Fig. 403.—An achromatic lens. C is of crown glass; F, of flint glass.
402. The Color of Bodies.—Project the spectrum of sunlight upon a white surface in a darkened room.
Now place in different parts of the spectrum objects of various colors. Red objects will show brilliant red when at the red end of the spectrum but look black at the blue end, while blue objects appear blue only at the blue end.
These facts indicate that the color of an object depends upon two things: (a) the light that falls upon it and (b) the light which it sends to the eye. A black surface absorbs all color while a white one reflects all wave lengths to the eye in the same proportion that they come to it. A white object will appear red in red light, and blue in blue light since it reflects both of these. A colored object reflects light of its own color but absorbs all others. The color then of a body is due to the light which it does not absorb, but which comes from it to the eye.
403. The color of transparent bodies, such as colored glass, is due to the presence of a dye or pigment contained in the body. This pigment absorbs a part of the light, the part transmitted giving the color. This may be shown by holding a sheet of colored glass in a beam of light either before or after it has passed through a prism. Some colors, as red, may be found to be nearly pure, only the red passing through, while green glass often transmits in addition to the green some yellow and some red light.
404. Complementary Colors.—If two prisms are placed in reversed position near each other (see Fig. 401), a beam of light dispersed by one is recombined into white light by the other. If now a card is held between the two prisms so as to cut off some of the colored light, say the red, the remaining light will be found to form a greenish blue. If the card is removed, the light becomes white again. That is, red and peacock blue light together form white. Any two colors that together form white light are called complementary. Other complementary colors are light yellow and blue, green and crimson, orange and greenish blue, violet and greenish yellow. We must not confuse the combining of colors (light) and the combining of pigments, the latter consisting of bodies that absorb light. Yellow pigment absorbs all but yellow and some green, while blue pigment absorbs all but blue and some green. Mixing these two pigments causes the absorption of all colors but green. Blue and yellow paint mixed produce green, while blue and yellow light give white.
405. The solar spectrum, as the spectrum of sunlight is called, may be observed in the rainbow. The latter is produced through the dispersion of light by spherical raindrops. Its formation may be imitated by sending a small circular beam of light through a screen against a round glass flask filled with water. (See Fig. 404.) The light passes through the water and is dispersed when it enters and when it leaves, producing a color upon the screen at R-V. The course of the light within the drop is indicated in Fig. 405. The violet ray comes to the eye more nearly horizontal and is therefore below red, as we look at the rainbow.
406. Fraunhofer Lines.—Some of the most important features of the solar spectrum are not seen in the rainbow or in the band of light usually observed upon a screen. By the use of a narrow slit and a convex lens to carefully focus the slit upon a white screen it is seen that the solar spectrum is crossed by many dark lines. These are called Fraunhofer lines, to honor the German scientist who in 1814 first accurately determined their position. Two experiments with a spectroscope will help to make clear the meaning of the Fraunhofer lines.