1. General Physics L20_color
Group Work
Why do cooler objects emit a greater fraction of their energy at long wavelengths (low frequencies)?

2. General Physics L20_color
Light and Color
This lecture takes lots of demonstration apparatus:
Transmission overhead projector with
Diffraction grating after focusing lens
Slit mask
Square mask
CMY filters
RGB filters
RGB filter wheels
Vertical-slit lamp and variac
Class set of diffraction gratings
R, G, B projectors

3. General Physics L20_color
Today’s Objectives
Explain how the human eye identifies colors.
Explain how colors are produced by absorption, dispersion, and scattering.

4. How Human Color Vision Works
General Physics L20_color
How Human Color Vision Works
424
530
560
Demonstrate
R, G, B slide projectors. Secondary colors formed from primaries, white from combination of all three.
Source: Griffith, Physics of Everyday Phenomena

5. General Physics L20_color
Question
What color do we see when green and red receptors are stimulated equally?
A. Cyan.
B. Blue.
C. Yellow.
D. Magenta.

6. General Physics L20_color
Question
What color do we see when blue and red receptors are stimulated equally?
A. Cyan.
B. Blue.
C. Yellow.
D. Magenta.

7. Bird and Reptile Color Vision
General Physics L20_color
Bird and Reptile Color Vision
Source: Scientific American, July 2006

8. Reflection and Transmission
General Physics L20_color
Reflection and Transmission
Demonstrate
Color filters R, G, B, C, M, Y with overhead projector slit and diffraction grating. Filters Remove colors from light.
Subtractive primary colors C, M Y with square mask and no diffraction grating on the projector. Combinations create additive primary colors and black.
(and absorption)

9. General Physics L20_color
Question
Chlorophyll absorbs blue and red light. What color does it appear?
A. red
B. yellow
C. green
D. violet

10. General Physics L20_color
Question
In color printing, which two color inks are combined to make the color orange?
Cyan and Magenta.
Cyan and Yellow.
Yellow and Magenta.
Demonstrate with CMY color wheel. Removing the C wheel still leaves a position of orange.

11. Fluorescent Light Not “Natural”
General Physics L20_color
Fluorescent Light Not “Natural”
Source:

12. Colors Split by Dispersion
General Physics L20_color
Colors Split by Dispersion
Source: Griffith, Physics of Everyday Phenomena, 4 ed.

13. Speed in Glass Depends on l
General Physics L20_color
Speed in Glass Depends on l
Index of refraction
n = c/v
c = vacuum speed
v = speed in medium

14. General Physics L20_color
Question
Which is faster in glass?
Blue light.
Red light.
Both have the same speed.

15. General Physics L20_color
Prism Separates White Light
Source: Griffith

16. General Physics L20_color
Board Work
What is wrong with this famous picture?

17. Dispersion Creates Rainbows
General Physics L20_color
Dispersion Creates Rainbows
Source: Griffith

18. Primary Rainbow: 1 Reflection
General Physics L20_color
Primary Rainbow: 1 Reflection
40°
42°
Source: Griffith, Physics of Everyday Phenomena

19. Secondary Rainbow: 2 Reflections
General Physics L20_color
Secondary Rainbow: 2 Reflections
54.5°
52°
Source: Griffith

20. General Physics L20_color
Viewing Both Rainbows
Source: Ackerman and Knox, Meteorology

21. General Physics L20_color Why the sky is blue and sunsets are red
Rayleigh Scattering
Why the sky is blue and sunsets are red

22. General Physics L20_color
There probably isn’t time, but this can be demonstrated nicely with a white light beam shining through dilute milk. The scattered light is bluish and the transmitted light is reddish.

23. Blue Scatters; Red Passes
General Physics L20_color
Blue Scatters; Red Passes
Source: Griffith