Let us recall our lesson on sunset and sunrise and remember the effect of atmosphere on light rays. Remember how the motes in the sky or mist intercepted the long blue rays and let the red through?
Now we will treat that 'mist' as a piece of glass or plastic; a solid yet semi -transparent medium.
In the first example below the reddish pigment particles suspended in the glass are sparsely arranged. As with the earth's atmosphere at sunset the pigment particles in the glass absorb the blue-green light rays ... and this has a dual effect. (Fig 1.)
1. It reflects the red light rays back to the observer making the glass appear red.
2. It allows some of the red rays through to strike the surface behind. Here, that surface (green), absorbs some of the red rays. This tends to slightly neutralize the color. If the green was stronger the shadow would become grey.
The solid object in Fig.2 blocks out all the light rays and casts a theoretical neutral shadow. Most painters find it useful to paint a warm shadow if the light source is cold and a bluish shadow of the source is warm. In this example the greenish background and reflected side light prevent an altogether neutral effect.
In the third example thicker glass is added to the center. This has the effect of both blocking the light and un-saturating the red. (see previous lesson on saturated color)
The principle regarding transparency is useful when painting with semi-transparent dark paint as the value and depth of the darks can be increased. As opposed to opaque darks, transparent dark allows light to penetrate the surface before reflecting back off what is underneath. This has the effect of filtering out light rays on the way in as well as on the way out thus allowing less of the light rays to escape and for our eyes to read richer, more ineresting darks.
THE SEMI-TRANSPARENT RED RUBY RING
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