Q.What are primary and secondary sources of light?

[!TLDR] Primary sources emit light of their own (the Sun, a flame, a bulb filament); secondary sources merely reflect or scatter light received from elsewhere (the Moon, and in fact nearly everything else we ordinarily see). [!ANSWER] Primary sources make their own light (by heat, current, or chemical/nuclear reactions); secondary sources only reflect or scatter light from a primary source.

A PRIMARY source of light emits light that it produces itself, through one of three broad mechanisms: high temperature (the Sun, stars, a flame, any glowing hot object), an electric current passed through it (a filament bulb, a tube light, a TV screen), or a chemical or nuclear reaction (fireworks, nuclear energy generators). A SECONDARY source, in contrast, produces no light of its own at all -- it becomes visible only by REFLECTING or SCATTERING light that has arrived from some other (primary, or another secondary) source. The Moon is the standard example: it shines purely because it reflects sunlight, having no light-generating process of its own. In fact, the overwhelming majority of what is seen in ordinary daily life -- other people, animals, plants, furniture, the printed page of this very textbook -- is secondary-source light, visible only because these objects reflect light originating from a genuine primary source such as the Sun or a room's lamps; secondary sources are also typically extended (spread over an area) rather than point-like. [!ANSWER] Primary sources make their own light (by heat, current, or chemical/nuclear reactions); secondary sources only reflect or scatter light from a primary source.

Recall the textbook's three-way classification of primary-source light-generating mechanisms (heat, current, chemical/nuclear reaction) versus the single defining trait of a secondary source (no light-generating mechanism of its own, only reflection/scattering).

Assuming most objects seen in daily life must be primary sources, when in fact the vast majority of everyday vision (people, objects, the Moon) is secondary-source light -- true primary sources are comparatively rare in ordinary experience.