Why can a laser only be focussed to within its wavelength? It says due to it being diffraction limited, what about focussing lenses and mirrors they don't involve diffraction. Please explain
Ah well - they do! Diffraction is not only limited to tiny holes and slots. It occurs whenever a wave is constrained to go through any hole or around any object - the diffraction pattern of large apertures is very near the conventional 'ray' diagram but lenses (telescopes and microscopes have a resolution that's fundamentally limited by diffraction effects). However 'sharp' you try to make the beam of a laser, at the place where the beam is said to be focussed, the actual location of where the spot occurs is still governed by diffraction. If the spot were much less than the wavelength then the image would have to consist of wavelengths that are shorter than the wavelength of the light. Also, how would you measure where the spot is?what about focussing lenses and mirrors they don't involve diffraction.