clc 
// Given that
R = 2 // radius of curvature in meter
// Sample Problem 12 on page no. 2.42
printf("\n # PROBLEM 12 # \n")
f = R // calculation for principal focal length of zone plate
printf("\n Standard formula used \n f = r^2 / lambda. \n r = sqrt(lambda*R). \n ")
printf("\n Principal focal length of zone plate = %f meter ",f)