Color

Chapter 28

The Color Spectrum Isaac Newton (yeah, him again!) was the first to

systematically study color Spectrum – the spread of all colors, in order:

red, orange, yellow, green, blue, and violet White Light – the combination of all light colors Black is the absence of light, objects appear

black when they have absorbed light of all frequencies

Black objects that you can see do not absorb all light that falls on them, because there is always some reflection off of the surface (you wouldn’t be able to see them without it)

Newton’s Color Spectrum

Color by Reflection The colors of most objects around you are due

to the way objects reflect light Electrons can be forced temporarily into larger

orbits by the vibrations of electromagnetic waves

Different materials have different natural frequencies for absorbing and emitting radiation

At the resonant frequencies of an object, light is absorbed

If a material absorbs light of most visible frequencies and reflects red, for example, the material will appear red

An object can only reflect light of frequencies present in the illuminating light

Color by Transmission The color of a transparent object depends

on the color of the light it transmits A red piece of glass appears red because

it absorbs all the colors that compose white light, except red, which it transmits

Pigment – the material in an object that selectively absorbs colored light

From an atomic point of view, electrons in the pigment atoms selectively absorb light of certain frequencies in the illuminating light

Stained Glass Windows

Sunlight

White light from the sun is a composite of all the visible frequencies

The brightness of solar frequencies is uneven, red light is not as bright as yellow-green light (humans see yellow-green the best due to this)

The graphical representation of brightness versus frequency is called the radiation curve of sunlight

Mixing Colored Light Light of all the visible frequencies mixed together

produces white White also results from the combination of only

red, green, and blue light When red and green overlap, yellow is produced When red and blue overlap, magenta is produced When green and blue overlap, cyan is produced Additive Primary Colors – red, green, and blue

can produce almost any color Color television is based on the fact that the

human eye can see combinations of three colors as a variety of colors

Complementary Colors

Red + Green = YellowRed + Blue = MagentaBlue + Green = Cyan

If you add the third color into these combinations, you will get white light

Complementary Colors – two colors that when added together produce white

Every hue has some complimentary color that when added will produce white

Same concept applies when subtracting colors from white

Complementary Colors

Mixing Colored Pigments The mixing of paints and dyes is an entirely

different process from the mixing of colored light Paints and dyes contain finely divided solid

particles of pigment that produce their colors by absorbing light of certain frequencies and reflecting light of other frequencies

When we mix colored pigments, we are subtracting the types of colors that the pigment will reflect (color mixing by subtraction), whereas when we mix light, we are adding colors (color mixing by addition)

Subtractive Primary Colors – the three paint colors that are most useful in color mixing by subtraction: magenta, yellow, and cyan

Color Subtraction

Why the Sky is Blue

Scatter – a redirection of a beam in multiple directions The nitrogen and oxygen scatter light of higher

frequencies more than lower frequencies Of the visible frequencies, violet light is scattered the

most, followed by blue, green, yellow, orange, and red, in that order

Our eyes are more sensitive to blue, so we see a blue sky (not a violet one)

The more larger particles there are in the atmosphere, the less blue the sky appears

The higher you go in the atmosphere, the less particles there are, so the darker the sky is

In a cloud, there are both large and small water droplets, so all frequencies are almost equally scattered, producing a white cloud

The Sky and Clouds

Why Sunsets are Red The lower frequencies of light are scattered

the least by nitrogen and oxygen molecules; therefore, red, orange, and yellow light are transmitted more readily through the atmosphere

Red light, which is scattered least, passes through more atmosphere without interacting with matter than light of any other color

At dawn and sunset, sunlight reaches us through a longer path through the atmosphere than at noon

The sunlight will look the reddest at these times, at noon it will be more yellow (less atmosphere to travel through

Sunset

Why Water is Greenish Blue

When we look at the surface of a deep blue ocean or lake, we are actually seeing the color of the reflected sky

The color of water is a pale greenish blue Water is transparent to nearly all the visible

frequencies of light, and the molecules absorb infrared

Water molecules resonate somewhat to the visible-red frequencies, causing a gradual absorption of red light by water and leaving behind red’s complimentary color, cyan

In deep water, many creatures are black or red, both of which cannot be seen easily in the deep waters

The Ocean

The Atomic Color Code – Atomic Spectra

Every element has its own characteristic color when made to emit light

Spectroscope – instrument used to analyze the light given out by glowing elements

When light from a glowing element is analyzed through a spectroscope, it is found that the colors are a composite of a variety of different frequencies of light

Line Spectrum – the spectrum on an element is not continuous, but appears as separate lines of color, each having a different frequency

The light from each element produces its own characteristic pattern of lines

Much of the information that physicists have about atomic structure is from the study of atomic spectra

Atomic Spectra of the Sun

Assignment

Read Chapter 28 (pg. 421-438) Do Ch. 28 Assessment #26-45 (pg. 440-

441)