Primary and Secondary Sources of Light

Every source of light seen in daily life can be classified as either PRIMARY or SECONDARY. A primary source emits light of its OWN making, through one of three broad mechanisms: (i) high temperature -- the Sun, the stars, any object heated hot enough to glow, or a flame; (ii) an electric current passed through it -- a tube light, an incandescent bulb filament, a television screen; or (iii) a chemical or nuclear reaction -- fireworks, or nuclear energy generators, where light originates directly from the reaction itself.

A secondary source, by contrast, produces no light of its own -- it merely REFLECTS or SCATTERS light that it has received from some primary (or another secondary) source. The Moon and the planets are classic examples (they shine only because they reflect sunlight), and so, in fact, is nearly everything else we ever look at -- other people, animals, plants, and ordinary objects around us are all secondary sources, visible to us only because of the light they reflect from primary sources like the Sun or a room's lighting. The overwhelming majority of light reaching our eyes at any moment, in ordinary …