Comparing short cameras and long cameras
Practical Activity for 14-16
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
- Cleaning tissue
- 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.
- Feel the lens with your fingers, to judge the curvature, and then clean it with the optics cloth or soap and water.
- 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-tested in January 2007