color of materials
/ KUH-ler uv muh-TEER-ee-ulz /
A ripe tomato is red, a lemon yellow, a leaf green — yet white sunlight contains every color equally. So where do the hues come from? Not from the light itself, but from what each material does to that light. The color you see is the leftover, the part of the rainbow a material sends back to your eye after taking its pick.
A material's color is set by which wavelengths of light it absorbs and which it reflects or transmits. White light arrives carrying all colors; the material's electrons swallow the wavelengths whose energy matches an available jump, and the remaining colors bounce back or pass through to make the hue you perceive. A tomato absorbs blue and green and reflects red; a sapphire absorbs everything but blue. The exact menu of jumps — set by the spacing of energy levels, the size of any band gap, or stray impurity atoms — fixes the color.
This understanding matters because color is one of the easiest windows into a material's hidden electronic structure: a chemist or geologist can often guess what is inside by what color comes out. The common misconception is that an object has a color the way it has a weight. In truth color is a relationship between the material, the light shining on it, and the eye — change the light, as under a sodium streetlamp, and the very same object can look a startlingly different shade.
Pure ruby and pure emerald owe their red and green to the very same impurity — a sprinkle of chromium atoms. The host crystal around the chromium is different in each gem, so it tunes the chromium's light-absorbing jumps differently, and the same ingredient yields two opposite colors.
The same chromium impurity makes ruby red and emerald green — the host crystal sets the color.
Not all color comes from absorption. Some of the brightest colors in nature — a peacock's feather, a soap bubble, a beetle's shell — are structural, made by light waves interfering on tiny ridges, with no pigment absorbing anything at all.