Convex lenses

  • #1
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Homework Statement


A thin double convex lens of focal length f is broken down into two halves at the axis. The two haves are then combined, as shown in the figure. What will be the focal length for the arrangements?

http://www.meritnation.com/app/webroot/img/shared/content_ck_images/images/Untitled(375).png [Broken]

Homework Equations




The Attempt at a Solution


For (i), I think that the answer is infinite because any incident ray will either go undeviated or will be laterally displaced - but the combination will not be able to converge it.
upload_2015-2-10_17-57-20.png

Is it right?

And, about the second arrangement - I think it must be 2f, but in my book, f/2 has been given as the answer!
upload_2015-2-10_18-2-55.png
 
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Answers and Replies

  • #2
Quantum Defect
Homework Helper
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Your answers look good to me. I think you even realize that f/2 is the correct answer, based upon your drawing...

The combined lens will be more focusing -- i.e. shorter focal length. 2f is a focal length 2X as long. Can that possibly be right, given your picture?
 
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Likes Prashasti
  • #3
63
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Ahh! I got it!
Power is inversely related to focal length. Now, because the combination is "more" converging (more power), its focal length will be less. :headbang:
I think it's correct now.
Thanks!
:oldsmile:
 

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