Measuring the speed of light using a microwave

Physics Lab Assignments

Transcript: PHY 21041 Lab 12

For this lab, you need to do a little bit of work ahead of time. You need a pair of glasses, like old people would wear. Not prescription glasses, not bifocals‐ they wouldn’t work, just ordinary, plain vanilla reading glasses. If you’re too embarrassed to borrow them from somebody, just go to Goodwill or something like that to get the cheapest reading glasses you can find. So go ahead and get some glasses and come right back! Unidentified voice: Hey, you young whippersnapper! Come back here with my glasses! And get off my lawn! Unidentified voice: Sorry, he made me do it! I’ll be right back. Unidentified voice: Here you go! Gotta run! Oh, Thank you! I guess. Now all we need is a place to set this up. We need a really dark room. I wonder where we could find one of those. Well, this is convenient. Let’s go on in. We found our dark room, now. Here are the glasses my student just got for me. And I’m using my laptop as a light source. I found a big, red arrow there as an image that we can use. I’m going to set it right here at the end of this long table, with the screen straight up and down. Way down there, at the other end of the table, I have the piece of index card that came from your packet. I have it attached to your book end down there. I have another book end right here that I can use to steady the lens so the image will be sharp and clear when I move it down there. Let’s see what you would do now in the lab. I’m going to hold the glasses, covering one lens. Only one lens is being used. I’m going to start a long distance away from the computer screen. The most common problem is in doing this lab is starting too close. So start a good distance away from the computer screen. And what I’m going to do now is move the lens further and further away until I see an image come to focus on my card over here. And there it comes, we’re almost there. And there is a nice sharp image in the card. You’ll need to then measure, this distance from the lens to the computer, that’s called the object distance, DO, because the computer screen is the object. We’ll also measure from the lens to the card. That’s the image distance, DI. From the instructions in Blackboard learn, you’ll see how to calculate the focal length of this lens using DO and DI. What I want you to do is several different trials like this, where you will move the cardboard screen and then move the lens to a new place to refocus again, in each case measuring DO and DI.