Color
• Isaac Newton passed a beam of
sunlight through a prism and
this resulted in a patch of colors
on a white piece of paper.
• He called this spread of colors a
spectrum.
Color
• Sunlight is an example of white
light. White light consists of all
the colors of the spectrum.
• White is not a color in itself,
but a combination of all colors.
• Black is not a color in itself but
the absence of light.
Color by reflection
• Colors of most objects are due
to the way that the objects
reflect light.
• When white light falls on an
object some frequencies are
absorbed and some are
reflected.
Color by reflection
• The frequencies that are reflected
combine to give the colors that we see.
Color by reflection
• The color of the object depends on
which frequencies are reflected.
• A white object reflects all frequencies.
• A black object absorbs all frequencies.
• A red object absorbs light at most
frequencies reflects primarily red light.
Color by reflection
• Only frequencies present in the
incident light can be reflected.
• Colors of objects depend on the
illuminating light.
• For example colors appear different
when seen in candlelight , fluorescent
light, and daylight.
Color
Color by transmission
• The color of a transparent object depends
on the color of the light transmitted.
• For example, red glass absorbs all the
colors of white light except for the red.
Blue glass absorbs all the colors of white
light except for the blue.
• Materials in glass that selectively absorb
light are called pigments.
Color by transmission
Sunlight
• The brightest colors in sunlight are in the
yellow green region.
• The yellow green combination is the one
that our retina is the most sensitive to.
Combining colors
• The combination of different colors results
in new colors.
Combining colors
• The combination of different colors results
in new colors.
Combining colors
• The combination of different colors results
in new colors.
• Combining red green and blue results in
white.
• Combining red and green results in yellow.
• Combining red and blue results in
magenta.
• Combining green and blue results in cyan.
Combining colors
• When two colors added together produce
white, they are called complementary
colors.
• If you begin with white light and subtract a
color from it the resulting color will be the
complement of that color.
Combining colors
• Mixing colored light is called color mixing
by addition.
• The primary additive colors are red, blue,
and green.
• Mixing different pigments that reflect
different colors is called color mixing by
subtraction.
• The primary subtractive colors are
magenta, yellow, and cyan.
Why is the sky blue
• Molecules of nitrogen and oxygen absorb
sunlight and reemit it in all directions.
Combining colors
• Of the visible frequencies
violet light is scattered the
most, followed by blue, green,
yellow, orange, and red.
Combining colors
• Our eyes are more sensitive to
blue than violet, so we see a
blue sky.
• If dust and other particles are
present, other colors are
scattered and the sky takes on a
whitish appearance.
Combining colors
• Water droplets in clouds are of
many different sizes, and
scatter many different
wavelengths of light. As a
result the clouds appear white.
Why are sunsets red?
• At sunrise and sunset sunlight
goes through a thicker layer of
atmosphere. This causes more
blue light to be scattered and as
a result the remaining light
appears to be more red.
Therefore the sun appears to be
red at sunrise and sunset.
Why are sunsets red?
Why is water greenish blue?
• Water molecules resonate somewhat in the
visible red frequencies.
• About 30m below the surface of water, all
the red color is absorbed from visible light,
and a greenish blue (cyan) color remains.
Summary
• The sky is blue because blue from sunlight
is reemitted in all directions by molecules
in the atmosphere.
• Water is greenish blue because red is
absorbed from sunlight by the water.
Atomic spectra.
• When atoms in a sample of matter are far
enough apart and their vibrations do not
interfere with each other, their true colors
are emitted.
• This happens when atoms are made to glow
in the gaseous state.
• In the solid state the atoms are crowded
together and their individual colors a
smudged together to produce a continuous
spectrum.
Atomic spectra.
• The light emitted by the atoms can be
analyzed by a device called a spectroscope.
• A spectroscope produces a line spectrum.
• Sophisticated spectroscopes are used to
study light from various sources in the
cosmos.
Atomic spectra.
Atomic spectra.
• Every element produces its own
characteristic line spectrum.
Summary
• Light is absorbed when its matches the
natural frequency of the electrons in the
material being illuminated.
• Transparent materials absorb some
frequencies and transmit others, and this
results in different colors being transmitted.
• Opaque materials absorb some frequencies
and reflect others, and this results in
different colors being reflected.
Summary
• Color mixing by addition is the mixing of
light of different frequencies.
• Color mixing by subtraction is the mixing
of colored paints or dyes that absorb all
frequencies except for the ones that give
them their color.
• Red green and blue are the primary
additive colors.
• Cyan magenta and yellow are the primary
subtractive colors.
Summary
• Scattering of blue and violet frequencies
gives the sky its blue color.
• When light travels through a long path in
the atmosphere the lower frequencies pass
through, and the longer ones are scattered
out.
• Atoms of each element have characteristic
line spectra.