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Finding Focal Length

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Posted by Robert May on October 27, 2002 20:32:57 UTC

The test you describe which is getting a focused image of the flashlight on a piece of paper is going to give you the focal length after a bit of math. The formula is 1/FL = 1/F1 + 1/F2 where F1 and F2 are the focal points where the source and destination are. What is meant here is that the source (the flashlight) is one focal point and you end up finding the other focal point by the use of the paper and find where the image of the flashlight is right and measuring the whole mess. The short way (assuming that you have the paper next to the flashlight) is that the focal length of the mirror is 1/2 the distance to the flashligt or paper. Unless you are really good at seeing the best focus or the mirror is very short for a focal ratio, measuring to a half inch will be about as close as the test will give.

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