| 2 comments ]

First, we need to find the angle deflection of light ray if it passes a convex lens.




Hence,




By using lens equation,

we can get



If the lenses are separated by angle of θ, then the total number of lenses is



If we want to trap a ray of light, we just need to have the light forms a (nearly) circle path. On one cycle of the path, the light passes N lenses, so the total deflection is




The total deflection must equal to ,





In order to get the longest focus length, variable h must be at its maximum value (i.e. aperture length, D)



That's my solution.
If you have any question, correction, or comment, please leave a comment below.
Thank you.

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2 comments

Sebastian said... @ March 26, 2009 at 3:40 AM

The maximum value of h is D/2, so the final result is half the one given in the above solutin

ANISH said... @ July 26, 2009 at 10:47 PM

If the angle of deviation becomes 2pi then how will the paraxial approximation hold true. If paraxial approximation fails then we cannot use the general lens equation 1/f = 1/V + 1/U.

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