# Reverse calculation of radius and principal planes of thick lens

1. Dec 24, 2012

### taimoortalpur

Dear All,

I need you kind support in a problem, which I am unable to figure out. It is regarding thick lens.

I am determining lens principal planes and radius using shaping factor and focal length. The forward problem I am using is,
Focal length
1/f = (n-1)*[1/r1 - 1/r2 +(n-1)*d/(n*r1*r2)]
Shaping factor
X = (r2 + r1)/(r2 - r1)
Solving these two equations for r1 and r2, I get.
1/r1 = (X+1)/ [f * (n-1)*(1+sqrt(1+(X^2-1)*d/(n*f)))]
1/r2 = (X-1)/ [f * (n-1)*(1+sqrt(1+(X^2-1)*d/(n*f)))]

These equations return complex number when (X^2-1)*d/(n*f) < -1. So I cannot use these equations for this domain and I have no idea how to determine radius and principal planes in such a case. Any body know a solution for this domain or more general equations?

Thanks in anticipation for reading and support.

2. Dec 25, 2012

### sankalpmittal

Does these sources help ? :

http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/priplan.html
http://www.math.ubc.ca/~cass/courses/m309-04a/sol4.pdf