In the Airy disk equation, is lambda the wavelength in medium or vacuum?

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mssm22
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hi

the airy disk equation is d= 1.22 lambda / NA
my question is:
lambda is the wavelength in medium or in vacuum?

thanks in advance
 

Discussion

Lambda is in whatever medium the light is propagating. Immersion microscopes use this fact to improve their resolution by shortening the wavelength by putting water or oil between the object being looked at and the lens. The newer generation of integrated circuit lithography tools also use water immersion to improve the resolution. Doesn't work very well for telescopes, however...