= Phase object
A specimen whose leading optical effect is a spatially varying <phase>, rather than <absorption (electromagnetic radiation)>. A thin specimen with <refractive index> $n_s$ in a medium of <refractive index> $n_m$ has, in a straight-path approximation,
$$
\phi(x,y)=\frac{2\pi}{\lambda}\int\bigl(n_s(x,y,z)-n_m\bigr)\,dz.
$$
Here $\lambda$ is the vacuum <wavelength>; the integral is an <optical path length> difference. For the chosen values $\lambda=550\,\mathrm{nm}$, thickness $1\,\mathrm{\mu m}$ and $n_s-n_m=0.005$, the <phase> is $\pi/55\simeq0.0571$ radians. This calculation does not identify thickness separately from <refractive index> contrast.<Introduction to Phase Contrast Microscopy by Nikon Instruments Inc.>[Formulas 2–4, optical path difference and phase shift]{ref}
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