Robert St. Angelo, age 11, of Niagara Falls, Ontario, Canada, for his question:

Why is the sky blue and the grass green?

This double barrelled question usually arrives in two sections. Sooner or later every young person wants the answer to both and, since they are somewhat related, it seems a good idea to tackle the two of them together. The sky is blue and the grass is green because all the colors are wavelengths, angled like threads from the bundle of energy in ordinary white light.

We see blue when the shortest wavelengths are sheared from the rainbow colors concealed is beams of visible light. This happens when light from the sun pierces down through the atmosphere above our heads. Its air, of course, is a congregation of gas molecules and floating fragments of dusty debris.

A sunbeam is a bundle of longer and shorter wavelengths of electromagnetic energy. The infinitesimal particles of matter in the atmosphere are just big enough to interfere with the shortest wavelengths in this radiant solar energy. These are separated from the rainbow mixture concealed in white, or colorless light and scattered over the sky. Our eyes happen to see these separated short wavelengths as blue.

We see the grass as green because its leafy blades contain a certain something that reflects a series of somewhat longer wavelengths. That something is the magical bio¬chemical called chlorophyll. It is contained in miniature bodies called chloroplasts and zillions of these midgets crowd into the cells of a single leaf.

As we know, all the visible substances around us are made of chemicals of some sort. The nature and inner structure of a chemical determine what happens when its surface is bathed in the radiance of white light. It selects and absorbs some of the pulsing wavelengths and reflects back others. We do not see those that are separated and ab¬sorbed. But we do see those that bounce back and scatter through the air. We see these longer and shorter wavelengths as the different colors in the rainbow spectrum of white light.

The wavelengths of electromagnetic energy have been compared to waves, pulsing up and doom. The assorted wavelengths in white light are measured by the angstrom unit, and there are about 250 million of these units in an inch. The heavenly blues of the sky are reflected from wavelengths that measure from 4,000 to 5,000 angstroms. The grassy greens that bounce back from a lawn have wavelengths ranging from 5,000 to about 5,500 angstroms.

At dawn and sunset, the sun is lower in the sky. Its sunbeams slope down a longer path to reach the earth and on the way they encounter perhaps 400 times more gaseous particles of matter. More of the longer wavelengths are bent and angled off into the sky. We see them as the gorgeous reds, golds and orange colors that glorify the sky in the evening and early morning.