normal circumstances unless under special circumstances you have to set up an apparatus of this type. So, have a look for interference, this is what you should do.

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