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Old Dec 7th 2009, 02:52 PM   #1
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Exclamation diameter of the smallest image that lens creates

A point source of white light is located 3 meters from the lens in the previous problem, on its axis. The locations of the point images are at different places for different colors because of chromatic aberration so that the combined image is a circle. If the aperture of the lens has a diameter of 2 cm, what is the diameter of the smallest overall image of the point source that the lens creates?
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Old Dec 30th 2009, 05:16 AM   #2
max
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Hi, What level is this question? I would work out the angle the blue light is refracted, and then the same for red light, then use trig to work out what this ends up showing at 3 metres.
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Old Dec 30th 2009, 07:58 AM   #3
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max, will i not need the focal lengths /refractive index for red and violet?
otherwise how will i calculate the radius of the zone of least confusion?
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Old Dec 30th 2009, 08:47 AM   #4
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I'm hoping that there is some info in the previous part of the question in which the lens is described. Sorry I havent heard of a zone of least confusion, we might just be using different terminology though. I've got some notes on chromatic aberation somewhere, i'll take a look.
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Old Dec 30th 2009, 09:13 AM   #5
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i mean the region where the image has least diameter. i think its just a matter of terminology.
if i know f red and f violet ,i will know the distance between the positions where the image has reddish rim and violet rim, and then use similarity relations to find the radius of the circle of least diameter between these two extremes?
max,for your convenience, here is the link to the previous question, it doesnt have the info, though.
http://www.physicshelpforum.com/phys...een-spots.html

Last edited by r.samanta; Dec 30th 2009 at 09:28 AM.
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Old Dec 30th 2009, 09:35 AM   #6
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I'd imagine that it will just be the diameter of the violet circle as it should all be symetrical and that will be the outermost. I am a little confused about it being in different places.
alex 83, can you say what the previous part of the question is? Or even better upload the whole question? It's easier to work out if it's in context.Thanks,
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Old Dec 30th 2009, 09:42 AM   #7
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http://www.opticampus.com/circle_confusion.gif
http://media-2.web.britannica.com/eb...4-FD6EE753.gif
this can help,max.

Last edited by r.samanta; Dec 30th 2009 at 09:45 AM.
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Old Dec 30th 2009, 10:05 AM   #8
max
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Thanks, I have to admit I havent seen that description before but it makes sense. I'm not sure if it is the same thing as the smallest image though, but rather the smallest image where the image is not distorted ie. all the pieces are in the right place. I think the smallest image will occur at the violet focal point and the size of the image will be that of the red circle. I could well be wrong though. Does the original question contain any of the focal lengths/refractive index information?

Last edited by max; Dec 30th 2009 at 10:14 AM.
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