Color

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.