diffraction

Definition: Optical phenomenon in which light appears to bend round corners or to spread out when a beam of light passes through narrow slit. * Part of beam of light passing the obstruction is deviated -- by an amount proportional to wavelength -- by the interaction of the obstruction with the wave-like nature of light. * The edge of the obstruction is the point at which new secondary wave-fronts are generated: this naturally spreads round the edge, making light appear to turn the corner.
Round the bend
This seemingly obscure bit of physical optics is actually photography's glass ceiling: it sets the limit of resolving power of lenses and indeed any optical system. Diffraction effects increase with longer wavelengths and smaller aperture setting (so avoid setting small apertures like f/22); and it sets more severe limits on the performance of long focal length lenses than on shorter lenses.

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