Water window

{{distinguish|Water hole (radio)|Water gate|Water vapor windows}}

The water window is a region of the electromagnetic spectrum in which water is transparent{{clarify|reason=on a quick search, the first source I found concerning soft x-rays in liquid water gave a tau to 50% attenuation that was microscopic|date=February 2023}} 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.{{Cite journal

| last1 = De Stasio | first1 = G.| last2 = Gilbert | first2 = B.| last3 = Nelson | first3 = T.

| last4 = Hansen | first4 = R.| last5 = Wallace | first5 = J.| last6 = Mercanti | first6 = D.

| last7 = Capozi | first7 = M.| last8 = Baudat | first8 = P. A.| last9 = Perfetti | first9 = P.

| last10 = Margaritondo | first10 = G.| last11 = Tonner | first11 = B. P.

| title = Feasibility tests of transmission x-ray photoelectron emission microscopy of wet samples

| journal = Review of Scientific Instruments | volume = 71 | pages = 11–14 | date = January 2000

| doi = 10.1063/1.1150151 | bibcode = 2000RScI...71...11D

| url = http://home.physics.wisc.edu/gilbert/publications/66.PDF | archive-url = https://web.archive.org/web/20170813225208/http://home.physics.wisc.edu/gilbert/publications/66.PDF | url-status = dead | archive-date = 2017-08-13 }}{{Cite journal

| last1 = Spielmann | first1 = C. | last2 = Burnett | first2 = N. H. | last3 = Sartania | first3 = S.

| last4 = Koppitsch | first4 = R. | last5 = Schnürer | first5 = M. | last6 = Kan | first6 = C.

| last7 = Lenzner | first7 = M. | last8 = Wobrauschek | first8 = P. | last9 = Krausz | first9 = F.

| title = Generation of Coherent X-rays in the Water Window Using 5-Femtosecond Laser Pulses

| journal = Science | volume = 278 | issue = 5338 | pages = 661–664 | date = 24 Oct 1997

| doi = 10.1126/science.278.5338.661 | bibcode = 1997Sci...278..661S

}} This is technically challenging because few if any viable lens materials are available above extreme ultraviolet.