photo credit: David Eppstein; image source

Fresnel Lens

If light bends at the air-glass surface, then what good is the glass inside the lens?  This question led to the development of the Fresnel lens--the photo above (hi-res version) shows an example, in a lighthouse. To learn how these lenses work, visit Fresnel Lens from HyperPhysics.

To see a hi-tech use of these lenses, check out Cheap, Superefficient Solar from MIT's Technology Review.

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Cheshire Cat

For a remarkable optical illusion, see Cheshire Cat from the Exploratorium. By watching a person's face with one eye and a wall--reflected in a small mirror--with the other, you can make most of the features of the face disappear, often leaving only the eyes or mouth. Try it!

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image © CERN; image source

Electrons and Positrons

In the photograph above (hi-res version), each pair of spirals that originate from the same point is the track of an electron-positron pair. The positron is the antiparticle of the electron--it has the same mass as the electron, but the opposite charge. These pairs are formed when a photon--which leaves no track--changes into particles with mass. For more information about this photo, see the "antielectron (positron)" section of the "Gallery of Bubble Chamber (BC) Pictures" at CERN's Introduction to the BC Site and "Special pictures and their interpretation: Photons, electrons and positrons" at CERN's Archived Bubble Chamber Teaching Materials.

You can find more on antimatter on this Exploratorium page.

Quarks Unbound

Here are some interesting sites from the Physics to Go collection on elementary particle physics:

• Quarks Unbound -- Find out why particle physics is so exciting (from the Division of Particles and Fields, American Physical Society)
• The Particle Adventure -- All about particle physics (from the Lawrence Berkeley National Laboratory)
• Fermilabyrinth -- Particle physics games and puzzles (from Fermilab)