Earth and Sky:Black and White　黑与白（在线收听）

Black and White黑与白

Sometimes you hear it said that white is a combination of all the colors of the rainbow -- and that black is the absence of color. Find out more about whether black and white are colors -- today on Earth and Sky.

JB: This is Earth and Sky. A listener wants to know, "Are black and white really colors?"

DB: Color isn't a physical quality. It's a phenomenon of light and perception. Matter and energy are colorless. It's really our eyes and brains that make a fire-engine red or chickens white.

JB: Colors are defined by their hue, saturation, and value. Hue is what we think of as making blue blue, and not orange or grey. Saturation refers to a color's vividness -- it's what makes a pale blue sky different from one that's deep blue -- or pink different from red. Value refers to a colored object's lightness, darkness, and brightness -- that's the contrast between, say, the shining moon and a glass of red wine.

DB: It gets pretty complicated to talk about what makes a color a color -- because colors can only be talked about in terms of themselves -- and how they differ from one another. Black is black precisely because it has no redness or greenness or any other hue. But, to answer your question, black and white -- and grey, for that matter, too -- are colors. They're neutral colors -- different from each other only in terms of their darkness and lightness. Like all colors, black and white are products of our visual processes -- they exist only because we're there to sense them.

JB: To ask us your question, come to earthsky.org. With thanks to the National Science Foundation, we're Block and Byrd for Earth and Sky.