We did another set of optics experiments this week, this time using lenses. Many years ago I bought an Optics Discovery Kit from the Optical Society of America (OSA), which is a great little kit that comes in an plastic box very similiar to old VHS tape boxes. Included in the kit are lenses, a bit of fiber optic, polarizers, a diffraction grating, a hologram and instructions for a number of experiments. This kit is perfect for a homeschool family studying physics and I’m happy to say its still available – follow this link to purchase the Optics Discovery Kit. This kit does NOT come with an optics bench but you don’t really need one.
I have a few other sets of lenses so was able to put together three lab setups for class but basically followed the instructions that come with the kit. Students used the lens to focus an image of a window (far away object) and measured the distance of the focused image from the lens to find the focal length of the lens. They also measured the magnification produced by one of the lens by dividing the image size by the object’s actual size.
One group also proved the lens equation by measuring the image distance, i, for a couple of different object distances, o. They had already measured the focal length, f, and could check to see if 1/f = 1/o + 1/i, which it did.
The optics bench in the photos above was the cheapest one I could find, and it shows when you try to use it. Its nice for being able to slide the components along the bars and there is a ruler along the side (far side in the photo so you can’t see it) but the screws that hold the posts don’t tighten very well which can be annoying. I’m really not that happy with it so I’m not going to provide a link. I also have two meterstick optical bench kits which are really cheap, only $15 or so, if you already have your own meterstick. These do NOT come with any optics so you have to purchase a few lenses. You can see the meterstick with stands in the photo below, but the plastic lenses in the photo are from the Optical Discovery Kit. I did not take a picture of the third set up. For our light source/object, one group used an iPhone and the other two groups used the LED Light Blox with wax paper taped over the end and in the case below, the number 4 draw on it. This made it easier to focus the image. If you just tried to focus on the bright bulb it was difficult to tell when it was in focus, and you couldn’t really tell if the image was upside down or rightside up. You could use any flashlight for this.
When students finished with the labs, they did some ray tracing worksheets for lenses. Here’s a good webpage describing ray tracing for lenses by Hyperphysics at Georgia State University.