http://confocal-microscopy-list.275.s1.nabble.com/Refraction-and-Dispersion-phase-contrast-tp7579532p7579550.html
Thank you so much for clearing this up so well. I think that the paradox
is within the cell. Eventually, I realized that this idea is incorrect,
that emanate from the single spot. In Murphy's original book, and now in
the new one, that point is discussed in terms of S and P waves. --and
> *****
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>
> Hi Claire and Joel,
>
> As Joel points out, 1/4 wavelength is the convenient average phase-shift.
> It is worth emphasizing that the phase-shift in question is between direct
> light and the diffraction orders produced by transparent object. Zernike in
> his nobel lecture shares many insights on how he first came up with and
> then refined the phase contrast method - (
>
>
http://www.nobelprize.org/nobel_prizes/physics/laureates/1953/zernike-lecture.pdf> ).
>
> He points out that when studying phase-gratings (not under microscope, but
> macroscopically), he found that when he used a telecope to precisely focus
> on the grating it disappeared. The phase-grating reappeared when the
> telescope was slightly defocused. He explains this behavior thus,
> a. To see the structure of phase grating, the diffracted light must be
> "thrown onto coherent background".
> b. The phase of diffracted light adds or cancels the amplitude of coherent
> background (i.e., uniform undiffracted light). When the phase-grating is in
> focus, the relative amplitudes and phase are such that the coherent
> background is not changed. Out of focus, the phases align better so that
> coherent background is visibly changed.
>
> The phasor diagram in above lecture points out that the average phase
> difference between direct and diffracted light is 1/4*Wavelength.
>
> Zernike first used a 'phase-slit' to achieve both a and b. Then to create a
> circularly symmetric contrast, he developed annulus. The lecture points out
> that microscopists following Abbe's theory remained wedded to thinking in
> terms of amplitude gratings and missed the point about phase-gratings.
>
> In short, Zernike's lecture is an absolutely fascinating read. I think all
> of the above is logically (rather than chronologically) laid out in Murphy
> and Davidson chapter - I tried looking up on Google Books, but page 120 is
> not part of preview!
>
> Cheers
> Shalin
>
> website:
http://mshalin.com> (office) Lillie 110, (ph) 508-289-7374.
>
> HFSP Postdoctoral Fellow,
> Marine Biological Laboratory,
> 7 MBL Street, Woods Hole MA 02543, USA
>
>
> On Mon, Jan 21, 2013 at 2:57 PM, JOEL B. SHEFFIELD <
[hidden email]> wrote:
>
> > *****
> > To join, leave or search the confocal microscopy listserv, go to:
> >
http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy> > *****
> >
> > 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 <
[hidden email]> 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:
>
[hidden email]]
> > > On Behalf Of Claire Brown, Dr.
> > > Sent: Monday, January 21, 2013 1:30 PM
> > > To:
[hidden email]
> > > 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:
[hidden email]
> > URL:
http://astro.temple.edu/~jbs> >
>
Joel B. Sheffield, Ph.D