Light and Shadow: Angle of Incidence and Compound Shadows
How the angle at which struck rays arrive colors a surface, and how shadow deepens toward its end
This folio of Notebook C pursues the science of light and shadow. A large diagram of crossing rays passing through a central body, together with a converging pyramid of light, illustrates how the angle at which the species of opposed bodies strike a surface governs how deeply that surface takes on their color, and how a spot is brighter or darker according to the sum of luminous or shadowy rays reaching it. Further figures treat a shadow trapped between two facing surfaces of equal quality and the compound derivative shadow cast by two lights of differing nearness.
On this page
A surface takes on more color where the struck angle is greater
The part of a surface struck by a greater angle of the species of the bodies set opposite it is more tinged with their color. The angle at 8 is greater than that at 4 because its base a-n is longer than e-n, the base of 4; the figure is bounded by a-n and 4 and 8.
Brightness and darkness follow the sum of rays reaching a place
As a thing touched by more luminous rays grows brighter, so it grows darker where struck by more shadowy rays. Place 4 is more luminous than 8 because 4 sees only the shadow i-n, while 8 is struck by both a-e and i-n and so is twice as dark.
Derivative shadows scale with the nearness of the two lights
With m and b as luminous bodies and a-n the shadowy body, the proportion of darkness among the derivative shadows a-n matches the proximity of the lights m-b that cause them; if the lights are of equal size, the sizes of the shadows' luminous circles follow the distance between the lights.