Optics Part 2: Chapter 27 Color

Color cones

light that enters the eye through the pupil ultimately strikes the inside surface of the eye known as the RETINA. it is lined with a variety of light sensing cells known as rods and cones. -while the rods and sensitive to the intensity of light, the cannot distinguish between lights of different wavelengths. -Cones are the color-sensing cells of the retina. A chemical reaction is activated that results in an electrical impulse being sent along the nerves of the brain. 3 kinds of cones each sensitive to it's own range of wavelengths pithing the visible light spectrum. RED CONES, GREEN CONES, AND BLUE CONES

Complementary Colors

• A color and its complement must combine to result in white. • R+B=M; R+G=Y; B+G=C • M +_g__=white • Y +_b__=white • C +_r__=white

Why the sky is blue varies in different locations under various conditions:

• Clear dry day—much deeper blue sky • Clear, humid day—blue sky • Lots of dust particles and larger molecules than nitrogen and oxygen in the atmosphere—less blue sky with whitish appearance • After heavy rainstorm (washing away of airborne particles)— deeper blue sky

Why are clouds white? - larger particles eg. water droplets - scatter all particles that combines to form white - really blue means really clean

- Clusters of various sizes of water droplets • Size of clusters determines scattered cloud color. - Clouds contain water drops of different sizes - Overall result is white clouds, scattering of all frequencies - Slightly larger clusters produce a deep gray. Thicker the cloud, less light gets through. - Still larger clusters produce raindrops.

Color

- Physiological experience; in the eye of the beholder • The color we see of an object does not reside in the object • It is the frequency of the light reflected or transmitted by the object • It depends on the frequencies of incident light, as well as the atomic structure of the material. • That is why objects appear to have different colors under different lighting; objects have different shades when wet or dry

Why the sky is blue

- Results of selective scattering of smaller particles than the wavelength of incident light and resonances at frequencies higher than scattered light - The tinier the particle, the higher the frequency of light it will re-emit. - Atmosphere contains small particles (O2 and nitrogen) - Blue and Violet get scattered the most :eye is more sensitive to blue (high frequency) -red/orange is left over (low frequency)

• White light is incident on a surface that absorbs blue. The surface will appear _________ • White light is incident on a surface that absorbs magenta. The surface will appear ________ • Yellow light is incident on a surface that absorbs red. The surface will appear ________ • Red light is incident on a surface that absorbs magenta. The surface will appear ________

-yellow -green -green -blue

Additive PRIMARY COLORS

Additive primary colors: colors that in equal intensities add to give the perception of white - RED, GREEN, BLUE - Produce any color in the spectrum - Red + Blue = ________ - Red + Green = __________ - Blue + Green =___________

Selective Transmission

Color of transparent object depends on color of light it transmits. Blue pigment in the glass only transmit blue light and absorbs the other frequencies. - Colored glass is warmed due to the energy of absorbed light illuminating the glass.

Monitors- additive color system RGB

Each pixel on a computer screen is composed of three small dots of compounds called phosphors. The phosphors emit light when struck by the electron beams produced by the electron guns at the rear of the tube. The three separate phosphors produce red, green, and blue light, respectively. • When no electrons strike the phosphors of a computer screen the phosphors emit no light and the screen appears black. On a white section of a screen all three phosphors are excited and produce light with about the same relative intensities as in sunlight so the light appears white. Gray parts of the screen have all three phosphors producing light, but at a much lower intensity.

What color is the human eye most sensitive to?

Green light. - the color we see also has to do with the sensitivity of our eyes. Not all perceive color the same way.

Why the sky is blue... Rayleigh Scattering

Rayleigh scattering by particles: when the particle size is smaller than the incident wavelength (<1/10th) and the resonance frequency is higher than the scattered frequencies of light, then scattering occurs predominantly of the higher frequencies present. The color of the sky depends on the particles present in the sky to scatter the sunlight. The earth's atmosphere consists primarily of nitrogen and oxygen molecules that have a resonance frequency in the ultraviolet region - hence Rayleigh scattering occurs and blue light is predominantly scattered.

