Welcome to another Unisir Physics Vodcast. Today we are going to be looking at a phenomena called Newton's rings, named after Sir Isaac Newton who studied the phenomena and in many scientific measurements. This podcast is actually in response to two questions emailed to us by Caitlin. The first being the relation between the curvature of a convex lens and the size of the rings. He said you tried the experiment at home and couldn't see anything, so we'll look a little bit into that and the reasons why. And the second, what does he affect? The different colours have on the rings. Does it change their size, their intensity, etc. Okay, well let's start by looking at the configuration and what do you actually need? In a textbook they have pictures like this. They'll have a little eye up the top and some light coming down, come from a different angle. Sometimes they've got weekly bits representing the light. Anyway, you can have a look in textbooks and see the different types of representations or just have a surf around on the net. Now what does this look like in reality? Well, it's depicting a magnifying glass. Here's one here and it is actually, if we align it up there, you'll see the shape. It's actually flat on this edge here, curved here. And on the bottom there's actually a plate. I've got a large glass plate there, so that's actually sitting there and our lens is sitting up next to it. So here's how the experiment will be set up. We have our plate. We see our lens on the top of it. Curveside down. Okay, so what do you get? Caitlin said she tried this and saw nothing. It certainly doesn't look anything like it does in the pictures or diagrams in the books. Instead of concentric rings bright and dark. But if you just get the light right in the angle and look really carefully, you will see something interesting. Especially if you just grab the lens and just press downwards on it a little bit. You can't see it on the video, but there's a little dot there. It looks like a little dark dot at the point where the lens touches the glass plate. If you rock it back and forward, you can see it moves back and forward across the plate. But you do need to get the lighting just right. It's only about a third of a millimeter across. That's when you're pressing reasonably hard on the top of the lens there. Now, if you get another magnifying glass actually, like one of these, and actually put it over the top of the lens and push down on it and look down, you can actually see it a little bit larger. You could also put it actually under a low-powered microscope and see the rings as well. Now, another part of Caitlin's question was what about the curvature? Well, if you actually grab a magnifying glass, it's a little thicker and put it on top and do the same thing, press down on it and look very carefully, rock it backwards and forwards. You'll also see a dot and what you'll notice, even between these two here, is that the dot associated with this one is slightly larger than the dot associated with this one. Now, I can hear you'll say, how can we see Newton's rings easily? Well, actually, I have a paper weight that does just this and here it is. Okay, let's have a look at it. It actually has two rings of metal, there's three thumb screws there, and sandwich in between the two pieces of metal are two pieces of glass. The bottom one being a flat piece of glass, it's actually frosted on the back and the top one is flat on the front with a very, very slight curve through the center there. So, when you actually hold it down, and if you adjust these knobs, you'll see just here, the rings and they move around. Let's have a look, I'll just bring it up into the light there and you'll see that there's some rings and they're called Newton's rings. Okay, let's investigate Caitlin's second question concerning the size of the rings under different coloured lights. Now, for the different colours, we'll use laser pointers. We have a red laser pointer, a green one and a blue one. For the experiment, they'll all be mounted on this tripod, and the laser pointer itself points up at a piece of paper. A piece of paper has been fold in half, and it's held by a tissue pig that's mounted over to a retort stand here. Now, we all know that laser pointers have little points of light, so what happens is, it'll form a little point of light on the back sheet of paper, the light will be reflected off, and it'll be filtered through the front sheet of paper, which will illuminate the Newton's ring device down here. So, I'll give it a nice diffused light over the whole surface of the glass plate there. Now, at the back here, we've got this thick glass plate. That's not really to do with the experiment. It's just sitting under the back of the apparatus here to hold it up at an angle, so we get a good view of the Newton's rings. Okay, well, let's just see how it works. I'll just switch on the laser, and you'll see over here, Newton's rings. Now, I can't really video this demonstration as the light levels are really critical, and my video camera gets very confused, because the lights are going to be off, and each of these lasers here have different intensities. We need to adjust the camera exposure correctly, so we can see the Newton's rings really clearly. So, what we'll do is take a series of photographs, and then we can examine each one of these. I'll do a red, a green, and a blue, and then I'll try to do something a little bit clever. I'll move this piece of paper downwards, and I'll half illuminate the rings with the laser light, and up here, I'll put a desk lamp about two meters back, and try to illuminate the top half of the Newton's ring device with white light, so that we can get a comparison between what the laser light's doing to the rings, and what the white light does to the rings. Okay, it's been about two hours, and I've taken about 30 photographs, and I'm going to show you the best of the best. The first one up is the one that's been illuminated with the white light, and if you look closely at the rings, you'll see they're made up of different colors. The outer edge of the rings red, the inner bit of the rings green, and the closest in edge is actually a movie blue. Now, the second one, we move to the coloured laser lights, we'll start off with the red, then the green, then the blue. So, Kateless question was to do with what happens as we change between the laser colours or the colours of light, and let's have another look, we'll look at the red, green, the blue. Now, you probably have seen that it seemed to have got smaller as we went up the colours, so from the red, and the green looked a little bit smaller, and the blue looked a little bit smaller again. Okay, under the second experiment I did, I illuminated half of the device with white light, and the other half with the laser light by moving the paper down as I told you about in the experimental setup. So, let's see how it went, we'll look at the red light, now you can see the red light seems to on the right hand side, we've got the colours, comes around the red, joins onto that in the red ring forms, we'll look at the green, we'll see that the ring seems to form off the edge of the green, and the blue. We'll just look at those again, red, green, blue. So, I hope you can see what happened there, but the ring seemed to get smaller, and the rings on the left hand side, the laser light illuminated rings, lined up with the rings that were formed from the white light with the same colour. Okay, well let's revisit Caitlyn's questions, the first in relation to the curvature and the size of the rings. Now, the less curve gave the bigger rings. As we saw, we didn't see, when we used the magnifying glass, just sitting on top of the flat plate, the rings were very small, like less than a quarter of a millimetre, but when we used my paperweight, the actual Newton rings demonstration, which has a very little curvature, we could easily make the rings large and visible. Okay, a second question was, what was the effect of illuminating the rings with different coloured lights? And as we could see, the rings shifted slightly. So, as we move from the red to the green to the blue, the rings got slightly smaller, but the really interesting thing was is how they lined up with the rings that were formed when we illuminated with the white light. So, two excellent questions, and you can examine the detailed photographs that accompany this podcast, and also do some extra research to learn even more about the physics of Newton's rings. Okay, I hope you enjoyed watching the Unisir Physics podcast, and thanks again to Caitlin for emailing in her excellent questions.