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.
-- For more on images, visit Geometrical Optics from the University of Pennsylvania.

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Physics at Home

Building A Telescope

Visit Building A Telescope to find out how to build your own reflecting telescope.  You'll need a long and a short focal length lens and some cardboard tubes.


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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 (see Geometrical Optics). This is the world's largest refractor.


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