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 Be the first pioneers to continue the Astronomy Discussions at our new Astronomy meeting place...The Space and Astronomy Agora Re: Measuring Actual Focal Length Forum List | Follow Ups | Post Message | Back to Thread Topics | In Response ToPosted by Tom Leech on January 10, 2001 20:23:42 UTC

hi marty, start from scratch for a moment, and forget what ever you have been told before. think of your mirror and a perfectly collimated (parallel) beam of light reaching it from an infinite distance away (the sun). now hold the mirror nearly in the shadow of a building while peaking around the corner at the sun. place a white fire resistant piece of paper high enough on the corner of the building (building preferably made out of brick), and step back with mirror in hand and focus the reflected image of the sun on the white fire proof paper. have a friend measure the distance from the image on the paper to the mirror surface. you now have the focal length of the lens. the radius of curvature of the lens is twice its focal length, so if you now go inside and with a tiny flashlight or candle if you also are a pyromaniac, hold the light source at distance twice the focal length away from the mirror, and focus the image of the light source on that same fire proof piece of white paper which would be placed right next to the light source. you now have found both the focal length and the radius of curvature. all that you have really done is refocus your mirror from a light at infinity to one that is at a distance equal to the radius of curvature (twice the focal length) of your mirror, just like taking a close-up shot with a closeup lens. tom leech