# Interference

by Jared Rovny
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00:01 Now that we have a terminology for discussing waves and periodic phenomena and have discussed sound and how sound is a wave and the properties of a sound. We're ready to move on to light and we are going to start with some of the historically interesting things that happened with discussing whether light is a wave or a particle.

00:18 So that's where we'll start. As an overview with light, we'll talk about the wave phenomena first and then move on to the properties of radiation, electromagnetic radiation, and then talk about some of the light spectrum properties.

00:31 First, with some wave phenomena, we'll start with interference and the idea that light waves can interfere with each other in the same way that sound waves interfere with themselves.

00:43 We go back to the historical question first, which is this light, this thing that we're seeing all the time and which we know and have known for many centuries is very important to our perception.

00:53 What is it? Is it a wave? Is it a particle? How does it behave? What can we do with it? Can we manipulate it or capture it? And this is a picture here of Thomas Young, who performed one of the very famous experiment to try to solve this hotly debated question as to whether light behaves like a wave or behaves like a particle.

01:13 Here's what he did, he took two screens like you see here.

01:17 One on the left, one on the right and in one of them he put two holes.

01:22 He cut two holes that were very small.

01:24 So we often call this the double-slit experiment because he cut two very small holes in this first screen.

01:30 And then he asked what would happen if I sent light through these two holes and look at what happen when it impacted the other screen, the screen that's on the far right here.

01:39 He wanted to deduce whether light was a wave or a particle by sending the light through these two slits.

01:45 Here's one that would help, suppose first of all that light were a particle, what would happen? If we sent a bunch of particles through these two slits what we would expect is that we'll have two streams of particles as you see here, impacting the back screen and we should see two bright spots on the back screen exactly where those two holes were that we created in the first screen.

02:05 What happens if light were a wave? We know from wave phenomena that if a wave impact on an opening in some barrier, maybe ocean waves hitting some opening, they would do it called diffraction.

02:17 We'll talk more about diffraction and where it comes from in a little bit but for now all we need to know is that when waves impact on an opening like this, they bend outwards in sort of a spherical shape.

02:30 What would then happen is if both of these waves are impacting this boundary and bending outwards in this spherical shapes, the parts of the waves that are going to be constructively interfering both that are peaks, will constructively interfere on the back screen creating a bright spot as the light constructively interferes.

02:48 On the other hand, if instead of... it constructively interfering with peaks of the light wave meeting peaks of the light wave.

02:55 We can also have the peaks meeting with the troughs and as we saw one we talked about wave addition.

03:00 When these peaks and troughs meet with each other, they destructively interfere because you add a positive with a negative.

03:07 In those cases, the light when it's destructively interfered would create these dark spots on the screen behind it.

03:13 And again, this is in contrast with the constructive interference which comes when peaks meet peaks or troughs meet troughs.

03:19 And so we have these two different phenomena.

03:22 So Thomas Young did exactly this, he sent light through these two slits, these two openings, to see what happens and found that light in fact behaved like a wave.

03:31 Instead of getting two bright bands, he instead observed these many, many bands which as we've just discussed comes from constructive and destructive interference of the light waves.

03:42 We can see on the screen on the back, both the bright bands and the dark bands.

03:46 And it is very important to conceptionally understand that the bright ones correspond with constructive interference and the dark ones with destructive interference.

03:55 So as we know, constructive causes the bright bands, destructive causes the dark bands.

04:00 So now that we know that light certainly has at least one wave phenomenon to it.

04:06 We can ask and discover some things about some other properties that we've seen many times in nature.

04:12 So for example, if light is going down and interferes with or impact on some medium, maybe it goes from air to some soapy layer that's on top of a layer of water.

04:22 What will happen is some of that light, as we'll see when we get to optics very soon, we'll go into the medium at a slightly bent angle and then come back out of the medium.

04:32 Whereas some of the other light will instead just reflect immediately rather than going in and then back out.

04:38 What happens in this case is that we say these two rays of light.

04:42 One that went into the medium and one that just bounced off the medium will have a different phase. We say that the phase is altered.

04:50 Remember our definition for the phase, the phase is how much the light is shifted or where it is in its path as the wave goes up and down.

04:58 So for example now, the wave that bounced off the surface might be at a peak while the wave that entered into the medium and came back out might have evolved in such a way that it is now at a trough or it could have been the exact opposite where the bouncing wave was at peak and the one that went in and back out was also at a peak.

05:16 And so we could have either of these two different types of phenomena.

05:19 When this happens, these waves interfere with each other.

05:22 Sometimes creating peaks and sometimes troughs depending on where the different phases were that we just discussed.

05:27 And we see this sort of diffraction or bending phenomena as light goes into and out of these thin surfaces.

05:34 And so we call this the thin film effect because as we just saw in our example it happens when you have a thin film of something.

05:41 Maybe a thin film of oil over water or a thin film of soap over water.

05:45 And we see this sort of shiny bands that you can see here coming from bubbles or soap or like we said oil.

The lecture Interference by Jared Rovny is from the course Light: Electromagnetic Radiation.

### Included Quiz Questions

1. It creates a diffraction pattern as a wave would
2. It creates two bright bands of light like particles
3. It creates two bright bands and a diffraction pattern as well, acting simultaneously as a wave and a particle
4. It will destructively interfere on the other end, creating a single dark spot
5. It will constructively interfere on the other end, creating a single bright spot
1. Constructive interference causes bright bands, while destructive interference causes dark bands
2. Destructive interference causes bright bands, while constructive interference causes dark bands
3. Different frequencies of light interfere to produce different levels of brightness
4. Different wavelengths of light interfere to produce different levels of brightness
5. Particles of light collide to combine or destroy their energy depending on frequency
1. Light reflected from the oil interferes with light which penetrates the oil and is reflected from the lower surface
2. The different colors of oil and water interfere to produce a sum of colors
3. The free flowing of oil causes a waving appearance
4. The light from below the water and the light from above the water collide to create random color variations
5. The light emanating from the oil enters the water and reflects from the ground below in different ways depending on the different parts of the ground
1. The phase of the light has shifted, causing it to be at a different location in its oscillations
2. The reflected light is constructive, while the transmitted light is destructive
3. The reflected light is destructive, while the transmitted light is constructive
4. The upper and lower surfaces are of different colors
5. The upper and lower surfaces are of different phases

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