Great question! Overall, this is a quite a
complicated question, without a straightforward
answer, as the issue deals with a mixture physics,
biology, psychology, and philosophy—but I will do
my best to answer.
Light—or more correctly, the
electromagnetic spectrum—is a continuous range of
all the possible frequencies of electromagnetic
radiation. Visible light (or, the light that
humans can see) covers just a tiny, tiny range of
the electromagnetic spectrum, from wavelengths of
about 400 nanometers to 750 nanometers. Since we
can only see light in this range of frequencies,
this means that humans cannot perceive the vast
majority of light in the universe. Now, when
electromagnetic radiation interacts with an
object, the light can be reflected, scattered,
absorbed, or transmitted through the object.
The light that leaves the
object will depend on the spectrum of light that
is illuminating the object, the reflective
properties of the object, the angle at which you
view the object, and the other nearby objects.
When you look at, for example, a pumpkin that is
illuminated by white light, you perceive that the
pumpkin is orange, because orange light (~620
namometer wavelength) is emanating from the
pumpkin to your eyes. However, your eyes can also
perceive a mixture of light to be the same color
as that pumpkin.
For example, television screens only produce
red, blue, and green light. So how do you see the
color orange on a TV? The TV screen emits the
correct mixture of red and green light, and your
brain perceives that mixture as the same color of
the pumpkin. So you see, our brains may perceive
many different combinations of the electromagnetic
spectrum as the same color.
In general, color is a
perceptual property of how our brains interpret a
small portion of the electromagnetic spectrum.
Over many years of human evolution, different
people and cultures have assigned color to be a
designation of how an illuminated object
stimulates the rod- and cone-cells in our eyes,
and how our brains interpret this stimulation.
Spectra that stimulate our light receptor cells in
a similar way are grouped together and designated
as a color. Each specific color that humans can
see may result from many different combinations of
the electromagnetic spectrum. So, “color” is the
best way for humans to group together things that
stimulate our bodies in a similar fashion, as we
cannot perceive the full detail of how light
interacts with all objects in our world.
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