If a prism B (Fig. 87) exactly similar to A in every way is placed behind A in a reversed position, it will undo the dispersion of A, bending upward the seven different beams in such a way that they emerge together and produce a white spot on the screen.  Thus we see, from two simple experiments, that all the colors of the rainbow may be obtained from white light, and that these colors may be in turn recombined to produce white light.

[Illustration:  FIG. 87.—­Rainbow colors recombined to form white light.]

White light is not a simple light, but is composed of all the colors which appear in the rainbow.

129.  Color.  If a piece of red glass is held in the path of the colored beam of light formed as in Section 127, all the colors on the wall will disappear except the red, and instead of a beautiful spectrum of all colors there will be seen the red color alone.  The red glass does not allow the passage through it of any light except red light; all other colors are absorbed by the red glass and do not reach the eye.  Only the red ray passes through the red glass, reaches the eye, and produces a sensation of color.

If a piece of blue glass is substituted for the red glass, the blue band remains on the wall, while all the other colors disappear.  If both blue and red pieces of glass are held in the path of the beam, so that the light must pass through first one and then the other, the entire spectrum disappears and no color remains.  The blue glass absorbs the various rays with the exception of the blue ones, and the red glass will not allow these blue rays to pass through it; hence no light is allowed passage to the eye.

An emerald looks green because it freely transmits green, but absorbs the other colors of which ordinary daylight is composed.  A diamond appears white because it allows the passage through it of all the various rays; this is likewise true of water and window panes.

Stained-glass windows owe their charm and beauty to the presence in the glass of various dyes and pigments which absorb in different amounts some colors from white light and transmit others.  These pigments or dyes are added to the glass while it is in the molten state, and the beauty of a stained-glass window depends largely upon the richness and the delicacy of the pigments used.

130.  Reflected Light. Opaque Objects. In Section 106 we learned that most objects are visible to us because of the light diffusely reflected from them.  A white object, such as a sheet of paper, a whitewashed fence, or a table cloth, absorbs little of the light which falls upon it, but reflects nearly all, thus producing the sensation of white.  A red carpet absorbs the light rays incident upon it except the red rays, and these it reflects to the eye.

Any substance or object which reflects none of the rays which fall upon it, but absorbs all, appears black; no rays reach the eye, and there is an absence of any color sensation.  Coal and tar and soot are good illustrations of objects which absorb all the light which falls upon them.