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I, too, am teaching a course in microscopy, and have run across the same
issue.  It has occurred to me that we actually can't see what is happening
to light when it is within an area of high refractive index until it
re-emerges into our normal world.  At that point, it resumes its original
speed/frequency, and so it's a bit like Schrodinger's cat.  At the same
time, I have come across much more detailed versions of the cause of the
phase effect.  Take a look at Murphy and Davidson's new book, "Fundamentals
of Light Microscopy and Electronic Imaging" for a discussion of a dual wave
model (the S and P waves) that derives from a diffraction-based analysis
rather than a velocity of light analysis.  I have to admit that I am still
struggling with that one, and would welcome any enlightenment.

As to your second question, the answer is "no".  Different structures will
cause different amounts of phase shift.  This is why the phase contrast
image is not binary, but shows gradations.  The 1/4 wavelength appears to
be just a convenient average, and a way to set the phase plate somewhere in
the middle.  In an early Reichert microscope that I had a chance to see
many years ago, the phase system was continuous, so that you could vary the
added shift from + to - 1/4, and reverse the contrast at will.

Joel

On Mon, Jan 21, 2013 at 1:53 PM, MODEL, MICHAEL <[log in to unmask]> wrote:

> *****
> To join, leave or search the confocal microscopy listserv, go to:
> http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
> *****
>
> Hi Claire - the speed of light does change but the eye responds only to
> frequency, it doesn't know anything about wavelength. And the frequency
> remains the same throughout all transformations of the wave.
>
> Mike
>
> -----Original Message-----
> From: Confocal Microscopy List [mailto:[log in to unmask]]
> On Behalf Of Claire Brown, Dr.
> Sent: Monday, January 21, 2013 1:30 PM
> To: [log in to unmask]
> Subject: Refraction and Dispersion-phase contrast
>
> *****
> To join, leave or search the confocal microscopy listserv, go to:
> http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
> *****
>
> I am teaching a class on light microscopy and have two questions:
>
> 1) If higher refractive indices materials slow down the speed of light
> does the wavelength also change so that frequency and energy are conserved?
> If this is true does is the wavelength shift so small that the colour does
> not change a great deal? The other explanation I had is that the speed of
> light never changes but short wavelengths take longer to travel through
> high NA materials because they interact with the material and travel along
> a longer path to reach the other side of the material. So the speed does
> not change, the wavelength does not change but the light takes longer to
> get through the material.
>
> 2) Does diffracted light shift by exactly 1/4 a wavelength in phase from
> incident light? If so why is it exactly 1/4 of a wavelength?
>
> Sorry for my basic questions but these sometimes seem harder to explain
> and understand than more complex concepts.
>
> Sincerely,
>
> Claire
>



-- 


Joel B. Sheffield, Ph.D
Department of Biology
Temple University
Philadelphia, PA 19122
Voice: 215 204 8839
e-mail: [log in to unmask]
URL:  http://astro.temple.edu/~jbs