July 16, 2007 Issue

Physics To Go 29 - Short/long focal length

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Physics in Your World

Properties of a Liquid-Drop Variable Lens image
Photo credit: Gisle Noel; photo courtesy of Philip Greenspun; image source

Properties of a Liquid-Drop Variable Lens

Notice that both the ant and the small image of the flower are in focus in this photograph (high-res version)--therefore, both must be at approximately the same distance from the camera lens. Since a drop of water has a small radius of curvature, its focal length is short, only about half a centimeter, so the image is close to the drop and is much smaller than the flower itself.

-- For related activities and information, see Properties of a Liquid-Drop Variable Lens.
-- Build your own water drop lens at Kitchen Science Experiments: Drop Magnifier.

(This feature was updated on July 15, 2013.)

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Physics at Home

Exploratorium: Snacks About Light

To do a simple experiment with a convex lens, go to Exploratorium: Snacks About Light, scroll down to the bottom of the list of activities, and click on "Water Sphere Lens." Try using a fishbowl of water as a lens. You can do the same activity with a magnifying glass, but be sure NOT to look through the magnifying glass into a bright light, or into the sun.

(This feature was updated on July 16, 2013.)


From Physics Research

40-inch Refracting Telescope image
photo credit: Yerkes Observatory; image source

40-inch Refracting Telescope

This photo (hi-res image) shows the moon, looking along the 40-inch refracting telescope at the Yerkes Observatory. Its focal length is long--about 19 meters--so the real image it makes will be big. This is the world's largest refractor.

(This feature was updated on July 15, 2013.)

Worth a Look

Make Your Own Telescope

Check out the Exploratorium's Make Your Own Telescope for instructions on how to build a telescope with simple materials.

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