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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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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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.

Astronomy across the Electromagnetic Spectrum

The NASA site It takes more than one kind of telescope to see the light shows how astronomers observe all different kinds of electromagnetic radiation, including:

• Radio Waves
• Infrared Waves
• X-Rays
• Gamma rays