Sky color (Mei scattering)

When the particles size increases to be comparable to the wavelength the scattering process is called Mie scattering and is not frequency dependent - so lower frequencies will also be scattered. The sky will be more whitish now. Pollutants in the atmosphere are larger sized particles that would produce this effect.

A blue object will appear black when illuminated with A. blue light. B. cyan light. C. yellow light. D. magenta light.

c

A variety of sunset colors is evidence for a variety of A. elements in the Sun. B. apparent atmosphere thickness. C. atmospheric particles. D. primary colors.

c

A white sky is evidence that the atmosphere contains A. predominantly small particles. B. predominantly large particles. C. a mixture of particle sizes. D. pollutants.

c

What can the human eye not see? A. Infrared radiation B. Ultraviolet radiation C. Both A and B. D. Neither A nor B.

c

A red rose will not appear red when illuminated only with A. red light. B. orange light. C. white light. D. cyan light.

d

If molecules in the sky scattered orange light instead of blue light, sunsets would be A. orange. B. yellow. C. green. D. blue.

d

Red, green, and blue light overlap to form A. red light. B. green light. C. blue light. D. white light.

d

When the color yellow is seen on your TV screen, the phosphors being activated on the screen are A. mainly yellow. B. blue and red. C. green and yellow. D. red and green.

d

To stay cool, stop stay warm?

stay cool-> reflect white light (wear a white shirt) in the sum stay warm -> wear opposite (black) absorbed all colors. - green sun? wear green to reflect and stay cool; wear anything else but green to absorb and stay warm - Magenta sun? wear Magenta or red and blue to stay cool; wear green (absence of magenta) to absorb and stay warm

Color and frequency of light

• Colorweseedependsonfrequencyoflight. - Lowest frequency—perceived as red - In between lowest and highest frequency— perceived as colors of the rainbow (red, orange, yellow, green, blue, indigo, violet) - Highest frequency—perceived as violet - Beyond violet, invisible ultraviolet (UV) * ROYGBIV -lowest to highest frequency (shortest to longest wavelength)

Why sunsets are red Rayleigh scattering -> what is left in the sun beam -> lower frequency red and orange

• Light that is least scattered is light of low frequencies, which best travel through air. - Red - Orange - Yellow • If sunlight has a short path through the atmosphere not much light is scattered out. • At sunset the path of sunlight through the clouds is maximized and most of the blue is scattered out so what is left as the light reaches the lower atmosphere, are the lower frequencies such as red and orange.

Selective reflection

• Objects reflect light of some frequencies and absorb the rest. - Rose petals absorb most of the light and reflect red. - Objects that absorb light and reflect none appear black. - Objects can reflect only those frequencies present in the illuminating light. So the true color of an object can be seen only in daylight where all frequencies are present.

Subtractive Primary colors

• Subtractive primary colors - Combination of two of the three additive primary colors of equal intensity: • red + blue = magenta (opposite of green) • red + green = yellow (opposite of blue) • blue + green = cyan (opposite of red)

Why is water greenish blue? - water= transparent = what goes through? Whatever is not absorbed -> red and infared - Sunlight = R + G + B - R (absorbed) = B +G = Cyan (color of ocean)

• Water molecules resonate somewhat in the visible red, which causes red light to be a little more strongly absorbed in water than blue light. • Red light is reduced to one-quarter of its initial brightness by 15 meters of water. There is very little red light in the sunlight that penetrates below 30 meters of water. • When red is removed from white light, the complementary color of red remains: cyan—a bluish-green color. • Particles and dissolved substances, depending on their size and physical properties (including the type of molecular bonds), will scatter or absorb visible light at different wavelengths and actually change the color of the water.

Observing colors (white, black)

• White is observed when light of all colors add together. eg) sunlight is the mix of all 7 colors. • Black is the absence of any reflected light • A color of an object (as viewed by the eye) is the frequency of the transmitted or reflect light.