We started our afternoon by recreating how we made a telescope last time out of two magnifying glasses, since we didn't get pictures then. Fortunately it was sunny, so on the way to the lab, we got to try focussing sunlight with different lenses, or even two lenses stacked up.

Back at the lab we reviewed Fermat's principle of "least time" (which can explain reflection and refraction), and learned about a new kind of reflection, total internal reflection.

Then we did a fun experiment with reflection, in which a laser beam makes an "optical lever" that can magnify the tiny movement of the hour hand of a clock. And finally, we made a "gradient index" tank with a mixture of two liquids, corn syrup and water, that have different indices of refraction.