The water window consists of the soft x-ray

X-ray

X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...

s between the K-absorption edge

Absorption edge

An absorption edge is a sharp discontinuity in the absorption spectrum of a material. These discontinuities occur at wavelengths at which the energy of an absorbed photon corresponds to an electronic transition or ionisation potential....

 of oxygen at 2.34 nm and the K-absorption edge of carbon at 4.4 nm. Water is transparent to these x-rays while nitrogen and other elements found in biological specimens are absorbing. These wavelengths could be used in a x-ray microscope

X-ray microscope

An X-ray microscope uses electromagnetic radiation in the soft X-ray band to produce images of very small objects.Unlike visible light, X-rays do not reflect or refract easily, and they are invisible to the human eye. Therefore the basic process of an X-ray microscope is to expose film or use a...

for viewing living specimens.