hello..... i have a student who is doing the DO course and I had graduated DO course about 10 years ago. I have forgotten all the formulas. Not sure how to explain or show the student.

Here is the questions:

  1. What is the sag of this surface over a diameter of 60mm?
  2. A pencil of light is made to converge to a point 20cm beyond a lens. Find the curvature of the wavefront:

a) on leaving the lens
b) 1cm from the lens,
c) 19.9cm from the lens
d) 21cm from the lens.

some guidance and help would be great.

Thank you.