A branch struck by rays: brightest at a right angle
Straight rays from a light source meet an undulating branch marked a to f along its underside.
A wavy tree branch is drawn diagonally across the leaf, its underside marked at points f, e, d, c, b and a, while straight luminous rays fan out from a light source at the upper left to strike it at different angles. The accompanying note gives the optical principle that the part of a body met by a ray closest to a right angle is the most brightly lit.
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Light nearest a right angle illuminates most
Straight luminous rays fan out from a source at the upper left and strike the undulating branch at different inclinations, its underside marked f, e, d, c, b and a. The note states the rule that the part of a body met by a ray closest to a right angle is the most brightly illuminated.