An Ice Lens

Almost every optical instrument, from binoculars to cameras to eyeglasses, contains lenses that were made by opticians. You can discover basic lens-making techniques in this activity.


  • A tennis ball
  • A sharp knife
  • Vegetable oil
  • A small juice glass
  • Distilled water (from the supermarket or drug store)
  • A freezer
  • A magazine


  1. Ask an adult to help you cut the tennis ball in half with the knife.
  2. Dip your finger in the oil and put a thin coating of oil on the inside of one of the halves of the tennis ball. The oil coating will make it easier to remove your ice lens from the mold without breaking it.
  3. Set the tennis ball-mold in the glass, hollow side up, and adjust it until it is level.
  4. Then fill the oiled mold with distilled water until the surface of the water is even with the rim of the ball.
  5. The lens should be as close to being a perfect hemisphere as possible.
  6. Put the glass, ball, and water in the freezer.
  7. Check every few hours to see if the water is frozen and clear. If you leave it in the freezer for too long, it may get cloudy.
  8. When the lens is frozen, pop it out of the tennis ball.
  9. Rub your hands over the flat part of the lens to smooth out any rough areas.
  10. Hold the lens over a magazine with the curved side facing your eyes to make text look bigger. Move the lens up and down to observe focusing effect.
  11. Hold the curved side of the lens toward your TV, and try to project an image of the TV on a piece of paper. Go back and forth to find best distances between the TV, the lens, and the piece of paper.


Opticians make lenses by pouring hot molten glass or plastic into molds. The water magnifying lens does not make as clear an image as a lens made out of glass. (You may have noticed this when you tried to use it to watch TV on a piece of paper.) However, the fuzzy image has more to do with the shape of the lens than with the material from which it's made: the ice lens has a much rounder curve than a glass magnifying lens. In most cases, when a magnifying lens has too rounded a curve, the image will be poor. Very skilled craftspeople also make lenses by grinding glass into curved shapes.