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## Homework Statement

## Homework Equations

1/s+n/s′=1/f

where

s is distance from source to diffracting surface,

s' is distance from diffracting surface to focus,

f is the focal length,

n is the refractive index.

## The Attempt at a Solution

Since we have parallel beams, we have s = infinity so the equation reduces to

n/s' = 1/F' <=> s' = n*F'

If our coordinate system is placed so that we have origo at F', we get:

s' = sqrt(x^2 + y^2)

inserting and solving for y we get:

sqrt(x^2 + y^2) = n*F'

x^2 + y^2 = (n*F')^2

y = +/- sqrt( (n*F')^2 - x^2 )

But the solution in the book says:

What am i doing wrong?

Also, i notice that my solution is a spherical surface with a radius n*F', but the books solution is ellipsical. From optics we know that it should be a spherical surface, so is the book simply wrong?

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