This is the physics 202 helium neon laser lead, so let's take a look at the parts. The glass tube you see in front here is the laser tube. It contains a mixture of helium and neon gas, and in operation an electric discharge pumps the neon atoms up to an excited state where they can participate in the lasing action. This isn't a complete laser. This is just the gain medium where the amplification occurs. And if you look carefully at it, you can see the ends aren't mirrors or angled at a very particular angle, which you might be able to figure out what that is from a previous lab. But to make a laser the light has to bounce back and forth between the two ends of the cavity. So to do that we add mirrors at each end. So here's in a certain of various mirrors we can add into the cavity. To help our alignment we've got a laser bouncing off a couple of mirrors again and going straight down the board of the cavity, straight down the board of the laser tube and passing through hopefully on the strike. Alright, so we're going to use that laser to help with our alignment. Now I want to add a mirror to the cavity in one end. I'm going to start with this mirror. The OC means it's an output coupler. That means the reflectivity is 97 or 98 percent. So it lets a couple of percent of light pass out the back of the mirror. The 60 means it's got a radius of curvature of 60 centimeters. So I'm going to put that over here and position it so the beam hits the center of that mirror and then I'm going to look for a reflection to go back through our laser tube and back into the laser. So I should see a spot of light on the front. The light, let me tighten this up and then see if we can zero it in. There it is. Alright, so we'll say that mirror is in place. Now about the other end of the laser, I want to add this mirror, at the end of the laser cavity, the HR means it's a high reflectivity. It reflects as much light as we can get out of it. It's probably greater than 99 percent. The 60 is again a 60 centimeter radius of curvature. We're going to mount it at the other end of the cavity over here. By bouncing our laser off the back end of the mirror, we can pick up the reflection over on the shutter in front of the laser over there so I can tighten that up and then zero it in. So my beam is going back into the laser and I like that. Now if the physics gods are kind to us, when I turn on the electric discharge, we'll see hopefully see some laser discharge coming out of the laser end. I'm not seeing anything coming out of the laser right now so I'm going to adjust the output of the laser a little bit and see if we see anything. There it is. All right, you can see on the spot over here our laser is now lasing lights bouncing back and forth in the cavity. Every time it goes through the gain tube over here, it's amplified a little bit. The amplitude builds up a couple of percent of the light leaks out and that gives us our spot over here on the screen. If you put a lens in front of it and blow up that spot, and you can see what we're getting out of it is basically just a Gaussian spot.