Physics To Go is an online monthly mini-magazine and a collection of more than 950 websites with physics images, activites, and info. You can view an archived version of our July 16, 2007 issue, Short/long focal length below, or click to see our September 1, 2013 issue, Two views of Earth.

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.)


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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. This is the world's largest refractor.

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


Worth a Look

Astronomy across the Electromagnetic Spectrum

Check out Astronomy across the Electromagnetic Spectrum to find out how astronomers use different telescopes to learn about the universe. At this site you can learn about each kind of electromagnetic radiation.


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


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