That's not the same principle. The diffraction limit pertains to angular size, not physical size. You are correct that you wouldn't be able to resolve things smaller than a given wavelength, but the diffraction limit can apply to arbitrarily large objects as long as they appear small. A 100,000-light-year-across galaxy will simply appear lower resolution in an IR telescope than a visible-light telescope of the same aperture/focal length.
exoplanetaryscience t1_iqsxnyj wrote
Reply to comment by mattjouff in If objects in space are far away, does light get scattered enough that it would look “low resolution” by the time it reaches us? by hau2mk7pkmxmh3u
That's not the same principle. The diffraction limit pertains to angular size, not physical size. You are correct that you wouldn't be able to resolve things smaller than a given wavelength, but the diffraction limit can apply to arbitrarily large objects as long as they appear small. A 100,000-light-year-across galaxy will simply appear lower resolution in an IR telescope than a visible-light telescope of the same aperture/focal length.