Comparing short cameras and long cameras
Comparing the curvature and shape of long and short focus lenses.
Apparatus and materials
For each student
Plano-convex or biconvex lens + 7 D or 150 mm focal length approx
Plano-convex or biconvex lens + 2.5 D or 400 mm focal length approx
Bath of water and detergent
Wall, paper or card to form a screen
Health & Safety and Technical notes
Soap and water are good for cleaning the lenses. Alternatively, use a soft cloth for cleaning spectacle lenses.
a Feel the lens with your fingers, to judge the curvature, and then clean it with the optics cloth or soap and water.
b Hold the + 7 D lens in your fingers and catch the image of a distant window (or lamp) on a wall or a sheet of paper. Repeat with the + 2.5 D lens.
You might ask students:
Which lens makes the larger image? Which lens needs a longer camera? What kind of lens would you expect to find in a tiny camera used by a secret agent?
The weaker lens produces the larger image but also needs a longer camera.
This experiment was safety-checked in January 2007