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Light and Color
A Summary
Light has a dual nature. This means that it exhibits properties of both waves and particles. The
current definition of light is a photon of energy carried by or upon a wave front.
A Photon is a discrete packet of energy. The visible spectrum is the range of wavelengths of
electromagnetic radiation which stimulate the human retina (4 x 10-7 m < λ < 7.5 x 10-7 m).
White light is a combination of all colors of the visible spectrum, while black is the absence of
all colors. White light can be separated into its component colors by diffraction which is the
bending of light. Since light is part of the electromagnetic spectrum, all light has the same speed
(3 x 108 m/s in a vacuum) and since v =fλ, frequency is inversely proportional to wavelength.
Hence colors of light which have the longest wavelength have the lowest frequency and those
with the shortest wavelength have the highest frequency.
The visible spectrum of light, has the following component colors listed from lowest frequency
(longest wavelength) to highest frequency (shortest wavelength)
R O Y
E R E
D A L
N L
G O
E W
G
R
E
E
N
B
L
U
E
I
N
D
I
G
O
V
I
O
L
E
T
Frequencies below the visible spectrum are called infrared and those above the violet are called
ultraviolet.
The instrument used to measure wavelengths of light is called a spectroscope.
Quantum Theory of the atom says that an electron in the electron cloud can only absorb energy
if that energy is enough to move it to the next level in the cloud. This moves the electron from
its normal or ground state to a higher energy or excited state. In order to lose this extra, set
amount of energy, the electron gives off light.
Max Planck found that the color or frequency of light given off corresponded to the amount of
energy given off according to the equation:
E = hf
Where h is a constant = 6.626 x 10-34 J•s and f = the frequency of light given off.
Hence energy is in joules. This atomic spectra or the spectrum of light given off by a specific
electron structure gives rise to the branch of chemistry called spectroscopy which is the
identification of chemical composition by examination of the light a compound gives off.
Primary Colors of Light vs. Primary Pigments
Primary colors of light are Red, Green, and Blue. When all three are combined you get white
light. When they are combined in pairs you get the complement of the third color.
Combination
Produces
Green and Blue Cyan
Red and Blue
Magenta
Green and Red Yellow
Which is the
complement of
Red
Green
Blue
Hence
Red + Cyan = White
Magenta + Green = White
Yellow + Blue = White
Pigments (i.e. inks and paints) are subtractive. This means that each color added causes less
light to be reflected! The primary pigments are Cyan, Yellow, and Blue. If you have ever
looked at a color print cartridge, these are the colors of the inks in the cartridge. If you add the
three primary pigments they produce the color black. Hence they have subtracted out all of the
reflected light. (Remember that black is the absence of reflections or light)
In pairs they produce their complimentary color by subtraction.
Combination
Magenta and Yellow
Magenta and Cyan
Yellow and Cyan
Which are made of
(Red + Blue) + (Green + Red)
(Red + Blue) + (Green + Blue)
(Red + Green) + (Green + Blue)
Test Review Questions:
Chapter 13, p. 482- 483
Problems: 2,3,5,6,7,8,9, 11, 15,16,17,18,19
Chapter 14, p. 520-521
Problems: 2,3,4,6,12,13,15,16,17,18
Produces
Red
Blue
Green