Water window

The water window is a region of the electromagnetic spectrum in which water is transparent to soft x-rays. The window extends from the K-absorption edge of carbon at 282 eV (68 PHz, 4.40 nm wavelength) to the K-edge of oxygen at 533 eV (129 PHz, 2.33 nm wavelength). Water is transparent to these X-rays, but carbon and its organic compounds are absorbing. These wavelengths could be used in an x-ray microscope for viewing living specimens. This is technically challenging because few if any viable lens materials are available above extreme ultraviolet.

Not to be confused with Water hole (radio), Water gate, or Water vapor windows.
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