Chapter 27: COLOR

*WHY Sunsets are RED*

A sunbeam MUST TRAVEL through more of the atmosphere at sunset THAN at noon. -As the Sun descends lower in the sky, *the PATH THROUGH THE ATMOSPHERE IS LONGER, MORE VIOLET and BLUE IS SCATTERED from the beam at sunset THAN AT NOON. *Q:* WHY the sky near the Sun is also RED at sunset?

*WHY the WHITISH SKY in BIG cities?*

Where the atmosphere *CONTAINS a lot of particles of DUST* AND *other particles LARGER THAN N2 and O2,* *LIGHT of the lower frequencies is also scattered strongly,* (which makes the sky less blue).

*WHY CLOUDS are WHITE*

*Clouds:* CLUSTERS of VARIOUS SIZES of WATER DROPLETS, which are BIG compared to the wavelength of light. *Mie SCATTERING:* When particles are BIG compared to the wavelength of light, ALL VISIBLE wavelengths ARE MORE or LESS EQUALLY SCATTERED.

*Selective Reflection*

DIFFERENT materials have different natural frequencies f0 for *ABSORBING and EMITTING electromagnetic radiation.* -At resonant frequencies (f light~ f0) at which the amplitudes of oscillation are LARGE, light is *ABSORBED*. -At frequencies below AND above the resonant frequencies (f light f0), light is *REMITTED*. -If the material is *TRANSPARENT*, the re-emitted light *PASSES* through it. -If the material is *OPAQUE*, the re-emitted light is *REFLECTED*. *Q:* WHY does something APPEAR white or black? Pass a beam of sunlight through a prism to form a rainbow-colored spectrum. -Petals appear? in ALL parts of the spectrum EXCEPT in the? *Selective reflection:* Objects REFLECT LIGHT of some frequencies *which give them the color and ABSORB the rest.* -Objects and REFLECT *ONLY* those frequencies present in the illuminating light. -Objects that REFLECT light of ALL the visible frequencies APPEAR the SAME COLOR as the light. -Objects that ABSORB ALL the visible light AND reflect NONE appear *black.* The appearance of a colored object DEPENDS on the kind of light that illuminates it. *-An incandescent lamp* EMITS MORE LIGHT in the lower than in the higher frequencies, enhancing any reds viewed in this light. *-A fluorescent lamp* is RICHER in the HIGHER FREQUENCIES and so blues are ENHANCED under them. *-"True color" defined as the color it has in daylight.*

*Mixing Colored Light*

The intensity of sunlight varies with *FREQUENCY* -MOST intense in YELLOW-GREEN portion (where our eyes are MOST Sensitive). *-ALL THE COLORS combined TO MAKE WHITE.* The perception of WHITE also results from the COMBINATION of only *red*, *green*, and Blue light.* -Solar radiation curve divided into THREE REGIONS (3) that *MATCH the color receptors in our eyes.* *A close examination* of THE IMAGE on a computer/TV screen WILL reveal the assemblage of *tiny spots* PIC: THREE (3) colors of *DOTS*: red, green, and blue. The MIXTURES of (RGB) colors *seen at a distance* PROVIDE *a complete range of colors.* *Q:* Lighting stage performance- WHY are the shadows of the golf ball CYAN, MAGENTA, and YELLOW? *Q:* A RED ROSE *WILL NOT appear* RED when illuminated ONLY with... *-CYAN light* *Q:* When the color YELLOW is seen on your TV SCREEN, the DOTS being activated on the screen ARE... *-RED* and *GREEN*. *Q:* A BLUE object will APPEAR BLACK when illuminated with... *-YELLOW light*

*SUBTRACTIVE primary Colors* (Mixing Colored PIGMENTS)

*(CYAN, MAGENTA, YELLOW)* -The FINAL color is achieved by starting with WHITE LIGHT (which contains ALL colors) AND then *SUBTRACTING AWAY* certain colors, *leaving other colors.* *-Various proportions of CMY dyes* will PRODUCE *nearly ANY COLOR in the spectrum.* -Color system CMY.

