Re: Lens and light questions

[Bob Sciamanda <sciamanda@worldnet.att.net>]

LUDWIK KOWALSKI wrote:
> The idea of well defined principal planes is in conflict with the "lens
> makers equation" which was used to calculate f1=f2. To use this equation
> is to accept the "thin" lens approximation.

You're completely correct, of course, Ludwig.  I felt this was implied 
in my statement, "the focal length is measured from this principal 
plane to the focal point" (having previously defined the focal point
location). 
I probably should have added : "this focal length will not agree with 
that calculated from the 'lens makers equation' unless the system is a 
thin lens".  Thanks for the clarification.

So, Tony, if you're going to use the lens makers equation to calculate 
the focal length then you are already assuming a single lens of
negligible 
thickness, which makes the principle planes coincident with the lens
itself.

BTW, although the principal plane construction abandons the thin lens 
assumption, the paraxial assumption is still applicable for small
apertures.
-- 
Bob Sciamanda				sciamanda@edinboro.edu
Dept of Physics				sciamanda@worldnet.att.net
Edinboro Univ of PA			http://www.edinboro.edu/~sciamanda/home.html
Edinboro, PA				(814)838-7185