Hi, my name is Stefan. Today, I'm going to be speaking on how to calculate the diameter of a human hair. First what you want to do is you want to get a piece of hair, a real thin hair and then you want to take an infrared beam and pass it through the hair and let it hit a wall or surface. Now when the light hits the surface, you're going to get a center point and then there will be some dim light and then there will be nothing and then you'll see light again and then nothing. Okay, you want to take this distance, we'll call it D1 and then you want to measure the distance to the surface or the wall and we'll call this D2. Now the diameter is going to be equivalent to, since we are using the infrared beam, 650/D1/D2. So when we multiply that out, that's 650 D2, distance from the hair to the wall/D1 which is the smallest distance from the center to the first dull point on the spot of the wall. And that's how you calculate the diameter of a human hair.