[Tfug] OT: Optics

[Michael Stenner]

On Sat, Dec 16, 2006 at 04:50:39PM -0700, Adrian wrote:
> My understanding of optics is fairly fundamental (I'm sure there are
> probably several UofA people on the list studing/doing it every
> day), but overall you are correct. The problem with a simple lens is
> refraction of light passing through it. That is, as light passes
> through the interface between two materials, it bends (an angle of
> refraction), and different wavelengths of light (colors) bend at
> slightly different angles (i.e. it also acts as a prism). A
> "perfect" mirror, on the other hand, reflects all the light at the
> same angle (the angle of incidence), instead of refracts, so the
> colors are not separated.
> 
> And so ends this evenings armchair physics lesson, no doubt complete
> with gross mischaracterizations of basic optical science
> priciples...

You're pretty much right on the money, actually.  Lenses and mirrors
both work by bending light.  The bend angles in lenses depend on the
index of refraction, which in turn depends on wavelength, and so you
get different colors doing different things.  This is called (various
flavors of) "chromatic aberration" or sometimes just "color".  A
mirror doesn't do that, the angle that light bends depends only on
geometry (angle of incidence = angle of reflection) and not on
wavelength.

Now, you can build lenses that do a pretty good job of minimizing
chromatic aberration.  They're usually called "achromats" or
"apochromats".  They're used when you need them, in cameras, etc.
However, they're bulkier and more expensive because they involve
multiple elements with different kinds of glass.

If I had to bet, I'd say they use mirrors because they're cheaper then
lenses, especially for big things and/or color imaging.

					-Michael
-- 
  Michael D. Stenner                            mstenner at ece.arizona.edu
  ECE Department and Optical Sciences Center                520-626-1619
  University of Arizona                                         ECE 524G