Major Concepts of Physics PHY 102 – Lecture #9 12015Syracuse University

Lecture #9Lecture #9How do the colors mix each How do the colors mix each

other up? other up?

February 16February 16thth Spring 2015Spring 2015

Prof. Liviu MovileanuProf. Liviu Movileanulmovilea@syr.edu

Room 211, Physics Bldg., 443-8078Room 211, Physics Bldg., 443-8078

Major Concepts of Physics Major Concepts of Physics PHY102PHY102

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1. Review/Continuation Blackbody 1. Review/Continuation Blackbody radiation.radiation.

2. Comments on Conceptual Examples2. Comments on Conceptual Examples

3. Color mixing/Dispersion3. Color mixing/Dispersion

4. Primary colors/Rules in color mixing4. Primary colors/Rules in color mixing

5. Lecture demonstration (Color mixing)5. Lecture demonstration (Color mixing)

6. Announcements6. Announcements

Lecture objectivesLecture objectives

For increasing temperatures, the black body intensity increases for all wavelengths.

The maximum in the energy distribution shifts to shorter (longer f) for higher temperatures.

maxT = 2.9x10-3 m K

is Wien’s law for the maximum intensity B(T)

providing an estimate of the peak emission

Wien’s displacement lawWien’s displacement law

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f

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Colors from the SunColors from the SunCalculate λp from Wien’s law. T = 6,000KResult: 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.

Suppose that we increase the temperature still further.

At 8000K, the value of λp 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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Blue-Hot Stars Blue-Hot Stars

While red and green are produced in prodigious amounts, blue and violet dominate, and the color that appears is blue.

Note that the Wien’s law allows us to estimate the surface temperature of a star, by simply observing the dominant colors it emits.

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Wien’s law

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Good absorbers are good emitters. Good reflectors are poor emitters. Emissivity of a SolidThe 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-Boltzman’s lawRecall 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 = eAT4

Emissivity of a solid

 

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What is white light?/Newton’s What is white light?/Newton’s observation observation

 

              

The dispersion of light The dispersion of light

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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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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 sun’s surface temperature: 6000K.Wien’s Law yields that the intensity of sunlight peaks at the color green.Wavelength is 450 nm.

Sunlight Radiation Sunlight Radiation

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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.

The primary AdditivesThe primary Additives

Rules in Color Mixing 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 BR, G, and B

Why are they the Why are they the primary additivesprimary additives??

Because experiment shows: Because experiment shows: Any perceived color can be matched by an Any perceived color can be matched by an additive combination of R, G and B.additive combination of R, G and B.

Just choose appropriate relative intensities.Just choose appropriate relative intensities.Reason: The eye has three types of sensors in Reason: The eye has three types of sensors in the retina. They are each sensitive to either R, the retina. They are each sensitive to either R, G, or B light.G, or B light.

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Primary AdditivesPrimary Additives

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Color TV 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.

FiltersA 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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Filters

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.Major Concepts of Physics PHY102 – Lecture #9 212015Syracuse University

QuestionQuestion

Mixing PaintsThis 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 ProblemsTwo colors are mixed finely. Or, two filters overlap.What is the resulting color? (Always assume white light is incident.)

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1. Workshop this week: Color mixing;1. Workshop this week: Color mixing;Example of problems and their solutions from Example of problems and their solutions from

previous lectures.previous lectures.

2. The material taught in this lecture cannot be 2. The material taught in this lecture cannot be found in the textbook!! Please take a look at print found in the textbook!! Please take a look at print outs and PHY 102 web site.outs and PHY 102 web site.

3. Homework #3 is due on this week.3. Homework #3 is due on this week.

AnnouncementsAnnouncements

Question:Question:Pigments of cyan and magenta are mixed together finely. Pigments of cyan and magenta are mixed together finely. What is their resultant color?What is their resultant color?