Comparing short cameras and long cameras

Class practical

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

Read our standard health & safety guidance

Soap and water are good for cleaning the lenses. Alternatively, use a soft cloth for cleaning spectacle lenses.

Procedure

1. Feel the lens with your fingers, to judge the curvature, and then clean it with the optics cloth or soap and water.
2. 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.

Teaching Notes

• 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