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Shape Up: Does the Shape of a Lens Affect its Magnification?

Author: Janice VanCleave

Problem

Does the shape of a lens affect its magnification?

Materials

  • clear plastic food wrap
  • clear drinking glass
  • tap water
  • sheet of newspaper

Procedure

  1. Tear off a 12-inch (30-cm) piece of plastic wrap.
  2. Line the inside of the glass with the wrap.
  3. Pour about 2 inches (5 cm) of water into the lined glass.
  4. Lay the sheet of newspaper on a table.
  5. Set the glass of water on top of the newspaper.
  6. Look down through the water and observe the newsprint.
  7. Compare the size of the letters viewed through the water with the letters that are not covered by the glass of water.
  8. Lift the plastic-wrap lining so that the bottom of the plastic is about 1 inch (2.5 cm) above the bottom of the glass. The bottom of the plastic wrap should now curve outward from the weight of the water.
  9. Look through the curved water surface and observe the printed letters.
  10. Describe any difference in the size of the letters viewed through the water with the letters that are not covered by the glass of water.

Shape Up

Shape Up

Results

When the surface of the water was curved, it magnified (caused to appear larger) the print; however, when the surface of the water was flat, there was no magnification.

Why?

The shape of the water's surface caused the magnification. When the surface of the water is curved outward, the water acts like a convex lens (a lens that is thicker in the center than at the edges; it magnifies images of objects viewed through it). Light rays moving through the curved surface of the water lens are refracted (change direction). The bent light rays produce an enlarged image of the object.

Let's Explore

  1. Does the depth of the water affect its magnification? Repeat the original experiment using different amounts of water in the glass.
  2. How does the amount of curvature of the water's surface curvature affect the magnification? Repeat the original experiment using larger and smaller glasses. Vary the distance the plastic wrap is lifted from the bottom of the glasses to change the shape of the bottom of the plastic wrap.

Show Time!

  1.  
    1. Would a drop that curves inward affect the magnification? Build a lens holder by cutting a 2-inch × 4-inch (5-cm × 10-cm) piece of aluminum foil. Fold the foil in half lengthwise. Use the point of a pencil to poke a hole, about half the circumference of the pencil, in the center of the strip. Fold down 1 inch (2.5 cm) of each end of the strip to form support legs. Lay a sheet of newspaper on a desk near a lamp, and place the foil magnifier on top of the paper. Use an eyedropper to fill the hole in the foil with water. Look through the water lens and observe the size of the print. Very carefully touch the bottom of the water drop with the tip of your finger. You should remove only a very small amount of water from the drop, which will either drop to bulge outward, producing a convex lens, or to cave inward, forming a concave lens. Repeat this procedure until a concave lens is formed.
    2. Repeat the previous experiment, this time making different-sized holes in the foil lens holder. Display the foil lens holder, along with diagrams of the images viewed through the different sizes and shapes of lenses.

Shape Up

Check It Out!

A lens is a curved, transparent object. Find out more about lenses. What is the difference between the shape of a convex lens and a concave lens? How do these lenses work? Do both lenses magnify? How does refraction of light by a lens cause magnification?

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