A pictograph of an eye stood on end — originally drawn horizontally with the pupil inside an oval, then rotated 90° for the vertical writing column. The rectangular outline is the eyelid; the two inner horizontals are the iris and pupil. As Kangxi radical #109 it is highly productive:
看,
眼,
睛,
睡,
瞎,
瞳,
盯 all index here. Self-radical char — single component, no further decomposition.
