The Physics of Color (Experiments in Physics by John Kolena and

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```					(Experiments in Physics by John Kolena and Hugh Haskell)

The Physics of Color
Name/partners: ________Period: ________

Goal: This la is a self paced tutorial designed to introduce the user to the physics of
additive and subtractive color mixing and of scattering-produced color.

The applications of the various principles learned here include color television, stage
lights, paints and dyes, color photography, color printing in magazines, and the color of
the sky and sunsets.

 P. Hewitt, CONCEPTUAL PHYSICS, 5TH EDITION, Chapeter 26,
 J. Pasachoff and M. Kutner, INVITATION TO PHYSICS, color plates 52-57
 Giancoli 23(13)

A) ADDITIVE COLOR MIXING (primary additive colors: red, blue, green)

PART 1: ADDING LIGHTS

Use three slide projectors to project the three primary colors in various combinations
onto a white wall. Put a different primary color filter in each different projector; then
partially overlap the color rectangles on the wall.

What color is produced when:

a) red is added to blue?

[Use Hewitt (bottom of right hand side of inside back cover) or Pasachoff (color
section just after page 280) to identify color names if you are not sure. ]

b) red is added to green?

c) green is added to blue?

d) Blue, green and red are all added together?

e) Magenta is added to cyan? (Why the answer is “light blue”?)
f) Magenta is added to yellow?

g) Yellow is added to cyan?

[To predict the answer to the three questions immediately above, think about
what magenta’s components are (i.e., R and B), what cyan’s components are, etc.;
then add the components and remember the answer to (d) above.]

h) Magenta, yellow, and cyan are all added together?

Explain how each of the following colors are produced by the addition of the three
primary colors (R, B, G):

a) Orange

b) Violet

c) Pink

d) apple green

Use the slide projectors to verify your predictions.

PART 2: APPLICATIONS

a) Look at a color TV (with the picture tube on!) with a magnifying lens.
How does a TV reproduce all the colors using only the basic three?

What tells the individual R, B, and G phosphors how bright to be?

A black and white TV uses two color phosphors to produce the perception of white
(and various shades of gray). What color are the phosphors?
b) Additive colors in art

Pointillist painters (Seurat, Signac, ……)

See Encyclopaedia Britannica Macropaedia: Neo – Impressionism

Macropaedia: Painting, Art of (Figure 1)

Mosaics, see Encyclopaedia Britannica Macropaedia: Mosaic (color plates)

c) Where else is the additive color process used?

B) SUBTRACTIVE COLOR MIXING (primary colors = magenta, cyan,
yellow)

PART 1: TRANSMITTED AND REFLECTED COLORS

Filters wwill TRANSMIT only one (or a narrow range of) color. For example, a red filter
place din front of a (white light) slide projector projects a red beam because the filter
transmits red but absorbs the blue and green in the white light.

Dyes and pigments will REFLECT only one (or a narrow range of) color. For example, a
red rose looks red (in white light) because the rose reflects red but absorbs the blue and
green that was in the white light.

To further illustrate the subtractive process, start with a white light slide projector in
which a magenta filter is placed. Then place a yellow filter in front of the lens. What
color emerges, i.e., passes through both the magenta and yellow filters?

[Use your knowledge of what primary additive color components (i.e. , R or B or G) a
magenta filter transmits and which it absorbs.]
Predict and then try the following combinations:

(the arrow indicates light about to pass through a filter, the color of which follows)

a) White →      M→        C        =       ?

b) white →       Y→        C       =       ?

c) white →       C→        Y       =       ?

Do the last two combinations produce exactly the same color?

How is the color produced in either of the last two combinations different from that
produced when Y and C light are added on the wall with two projectors?

Predict and then try experimentally:

d) white →        Y→       B           =   ?

e) white →        R →         C        =   ?

f) white →        M →          R       =   ?

g) white →         R   →       B =         ?

There are also many other combinations that you can try on your own. Are the results
always the same as you predict?
Why not?

Decide the “content” of either the “orange” or “purple” filters by testing which primary
(additive and/or subtractive) colors pass through and which don’t.

C) COLOR BY SCATTERING
Set up the following experiment:

Look
This way            −−→              Glass tank                         slide projector
To see                               filled with water
Transmitted light

↑
|
|

Look this way to see
scattered light

Gradually add a bit of milk to the water and mix. Don’t add too much at a time; how do
the colors of the transmitted and reflected light change as more milk is added?

Why does the color of the scattrered light change from the bluish to white as more milk is
added? (see next page)
What color is the transmitted light? Why?

And does have anything to do with the blue sky, red sunsets, and white clouds? Explain
with a diagram.

THE COLOR OF SCATTERED LIGHT AND HOW IT DEPENDS ON
WAVELENGTH

IF Wavelengths of light >> size of the                       THEN Scattering ≈ λ-4
scattered               scattering particles

e.g. SUNLIGHT                 ATMOSPHERIC            i.e., very strong color effects
MOLECULES             (Rayleigh scattering)

(4000 A < λ < 7000 A )         Size =1A
IF Wavelengths of light << size of the                 THEN Scattering does not depend
scattered               scattering particles                  on wavelength

e.g., SUNLIGHT             CLOUDS, BIRDS,            i.e., no color effects
PLANES

λ ~ 10-6 m              Size = 1 – 100 m
IF Wavelengths of light ≈ size of the                       THEN Scattering ~ 1/ λ
scattered               scattering particles

STARLIGHT                  DUST PARTICLES            i.e., weak color effects
AROUND STARS
(reflection nebulae)

QUESTIONS ABOUT COLOR MIXING AND COLOR SCATTERING

1. If sunlight were blue instead of white, what color clothes would be most advisable on a
sunny day? What about on a very cold day?
2. What color would red cloth appear if illuminated by sunlight? By a neon sign? By a
cyan light?

3. How could you use the spotlights at a play to make green clothes of a performer
suddenly change to black?

4. If the sky were composed of atoms that scattered red light best, instead of blue, what
color would sunsets be?

5. Why does the eclipsed full moon appear reddish orange?
(How does sunlight get to the moon if it’s eclipsed anyway?)

6. The blue of a kitten’s nose, of the feathers of a blue jay or bluebird, of the rump of a
baboon, or of the neck of a turkey is NOT from a blue pigment. What is the blue color
due to?

Why/how does industrial pollution change the color of sunsets?

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