Jamieson, Fausset and Brown on Matthew 6:22

22. The light--rather, "the lamp."

of the body is the eye: if therefore thine eye be single--simple, clear. As applied to the outward eye, this means general soundness; particularly, not looking two ways. Here, as also in classical Greek, it is used figuratively to denote the simplicity of the mind's eye, singleness of purpose, looking right at its object, as opposed to having two ends in view. (See Pr 4:25-27).

thy whole body shall be full of light--illuminated. As with the bodily vision, the man who looks with a good, sound eye, walks in light, seeing every object clear; so a simple and persistent purpose to serve and please God in everything will make the whole character consistent and bright.