So here's our interferometer. Let's take a look. Starting from the laser, beam travels to the mirror, second mirror through a lens. The beam splitter cube is the part that actually takes the beam and splits it up into two separate beams. One going straight through, one going out from a right ankle. Each of the two beams hits a mirror and gets reflected straight back where it came from. So the beam from this mirror goes back into the cube, gets split up again passing part of the beam through and part of the beam coming out this way. The beam from this mirror reflected back towards the cube. Again, part of it splitting off towards the mirror and part of it passing straight through. So we end up with two beams coming out in this direction to hit our screen. So we see an interference pattern giving us the familiar bullseye that you've probably seen in previous course. This mirror, you'll note, is that this especially mounted on a translation stage so we can actually easily move it back and forth in this direction by turning this knob. The knob is marked off in a millimeter scale. We can see it's reading just about 10 millimeters now, meaning I've got about 10 millimeters worth of travel where I can still push this mirror in this direction. So let's look at the interference pattern and see what happens when we translate this mirror around a little bit. First we see the interference pattern or bullseye pattern and you think about why, why rings? Why does it look like a bullseye pattern? If I'm interfering a plane wave with a plane wave, the whole thing should either go light or dark all at once. But it's not plane waves revealing with any more. After the light goes through the lens, right, it comes to focus right here. The light is actually a spherical wave. So think about what happens when you interfere one spherical wave with another spherical wave. Alright, so if I reach in here and push with my finger on the back of this mirror just a little bit, you can see the ring pattern contracts towards the middle as I exert pressure on that mirror. All the rings move inward. So remember that, right? When the mirror is current positioned and I push on the back, the rings move inward. Now I'm going to turn the knob and move the mirror continuously in that direction and let's see what happens to the interference. The first thing you notice is as I'm turning the knob, the ring pattern becomes invisible because the fringes are flying by so quickly you can't resolve them. The second thing you notice is I move that mirror closer to the beam splitter is the ring pattern gets larger. Now our rings actually don't look too much like rings anymore. They're starting to get a little distorted. So at this point, looking at rings anymore, we just kind of have a squiggly line over there. But at this point, more or less made the length of this arm exactly equal to the length of that arm. Because our optics aren't perfectly flat anymore, that takes our nice symmetric waves and kind of distort them because the beams, the rays are not reflecting off flat surfaces. Our scale now reads something like a little more than seven millimeters. So I've translated the mirror about three millimeters closer to the cube and now we have a somewhat different ring pattern. I'm going to keep translating it in this direction a few more millimeters and we see as I do that our ring pattern now reappears. I'm going to make a slight adjustment to the mirror to get it centered on this ring. Alright, now we're back to looking at a ring pattern again. Except now when I press with my finger on the stage over here, the rings go outward instead of inward. So this is different behavior from what we saw when this mirror was on the other side of where the two arm lengths are the same length.