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)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 LightDemo – 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 ShadowsDemo - Color Addition and Colored Shadows Color Addition Schematic Red, green, and blue are called the additive primaries.

RED, GREEN, and BLUE when added also produce white. Demo - Color Addition and Colored ShadowsDemo - Color Addition and Colored Shadows

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 single colors 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.

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

To summarize, see color addition circles on next slide.

Color Addition Circles This is what you would get with three partially overlapping spotlights reflecting off of a white screen.

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

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

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

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

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

To summarize, see color subtraction circles on next slide.

Color Subtraction Circles This is what you would get with three partially overlapping transparencies on an overhead projector. 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.

White Red Green Blue Yellow Cyan Magenta Primary Colors

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.

Blue in this direction Sun Earth Top of Atmosphere

6.WHY SUNSETS ARE RED 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. Sunset Demo - Blue Sky and Red SunsetDemo - Blue Sky and Red Sunset Because of scattering of blue light the sun appears more yellowish at noon than it really is. Earth Sun

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.black

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 – URLSlide – Colorblindness Tests – URLURL

10. AFTER IMAGES Slides - After ImagesSlides - After ImagesSlides 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

Most of the light that we see has undergone (a) selective interference (b) selective transmission (c) selective reflection (d) selective refraction (c) selective reflection

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 (c) complementary colors

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

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 (b) resonators which scatter blue light

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 (c) cyan To Chapter 28

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