Undergrad Waves and phase shifts (attempting to learn physics)

[Nick tringali]

TL;DR

Im reading an article that my professor gave me that she figured id find interesting and im trying to understand it.

So I haven't taken a physics class in 4 years and forgot a lot about it. I am trying to understand an article but I am stuck on a core concept "Because essentially no amplitude reduction of the light occurs to create contrast and because the phase delays are usually very small, the generation of sufficient contrast for visual discrimination requires conversion of phase shifts into brightness variation. (Page 730)" So my question is can someone explain to me what a phase shift is in this context. This is about phase contrast microscopes. I want to learn about how they work exactly. And also why do phase shifts happen when light interacts with something. Thanks.http://pre.mntp.pitt.edu/MNTP_Prtcp_res_2010/teaching/Optics_Chapter_95_LanniKeller.pdf

 

[Drakkith]

The very small thicknesses of cells and cellular structures, coupled with the low refractive indices which are also nearly the same as the refractive index of the surrounding water, only generates very small regions where parts of the wavefront are moving at different speeds. This very small retardation of the speed of a segment of the wavefront shows up as a small phase shift compared to the rest of the wavefront.

This small phase shift isn't enough to generate detail (contrast) when imaged, so the microscope manipulates the wavefront to do this 'artificially'.

And also why do phase shifts happen when light interacts with something.

When parts of a wavefront pass through a medium with a higher refractive index than what the rest of the wavefront passes through, there is a delay of that part of the wavefront, since it moves at a slower speed. So that part of the wavefront falls behind and its crests and troughs get out of sync with the rest of the wavefront, which is a phase shift.

 

[Nick tringali]

The very small thicknesses of cells and cellular structures, coupled with the low refractive indices which are also nearly the same as the refractive index of the surrounding water, only generates very small regions where parts of the wavefront are moving at different speeds. This very small retardation of the speed of a segment of the wavefront shows up as a small phase shift compared to the rest of the wavefront.

This small phase shift isn't enough to generate detail (contrast) when imaged, so the microscope manipulates the wavefront to do this 'artificially'.
When parts of a wavefront pass through a medium with a higher refractive index than what the rest of the wavefront passes through, there is a delay of that part of the wavefront, since it moves at a slower speed. So that part of the wavefront falls behind and its crests and troughs get out of sync with the rest of the wavefront, which is a phase shift.

Thank you! Very clear now.