Major Concepts of Physics PHY102

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• Major Concepts of Physics PHY102

Lecture 9Color mixing February 16th Spring
2009 Prof. Liviu Movileanu http//physics.syr.edu
/courses/PHY102.07Spring/index.html
lmovilea_at_physics.syr.edu Room 211, Physics
Bldg., 443-8078
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Major Concepts of Physics PHY 102 Lecture 9
2009?Syracuse University
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Lecture objectives
1. Review/Continuation Blackbody radiation. 2.
Comments on Conceptual Examples 3. Color
mixing/Dispersion 4. Primary colors/Rules in
color mixing 5. Lecture demonstration (Color
mixing) 6. Announcements
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Wiens displacement law
For increasing temperatures, the black body
intensity increases for all wavelengths. The
maximum in the energy distribution shifts to
shorter ? (longer ?) for higher temperatures. ?pT
2.9x10-3 m K is Wiens law for the maximum
intensity B?(T) providing an estimate of the peak
emission
f
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Colors from the Sun

Calculate ?p from Wiens law. T
6,000K Result 480 nm. The wavelength 480 nm is
in the green (middle) part of the
spectrum. Conclude At 6000K, the radiation
produced is a mixture of red, green and blue
light, with intensity peaked in the green. But
The eye perceives equal intensities of red, green
and blue as white light. This explains why white
light is perceived at the highest temperatures.

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Blue-Hot Stars
Suppose that we increase the temperature still
further. At 8000K, the value of ?p is in the
blue part of the visible spectrum. Stars whose
surface temperature is more than 8000K are called
blue-hot stars.
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Wiens law
While red and green are produced in prodigious
amounts, blue and violet dominate, and the color
that appears is blue. Note that the Wiens law
allows us to estimate the surface temperature of
a star, by simply observing the dominant colors
it emits.
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Emissivity of a solid
Good absorbers are good emitters. Good reflectors
are poor emitters. Emissivity of a
Solid The emissivity, e , of a solid measures the
ability of the solid to emit radiation at a
given temperature T. For a perfect black body, e
1. For a perfect reflector, e 0. Stefan
Boltzmans law Recall the law For a black body,
area A, at temperature T ?Q/?t
?AT4 Its extension for a general solid Insert
the factor e on the right side of the law
?Q/?t e?AT4
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Major Concepts of Physics PHY102 Lecture 9
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What is white light?/Newtons observation
                       
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The dispersion of light
                           
 
Conclusion White light is a composite of the
primary colors. Prism reveals this by separating
them out. The experiment is the dispersion of
light. Later discovery Each primary color has
its own wavelength range. Conversely, light of a
single wavelength must be that of a primary
color.
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Sunlight Radiation
Experimental Finding The shortest wavelengths
are bent the most. Violet light (shortest
wavelength) is bent the most. Red light
(longest wavelength) is bent the least. Examine
Sunlight The suns surface temperature
6000K. Wiens Law yields that the intensity of
sunlight peaks at the color green. Wavelength is
450 nm.
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The primary Additives
Approximate model for sunlight Assume the
intensities for red (R), blue (B) and green (G)
are roughly equal. Also Ignore its emissions in
the colors orange, yellow, and violet. We call
the colors R, G, and B the primary additives.
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Rules in Color Mixing

• Additive Combining
• You need to know these four facts
• R G Y (yellow).
• R B M (magenta).
• G B C (cyan).
• R G B W (white).
• The color circles demonstration shows these four
  facts explicitly.
• Important note We are combining light sources.
• That is, each color in the sum is entering our
  eye, simultaneously.

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R, G, and BWhy are they the primary
additives?Because experiment shows Any
perceived color can be matched by an additive
combination of R, G and B.Just choose
appropriate relative intensities.Reason The eye
has three types of sensors in the retina. They
are each sensitive to either R, G, or B light.
Primary Additives

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Color TV
Example Color TV Most TV tubes produce color as
follows Use three different phosphors on the
screen, arranged in tiny dots. The three
phosphors produce red, green or blue light, when
struck by the electron beam of the tube. The eye
integrates the tiny dots to produce a mixture of
the three primaries. The three types produce
red, green or blue light when an electron beam
impinges.
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Filters
Filters A filter absorbs light of certain colors.
It lets through the other colors. Example An
ideal red filter absorbs all colors but red
light. It transmits red light. In the model W
R G B, the red filter absorbs blue and green
light, transmits red. Demonstration A yellow
filter is put on overhead projector above a
slit. The light is separated by a diffraction
grating. What primary colors are absent? Why?
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Question
White light is incident on a pair of overlapping
green and red filters. What light
emerges? Important note If we overlap two
filters, we are not combining light additively.
Each filter subtracts light. Together, no light
gets through. We call this subtractive
combining.
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Mixing Paints This is another example of
subtractive combining. Example We make a fine
mixture of R and B paints. What color is the
mixture? Any color of light that can be absorbed
by either pigment is absorbed. This is
subtractive combining. Subtractive Mixing
Problems Two colors are mixed finely. Or, two
filters overlap. What is the resulting color?
(Always assume white light is incident.)
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Announcements
1. Workshop We will have a recitation session
this week Example of problems and their
solutions from previous lectures. 2. You will
receive by e-mail a reading text regarding color
mixing. The material taught in this lecture
cannot be found in the textbook!!
Question Pigments of cyan and magenta are mixed
together finely. What is their resultant color?
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