The ability of optical systems to distinguish between closely spaced objects is limited because the wave nature of light. Consider two light sources far (distance L) from a cirsular aperature with diameter D. Because of diffraction, each source is imaged as a bright central region flanked by weaker bright and dark bands. What is observed is the sum of two diffraction patterns. If the two sources are far enough apart to ensure that their central maximum do not overlap, their image can be distinguished and are said to be resolved. The following simulation let you play several parameters to get the feeling of it. You can play with "distance between the two sources, Diameter of the optics aperature and distances from source and optics aperature.

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Resolution of circular apertures
Designed by Fu-Kwun Hwang (http://www.phy.ntnu.edu.tw/ntnujava/)
Deptartment of Physics, National Taiwan Normal University, Taiwan