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posted by  KatD on 7/28/2008 2:13:51 PM  |  status: Live  

Conceptual telescope problem. Urgent! Will rate well!

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1.)The images of the astronomical and terrestrial telescopes used in an experiment are colored around the edges. Why? How can this "chromatic aberration? be avoided in practice?

2.)An optical lab stocks a large collection of lenses with both positive and negative focal lengths. A lens with positive focal length larger than the largest available positive f lens is urgently needed. Suggest how this can be achieved using a combination of available lenses.

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Oracle
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posted by Steve (retired) on 7/28/2008 3:32:50 PM  |  status: Live
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KatD's comment:
"Great explanation..thanks!"
Response Details:
For (1)

Chromatic aberration is the abberation caused due to different wavelengths being brought to focus at slightly different points. This causes objects like stars to ve surrounded by fuzzy, rainbow coloured halos.
Chromatic aberation can be corrected by using a second carfully designed lens mounted behind the main objective lens of the telescope to compensate for the chromatic abberation and cause all wavelengths to focus at the same point.
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Oracle
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posted by Steve (retired) on 7/28/2008 3:34:18 PM  |  status: Live
Asker's Rating: Helpful   
KatD's comment:
"Thank you !"
Response Details:
For (2)
    
 
When two lenses are combined together, the focal length of the combination
                                1 / F = 1 / f1 + 1 / f2
where f1 and f2 are the focal lengths of the individual lens
We take a Converging lens have the maximum focal length
and a Diverging lens having the maximum focal length and combine them .
The focal length of the combination
                                F = [f1 *( -f2 )] / ( f1- f2  )
the denominator will be a minimum and therefore F will be a maximum.
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