This is the classic lab where students use a candle and convex lens to project a real image of the candle (which acts as both object and light source) onto a screen and then use the measurements for the distance of the image and the distance of the object to calculate the focal length of the lens used. In this lab, students will use two different convex lenses (of two different curvatures; one thicker than the other) and calculate the focal length for each.
Suggestion: Turn lights down in the classroom and once students have projected a clear, sharp image onto the screen, have them wiggle their fingers just behind the flame. They should see a clear, sharp image of their fingers appear on the screen. This always seems to elicit a big "WHOA!"