Definition of a light source: A
light source is an object that emits light around itself
It can be distinguished two kind
of light source:
sources of light:
These objects create light they emit.
Primary sources can have high
temperature like the Sun, flames, the filament of an incandescent lamp.
etc. They can also be at ambient temperature using electrical or
chemical processes to produce light like fireflies, some aquatic
organisms ( in this case we use the term of bioluminescence ), TV,
computer or mobile screens.
- Secondary sources of light
These objects don't produce the light they emit: they have received
light from another source first, then they can send it back (
partially or totality ): we say they diffuse light ( or they reflect
it, for mirrors).
Secondary sources diffuse light in all directions.
Thus, a secondary source can be considered as a source of light only
when it is lighted ( by a primary or secondary source ).
The Moon ( that receives light
from the Sun ) and planets are secondary sources that diffuse some
light like cinema screen lighted by a projector.
all objects ( including people ) around us can be considered as
secondary sources of light if they are in a room with a shining lamp.