Phase object

A specimen whose leading optical effect is a spatially varying phase, rather than absorption. A thin specimen with refractive index $n_s$ in a medium of refractive index $n_m$ has, in a straight-path approximation,Here $\lambda$ is the vacuum wavelength; the integral is an optical path length difference. For the chosen values $\lambda =550\mathrm{nm}$, thickness $1\mu \mathrm{m}$ and $n_s-n_m=0.005$, the phase is $\pi /55\simeq 0.0571$ radians. This calculation does not identify thickness separately from refractive index contrast.^[ref]

A phase object for which the relevant relative phase variations are much smaller than one radian. The Taylor expansion $e^{i\varphi }=1+i\varphi +O({\varphi }^2)$ separates a direct reference term from a small specimen term. A constant overall phase can be removed before making this approximation: phase-contrast microscopy measures spatial variations, not an absolute common phase.