Re: Why do thin glass plane mirrors not show colour interference effects

Man, that was some question you asked. I thought about it for a long time
and then wound up asking a lot of people who didn't know either. This
answer comes to you courtesy of my friend Dean McLaughlin, a postdoc in
Astronomy at Berkeley. 

One thing that's
important for the soap film is that it's thin, relative to the wavelength
of the light passing through it. If a plane mirror is much thicker than
one wavelength (as it obviously is), then the angular separation between
consecutive interference minima and maxima will be something like (this is
probably wrong, but you get the idea):

sin(theta_max) - sin(theta_min) = lambda/2d

where d is the thickness of the mirror glass. If that's even 1 cm, then
for visible lambda=550 nm, it looks to me like the minima and maxima are
separated by only 0.00003 rad. At the same time, large angles theta_max
for an individual fringe are obtained only for the highest orders, which
are way down in intensity. So while the effect has to be there, maybe the
answer is just that it's unobservable in the plane-mirror case. (The
canonical soap film has d comparable to lambda.)   

Thanks again for your question.