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Q & A: Focal points of lenses

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Most recent answer: 07/14/2015
Is there a difference between the focus of a mirror and the focus of a lens? When i explain "focal point" to my son by saying that "any object will form multiple images when viewed through a lens. each of these images will be offset from one another by just a little bit. consequently the overall effect will be blurry and dim. however when the screen is moved so that it is at the focal point of the lens, then all these images will overlap perfectly and the resultant image will be sharp and bright." am i correct and is the correctness only limited to lenses but not to mirrors? What is the correct analogy to give for a mirror?
- rajeev (age 41)
New Delhi, India

Hello Rajeev,

You are correct in saying that both mirrors and lenses have similar properties.   They obey the same rules of changing the direction of incoming light rays.  Perhaps a better way of explaining to your son is to adopt the "ray" point of view and to see where a bundle of rays converge to a point.   There are many excellent sites that have nice diagrams explaining both concave and convex lens systems.   Try   as a starting point.   Also you can go to our website and type lenses into the search box.


(published on 07/14/2015)

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