Professor Dave here, let's talk about colors.
We previously discussed
the electromagnetic spectrum, and if we
zoom in on the visible portion we see
some familiar colors. The order of these
colors can be remembered with the
mnemonic roygbiv. This stands for red,
orange, yellow, green, blue, indigo, and
violet, which lists the visible colors
from lowest frequency to highest
frequency. So how does this correlate
with the colors of objects? Well as it
happens, most illuminated surfaces will
absorb some light and reflect some light
depending on the material's properties.
When reflection occurs we can always
define an angle of incidence and an
angle of reflection, where the angles can
be measured with respect to a vertical
line perpendicular to the reflecting
surface, and these angles will always be
equal to one another. But as we said,
light can also be absorbed, and many
materials will absorb only particular
colors while reflecting others. The
colors that are reflected are the ones
we see, so when you see a green object
like a leaf, it's because green is the
predominant color that is not absorbed
but rather reflected to meet your eyes
and not because there is some inherent
greenness to the leaf. If an object
absorbs all wavelengths of visible light
it will appear black. If it reflects all
visible wavelengths it will appear the
same color as the light that is
illuminating it. An interesting
phenomenon is the way light bends as it
travels from one medium to another. This
is called refraction. If light goes from
one transparent medium to another, like
from air to water, the light ray changes
direction at the boundary. The angle of
incidence will be measured as we
mentioned before, but instead of an
identical angle of reflection there is a
new angle of refraction. This will happen
any time light's velocity changes, as light
moves a little bit more slowly through water
than air, which explains why objects in
the water sometimes appear to be closer
than they actually are. Light travels
even a bit more slowly through glass.
This is why a prism is able to split up
white light into all the individual
colors it contains, because when light
hits the prism at an angle, the different
wavelengths of visible light are slowed
down by different degrees, and thus have
slightly different angles of refraction.
This causes the colors to fan out such
that they are individually visible. Since
white light can be refracted to separate
all the colors, it should follow that all
the colors can be combined to get white
light. This is true of the additive
primary colors, which are red, green, and
blue. If we pass light through a red
filter and combine it with light through
a green filter, we get yellow light. If we
combine this with light passed through a
blue filter we get white light, which
makes yellow, the complementary color of
blue. In this way each pair of primary
additive colors combines to give the
complement of the third. It is the
combination of these colors that allows
us to see all the others, because our
eyes have three kinds of color receptors,
each of which is sensitive to red, green,
or blue light, which can be stimulated in
various combinations. When we create art
with various materials, these
combinations will be different, because
paint and wax and other material
substances exhibit a certain color
because of the light they reflect, unlike
the actual beams of light we discussed.
So when mixing paints, blue and yellow
will give green rather than white.
All three of the subtractive primary colors,
cyan, magenta, and yellow, when combined
will subtract all of the colors from
white light and the mixture will appear
black. So the next time you do some
painting or see a rainbow, give a
moment's thought to the electromagnetic
waves that produce all of the vibrant
colors we have grown to appreciate.
Let's check comprehension.
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