Re: ray tracing assumptions

["John S. Denker" <jsd@MONMOUTH.COM>]

Wolfgang Rueckner wrote:
> ...students often ask why it is that we can
> treat the image of the first lens as the object of the second when,
> in fact, the image is not where we're pretending it is (because the
> second lens is actually in the way).

<nit>
Actually I think it is the first lens that is in the way.
</nit>

Answer: Because we're doing ray tracing.  Rays are straight lines.
Two points determine a line.   Or intercept and slope determine
a line.  Stating the location of the image is sufficient to
determine the slope and intercept that the ray will have when
it strikes the second lens.  The question of what produced this
straight line is irrelevant once we know the intercept and slope.