Re: [ATM] mirror or lens?

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Bob May wrote:
> Walk him up to the mirror in the hallway and ask him if he's
> looking through a lens or a mirror.
> He can play nuances all day long with various tricks of the
> language but reality is something that he's got to agree with.

Hmmm.... for a hallway mirror chances are it's both - the metallic 
coating is usually on the back side, so light goes through the glass 
(a lens) and reflects off the mirrored surface, and then back through 
the glass again.  I guess it's a lens and a mirror.  It was a trick 
question.  :)

> Michael Peck wrote:
>> "When a plane wave falls on to a boundary between two homogeneous media 
>> of different optical properties, it is split into two waves: a 
>> transmitted wave proceeding into the second medium and a reflected wave 
>> propagated back into the first medium. The existence of these two waves 
>> can be demonstrated from the boundary conditions,..." (p. 38)
>> 
>> So, from the point of view of classical electrodynamics reflection and 
>> refraction are different aspects of the same phenomenon. Presumably you 
>> could derive the same laws from quantum electrodynamics. But I wasn't 
>> thinking about classical electrodynamics, let alone solid state physics 
>> or QED. I was thinking about current optical engineering practice. As 
>> far as I can tell, to a modern optical engineer a lens is an image 
>> forming instrument, and it doesn't matter if it's made of refractive or 
>> reflective elements, or a combination of both. That's how Warren Smith 
>> uses the word in "Modern Lens Design" for example, which according the 
>> back cover blurb gives "more than 280 worked-out lens designs" including 
>> reflecting and catadioptric telescopes.

Exactly.  A lens makes use of the transmitted wave, and a mirror makes 
use of the reflected wave.  In my book, that makes them very different.

Time to offer the astrophysicist a beer, call a truce, and get on with 
life.

	Mike Lockwood

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