6551371_origI started class this week with a quick slideshow on refraction of light and explaining the mystery of the headless polar bears. (photo from http://teamevergreen.weebly.com/light-and-shadow.html)  Refraction, the bending of light at an interface occurs because the speed at which light travels depends on the medium its traveling through.  If light passes from air into glass, the speed of light slows down and the light ray bends closer to the normal (angle of refraction < angle of incidence), but if light is traveling in glass and then enters air, it speeds up and bends away from the normal (angle of refraction > angle of incidence).  For the polar bear in the photo, the light bouncing off its head is passing from air into some very thick windows and then back to air before reaching our eyes.  The light coming from the body of the polar bear is traveling through water and then the thick windows before reaching the air and our eyes.  The index of refraction of water is a closer to that of the window then to air, so there is a bigger shift when the light rays goes from Air/window/Air then there is for Water/window/Air, so the head and body don’t match up!

.198x297x57722-004-17350912.jpg.pagespeed.ic.tEqa88HDBl We practiced ray tracing through thin lens, which is very similar to the ray tracing for mirrors that we did last week and actually part of the same set of handouts.

This handout has a nice discussion of the thin-lens equation and has the basically the same lab that we did in class – take a converging lens and find its focal length by using an object very far away (a bright window), then the image distance will be approximately equal to the focal length.  Then the students put a light source (bright LED) at different distances from the lens and found the image distance and magnification of the image.

I also showed this video in class, which shows how mirrors and lens focus light. 

The equipment we used today included two meter sticks being used as optical benchs. The equipment can be purchased from hometrainingtools.com.  This set up works pretty well for most of these basic optics experiments.  I also recently bought this optical bench, but it was kind of frustrating to use, it was hard to tighten the screws enough to hold things steady – bad system.  So I wouldn’t recommend it.IMG_2515