*Additive PRIMARY colors* (Mixing Colored Light)

*(RED, GREEN, BLUE)* *-COMBINATION of TWO* of these THREE ADDITIVE PRIMARY colors of EQUAL brightness. -BLUE + GREEN= *CYAN* -RED + BLUE= *MAGENTA* -RED + GREEN= *YELLOW* -RED + GREEN + BLUE= *WHITE* *-Various amount of (RGB) produce ANY COLOR in the spectrum.* -Color system RGB

*Mixing Colored PIGMENTS*

*PIGMENTS* are *TINY PARTICLES* that ABSORB specific colors. *Color printing-* the variety of colors in the colored photographs in ANY book are the result of CMYK dots. The APPROXimate RANGES of the frequencies we sense as the (ADDITIVE Primary colors RGB) AND (SUBTRACTIVE primary colors CMY.) *Q:* *When WHITE LIGHT shines on CYAN pigments,* WHAT color of light is ABSORBED? *-RED light* *Q:* A *MIXTURE* of *YELLOW* and *MAGENTA* pigments APPEARS... *-

*Selective TRANSMISSION*

*Q*:* HOW does light change color when going through colored glass? *Selective Transmission:*-- Colored glass *CONTAINS dyes OR pigments*--fine PARTICLES that *SELECTIVELY TRANSMIT* light of certain frequencies AND ABSORBS OTHERS. -Color of TRANSPARENT OBJECT *depends on color of light it TRANSMITS.*

WHY red rose petals *appear RED* while *Leaves appear GREEN?*

*SELECTIVE REFLECTION* Objects *SELECTIVELY REFLECT* certain frequencies of light AND absorb the others. PIC: Objects appear to have colors since *they are able to SELECTIVELY ABSORB and REFLECT* certain wavelengths of visible light.

*WHY is the SKY BLUE?*

*Scattering:* DIFFERENT from reflection, where *radiation is DEFLECTED in ONE DIRECTION,* some particles AND molecules found in the atmosphere *have the ability to SCATTER solar radiation IN ALL DIRECTIONS.* *Rayleigh SCATTERING (SELECTIVE scattering):* when particles (i.e. N2 and O2) are MUCH SMALLER than the wavelength of radiation, particles are MORE EFFECTIVE at SCATTERING a *particular wavelength of light.* *-ULTRAVIOLET* is scattered the MOST in the atmosphere, followed in order by *violet*, *blue*, *green*, *yellow*, *orange*, and *red*. -Our eyes *are NOT very sensitive to violet light,* SO the blue scattered light is what predominates in our vision. *Q:* WHY the sky is always DARK when viewed from the moon -EVEN when the sun is ABOVE the horizon? *The moon has NO appreciable stmosphere to do this scattering, SO the SKY APPEARS DARK.*

*Complementary colors:* (Mixing Colored Light)

*TWO COLORS* added together AND producing WHITE. *-CMY are the COMPLEMENTS of additive primaries RGB.* -CYAN + RED= *WHITE* (=*B*lue + *G*reen + *R*ed) -MAGENTA + GREEN= *WHITE* (=*R*ed + *B*lue + *G*reen) -YELLOW + BLUE = *WHITE* (= *R*ed + *G*reen + *B*lue)

*CYAN*

-*BLUE* + *GREEN*= CYAN

*MAGENTA*

-*RED* + *BLUE*= MAGENTA

*YELLOW*

-*RED* + *GREEN*= YELLOW

*Color in Our World*

Color we see depends on *frequency of light*. Light of DIFFERENT frequencies are perceived as different colors. -From *LOWEST to HIGHEST frequency*, light is perceived as red, orange, yellow, green, blue, indigo, violet. -Lights of these colors TOGETHER appear *WHITE*. The Sun which appears white is essentially ALL COLORS MIXED together. Light sources as the Sun AND lamps *EMIT LIGHT*. *Q:* HOW can we see things that DO NOT EMIT visible light? MOST objects *REFLECT RATHER THAN* emit visible light. -We see things because they *REFLECT* light into our eyes:

*WHY WATER is GREENISH Blue?*

DEEP BLUE at the surface: *the REFLECTED color of the sky.* *GREENISH BLUE under water* -WATER is TRANSPARENT to light of NEARLY all the visible frequencies. -WATER molecules resonate to the frequencies of *infrared AND red lights.* *-RED LIGHT is REDUCED* to 1/4 of its initial brightness by 15m of water. VERY LITTLE RED light penetrates below 30m of water. -CYAN= *WHITE - RED* MANY crabs AND other sea creatures that APPEAR BLACK in DEEP WATER *are found to be RED* when *they are RAISED TO THE SURFACE.* Why? The EXTRAORDINARY BLUE (color) of lakes in the Canadian Rockies *is PRODUCED by scattering from EXTREMELY fine particles of glacial silt* SUSPENDED in the water.