Editing Refractive index (section)

===Refractive index variations===
{{Main|Phase-contrast imaging}}
[[File:S cerevisiae under DIC microscopy.jpg|thumb|alt=Budding yeast cells with dark borders to the upper left and bright borders to lower right|A [[differential interference contrast microscopy]] image of [[budding yeast]] cells]]

Unstained biological structures appear mostly transparent under [[bright-field microscopy]] as most cellular structures do not attenuate appreciable quantities of light. Nevertheless, the variation in the materials that constitute these structures also corresponds to a variation in the refractive index.  The following techniques convert such variation into measurable amplitude differences:

To measure the spatial variation of the refractive index in a sample [[phase-contrast imaging]] methods are used. These methods measure the variations in [[phase (waves)|phase]] of the light wave exiting the sample. The phase is proportional to the [[optical path length]] the light ray has traversed, and thus gives a measure of the [[integral]] of the refractive index along the ray path. The phase cannot be measured directly at optical or higher frequencies, and therefore needs to be converted into [[intensity (physics)|intensity]] by [[interference (optics)|interference]] with a reference beam. In the visual spectrum this is done using Zernike [[phase-contrast microscopy]], [[differential interference contrast microscopy]] (DIC), or [[interferometry]].

Zernike phase-contrast microscopy introduces a phase shift to the low [[spatial frequency]] components of the [[Real image|image]] with a phase-shifting [[annulus (geometry)|annulus]] in the [[Fourier optics|Fourier plane]] of the sample, so that high-spatial-frequency parts of the image can interfere with the low-frequency reference beam. In {{abbr|DIC|differential interference contrast microscopy}} the illumination is split up into two beams that are given different polarizations, are phase shifted differently, and are shifted transversely with slightly different amounts. After the specimen, the two parts are made to interfere, giving an image of the derivative of the optical path length in the direction of the difference in the transverse shift.<ref name=Carlsson/> In interferometry the illumination is split up into two beams by a [[Beam splitter|partially reflective mirror]]. One of the beams is let through the sample before they are combined to interfere and give a direct image of the phase shifts. If the optical path length variations are more than a wavelength the image will contain fringes.

There exist several [[phase-contrast X-ray imaging]] techniques to determine 2D or 3D spatial distribution of refractive index of samples in the X-ray regime.<ref>{{Cite journal | first = Richard | last = Fitzgerald | title = Phase-Sensitive X-Ray Imaging | journal = Physics Today | volume = 53 | page = 23 | date = July 2000 | doi = 10.1063/1.1292471|bibcode = 2000PhT....53g..23F | issue = 7 | s2cid = 121322301 | doi-access = free }}</ref>