Re: PHYS-L digest 425

[William Beaty <billb@eskimo.com>]

On Thu, 29 May 1997, home wrote:

> Could someone explain the basic principles behind using different
> glasses to create a color corrected lens. This is always stated in text
> books but I do not see how it works.

It helps to look at a simpler case first, the prism, then apply the same
idea to lenses.

A prism bends light, but because of atomic resonances in the glass
molecules, it bends different frequencies by different amounts.  Prisms
create rainbow effects, but what if the goal is to bend light WITHOUT
generating rainbows?  One way to do this is to combine two prisms which
are composed of glass which has unequal properties of frequency-
dispersion.
    _________
|\  \        |
| \  \       |
|  \  \      |
|   \  \     |
|    \  \    |
|     \  \   |
|      \  \  |
|       \  \ |
|________\  \|

Imagine two prisms placed in series, with one prism upside-down, and with
the prisms touching together.  One prism bends light in one direction, the
second prism bend it back again, and if the prisms are identical, the net
angle of light is zero (the effects of the second cancel the first).  The
above prisms constitute a solid glass plate w/parallel faces, no?  But if
the prisms are made of two different types of glass, "crown" and "flint",
then two independant things are going on: the amount of bending, and the
amount of frequency-spreading.

The angles of the faces of the two crown/flint prisms can be
reground/polished so that the net angle of light is again zero.  The
result is a composite prism which spreads frequencies and makes rainbows
without deflecting light!  (such straight-through prisms are useful in
optics systems.)  Or, the prism faces can be reground so the frequency-
spreading effects of the second prism cancel the first.  Since the types
of glass aren't identical, doing this will not necessarily give us a zero
net deflection angle.  The result then is a composite prism which deflects
light but has no frequency spreading.  We've build a "color corrected"
prism.

Lenses are essentially smoothly-joined arrays of microscopic prisms.  It
is possible to combine two lenses, one of them "upside-down" (one concave
and the other convex), so that the composite focal length is less than
that of just one lense, but the frequency-dispersion effects are
minimized.  The result is the "color corrected" lens. 

......................uuuu / oo \ uuuu........,.............................
William Beaty  voice:206-781-3320   bbs:206-789-0775    cserv:71241,3623
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