Chapter 27 Color 1 SELECTIVE REFLECTION Most objects

Chapter 27 Color

1. SELECTIVE REFLECTION • Most objects "reflect" rather than emit light. • The spring model of the atom works well in explaining reflection. • Radiations that match the resonant frequencies of the atoms are absorbed. • Frequencies of the radiations on either side of the resonant frequencies are “reflected. ”

• Objects can only reflect the light that is in the source illuminating the object. • Demo – Razorback Football in Cyan Light (Next Slide)

2. SELECTIVE TRANSMISSION • As light passes through materials some frequencies of light are removed (absorbed) while other frequencies are transmitted. • The degree of transmission depends on how transparent the material happens to be.

• Color filters are good examples of selective transmission. • Demo – Color Filters and White Light

3. MIXING COLORED LIGHT • All visible frequencies make up white light. • Example: The sun emits all frequencies and its light is white. • (Actually it is slightly yellowish to us on Earth, which possibly explains why we are more sensitive to light in the middle of the spectrum. )

• RED, GREEN, and BLUE when added also produce white. • Demo - Color Addition and Colored Shadows • Color Addition Schematic • Red, green, and blue are called the additive primaries.

Color Addition

Through color addition you are able to see a wide range of colors from a color TV or color projector which actually only emit three different colors. These colors are red, green, and blue. They are called the additive primaries.

Your vision system “adds” these together to see a single color from a single location illuminated by more than one color. You even see colors that don’t appear in the continuous emission spectrum of the sun. Red, green, and blue are used as the additive primaries because this set of three will produce the widest range of colors that you visually experience.

On the next slide you will see what happens as you add colors to produce other colors.

Colors in White Light White Red Green Blue You can see that these three add to give white. Yellow Note that yellow is the addition of red and green. Cyan Magenta Note that cyan is the addition of green and blue. Note that magenta is the addition of red and blue.

To summarize, see color addition circles on next slide.

Color Addition Circles What you are about to see is what you would get with three partially overlapping spotlights reflecting off a white screen. Yellow Red Green Cyan Magenta Blue

Complementary Colors • Any two colors that add to give white are said to be complementary colors. • Demo - Complementary Colors

4. MIXING COLORED PIGMENTS • Subtractive primaries - YELLOW, CYAN, and MAGENTA • Example - Mixing paints, zip-lock sandwich bags, color printing • Demo - Color Subtraction • Overhead - Foretravel Advertisement

Color Subtraction

Through color subtraction you are able to see a variety of colors from printings, paintings, etc. If you have ever bought printer inks, you will notice that the ones used to provide a variety of colors in printing are yellow, cyan, and magenta. They are called the subtractive primaries.

In subtraction, colors are eliminated by the absorption of colors that were in the original illuminating source. This particular set of three colors, yellow, cyan, and magenta, will produce the widest range of colors that you visually experience.

On the next slide you will see what happens as you remove different colors from white light.

Colors in White Light You get blue. White Blue Yellow Take away yellow and what is left?

Colors in White Light You get red. White Red Cyan Take away cyan and what is left?

Colors in White Light You get green. White Green Magenta Take away magenta and what is left?

To summarize, see color subtraction circles on next slide.

Color Subtraction Circles What you are about to see is what you would get with three partially overlapping transparencies on an overhead projector. Green Cyan Yellow Blue Red Magenta Slide - Raincoat Closet

• It should be noted from the previous that objects that reflect a particular color are themselves good absorbers of the complimentary color of that particular color. • For examples: • A red object is a good absorber of cyan and vice versa. • A blue object is a good absorber of yellow and vice versa. • A green object is a good absorber of magenta (blues and reds) and vice versa.

Primary Colors White Red Green Blue Yellow Cyan Magenta

5. WHY THE SKY IS BLUE • One man’s view.

• Just as resonating tuning forks scatter sound, so do particles in our atmosphere scatter light.

• N 2 and O 2 scatter high frequencies which are near natural frequencies of N 2 and O 2. • (Natural frequencies are in the UV. ) • This scattering produces the bluish sky. • The blue end of the spectrum is scattered ten times better that the red end.

Sun Top of Atmosphere Blue in this direction Earth

6. WHY SUNSETS ARE RED Sunset • Ten Most Beautiful Sunsets If the atmosphere becomes thicker or the paths of light through the atmosphere become longer, more of the longer wavelengths of light will be scattered.

Sun Earth

• Demo - Blue Sky and Red Sunset • Because of scattering of blue light the sun appears more yellowish at noon than it really is.

7. WHY CLOUDS ARE WHITE • Droplet size dictates which colors are scattered best. • Low frequencies scatter from larger particles. • High frequencies scatter from small particles.

• Electrons close to one another in a cluster vibrate together and in step, which results in a greater intensity of scattered light than from the same number of electrons vibrating separately. • Large drops absorb more and scatter less.

8. WHY WATER IS GREENISH BLUE • Water quite often looks bluish. • This is due to reflected “sky light. ” • A white object looks greenish blue when viewed through deep water.

• Water is a strong absorber in IR and a little in red. • Remove some of the red and cyan is left. • Crabs and other sea creatures appear black in deep water.

9. COLOR VISION AND COLOR DEFICIENCY • Colorblindness (color deficiency) affects about 10% of population • Red-green is predominant • Yellow-blue - a few • Total – some • Slide – Colorblindness Tests – URL

10. AFTER IMAGES • Slides - After Images

• After images are due to conal fatigue. • Cones that have been “firing” for a while will not “fire” as well as “rested” cones when all are exposed to white light.

Chapter 27 Review Questions

A mixture of magenta and green lights give white light. These two colors are (a) additive primaries (b) secondary colors (c) complementary colors (d) fluorescent colors (e) interference colors

Mixing yellow paint and magenta paint gives what color? (a) red (b) green (c) blue (d) cyan

What color would red cloth appear if it were illuminated by cyan light? (a) cyan (b) red (c) yellow (d) green (e) black

The sky is blue because air molecules in the sky act as tiny (a) mirrors which reflect only blue light (b) resonators which scatter blue light (c) sources of white light (d) prisms (e) none of these

When you stare at a red object for a long time without moving your head and eyes and then suddenly look away at a white screen, you will see a image of the object. (a) red (b) blue (c) cyan (d) green (e) white