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)
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