Introduction

Benjamin Franklin was a scientist and an inventor. As he got older, he noticed he needed glasses for reading and seeing things far away. Franklin solved this problem by inventing bifocals, which were glasses made with two different lenses, one for seeing things up close, and the other for seeing things far away.

Water acts just like corrective lenses made of plastic or glass. The water refracts or changes the direction of a light beam. A water lens is known as a convex lens because it curves out in the middle. Franklin used convex lenses for both parts of his bifocals. One part of the lens was for reading up close, and the other helped him see at a distance.

Materials

  • Clear plastic wrap
  • Newspaper
  • Tap water

Activity

  1. Lay the plastic wrap on the newspaper.
  2. Have the students put a droplet of water on the plastic wrap. Ask them to explain what happens to the print underneath it.
  3. Add more water. Ask what happens to the print now that the shape has changed.

Conclusion

Use your experiment results to draw conclusions about Franklin’s bifocals. You can ask the students to determine which lens was more convex (more curved in the middle), the one for reading or the one for distance.

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