Retroreflection by common surfaces occurs much more frequently and in a much wider variety of curve shapes than existing retroreflection mechanisms (shadowing among opaque surface irregularities) can explain. This paper proposes several new mechanisms. Surfaces may retroreflect from (i) right angle corners and troughs, (ii) optically smooth, convex sections of interface below which scatterers occur, and (iii) opaque inclusions below the air-material interface (below-surface shadowing). Derivations of special cases give retroreflections with observable magnitudes and with curve shapes covering the variety of those observed. The paper also demonstrates a vast variety of ways in which these mechanisms can occur and establishes the existance of the necessary combinations of structures on many of the most commonly exposed natural surfaces. This comprises considerable evidence that the proposed mechansims (perhaps combined with some surface shadowing) can explain the ubiquitous occurrence of retroreflection from common surfaces.
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