2  If you hold a typical pair of glasses over a table, with overhead lighting, the lenses produce a shadow on the surface.  Why should clear lenses produce a shadow?

5 . A common design for a spotlight uses a lens and a mirror, combined with a gas-discharge lamp, to project a powerful parallel beam of light.  Where are the lens and mirror positioned?   

6.  A professor is trying to quickly estimate the focal length of a 2.5 cm diameter biconvex lens.  He takes the lens and finds the image of the overhead ceiling lights on his table to be 3 cm below the lens, so he estimates the focal length as about 3 cm.   His student objects that at the focus there should be NO image formed, so this must be wrong.   Why can the prof make this approximation successfully?