2Dispersion Separation of visible light into its different colors Newton used a prism to show how light is separated into its component partsWhen all the wavelengths of the visible light spectrum strike your eye at the same time, white is perceived
3DispersionPrismoptical device having a triangular shape, made of glass or quartz; used to deviate a beam of light or invert an imageA Prism can be used to separate light into the visible spectrum of colors
5The Color Spectrum Spectrum the spread of colors seen when white light is passed through a prismthe spread of radiation by frequency, so that each frequency appears at a different positionVisible spectra occurs at Wavelengths nanometers (nm) to 400 (nm)
7The Color SpectrumMixing all of the colors of the spectrum will produce white light.Black is not a color but is the absence of light.Objects appear black when they absorb light of all visible frequencies.Carbon, soot and black velvet are excellent absorbers of light.
103 – Do Now Problems 1. Define the term spectrum. 2. Are black and white ‘real’ colors? Explain your answer.3. What happens to light of a certain frequency that encounters atoms of the same resonant frequency?HomeworkRead and notes 28.4 – 28.7
11DispersionSo when you are in a room with no lights and everything around you appears black, it means that there are no wavelengths of visible light striking your eye as you sight at the surroundings
13Only blue light is reflected from the surface White light composed of all wavelengths of visible light is incident on a pure blue objectOnly blue light is reflected from the surface
14When visible light strikes an object and a specific frequency becomes absorbed, that frequency of light will never make it to our eyes
15Any visible light which strikes the object and becomes reflected or transmitted to our eyes will contribute to the color appearance of that object
16So the color is not in the object itself, but in the light which strikes the object.
17The only role that the object plays is that it might contain atomscapable of absorbing one or morefrequencies of the visible lightwhich shine upon it
18What color will an object appear if it is able to absorb all the frequencies of visible light except yellow?Yellow! But only if the source light contains ROYGBIV
19Color by ReflectionLight- is a type of electromagnetic wave which stimulates the retina of our eyesLight is reflected from objects in a similar way to how sound is reflected.
20Color by ReflectionAt resonant frequencies where amplitudes are large, light is absorbed.When the frequencies are below and above resonant frequencies what happens to the light?The light is reemitted.If transparent, the reemitted light passes through it.
21Color by ReflectionIf opaque, light passes back from where it came (reflection)Materials absorb some light & reflect the restIf a material absorbs all the light, it is blackAn object can only reflect light of frequencies present in the illuminating light
22Color by TransmissionThe color of a transparent object depends on the color of the light it transmitsPigment-A material that selectively absorbs colored light
23Color by TransmissionElectrons in the pigment atoms absorb light of certain frequencies from the illuminating lightLight of the other frequencies are reemitted
24This is why sunlight appears yellowish Yellow-green light is the brightest part of sunlight.The violet portion of sunlight is the least bright.This is why sunlight appears yellowish
25Mixing Colored Light Additive primary colors-Red, green, and blue. Complementary Colors- when two colors are added together to produce white.Red + Green = yellowRed + Blue = magentaBlue + Green = cyanYellow + Blue = whiteMagenta + Green = whiteCyan + Red = white
29Pigments Pigments absorb light. Pure pigments absorb a single frequency or color of light.The color of light absorbed by a pigment is the complementary color of that pigment.
30Mixing Colored Pigments Paints and dyes contain pigmentsThey produce colors by absorbing light of certain frequencies and reflecting light of others.Pigments use color mixing by subtractionMixing colored light is color mixing by addition
31Mixing Colored Pigments For example: Mixing paints is color subtracting.For example:Blue paint reflects mostly blue light, but also violet and green; it absorbs red, orange, and yellow light.
32Yellow plus Cyan yields Green Mixing PigmentsYellow plus Cyan yields GreenYellow plus Magenta yields RedCyan plus Magenta yields BlueEqual quantities of Yellow, Cyan and Magenta yieldsBlack
33Color Subtraction Example 1 Magenta light shines on a sheet of paper containing a yellow pigment. Determine the appearance of the paper.Magenta light consists of red light and blue light.A yellow pigment is capable of absorbing blue light. Thus, blue must be subtracted from the light which shines on the paper.This leaves red light. If the paper reflects the red light, then the paper will look red.M - B = (R + B) - B = R
34Color Subtraction Example 2 Yellow light shines on a sheet of paper containing a red pigment. Determine the appearance of the paper.Yellow light consists of red light and green light.A red pigment is capable of absorbing cyan light; that is, red paper can absorb both green and blue primary colors of light (recall that cyan light is a mixture of green and blue light).So red and green light shine on the paper; and green light must be subtracted. (There is no need to worry about blue light since blue light is not shining on the paper.)This leaves red light to be reflected. If the paper reflects the red light, then the paper will look red.Y - C = (R + G) - (B + G) = R
36How we see colorThe color of an object is not actually within the object itselfThe color is in the light which shines upon it that ultimately becomes reflected or transmitted to our eyes
37Quiz1. Describe the function of the dye in blue jeans. What does the dye do to each of the various frequencies of visible light which strike the pants?The dye is intended to absorb all the colors of visible light except blue. If only blue is reflected then the jeans are “blue” jeans.2. Explain why a red shirt looks red when visible light ("ROYGBIV") shines upon it.The dye is intended to absorb all the colors of visible light except red. Since red is reflected to our eyes the shirt looks red.
38Why is the Sky Blue?The high frequencies (BIV) are more easily scattered by atmospheric particlesWhile the lower frequencies (ROY) are most likely to pass through the atmosphere without a significant alteration in their direction
39Why is the Sky Blue?The interaction of sunlight with matter can result in one of three wave behaviors: absorption, transmission, and reflectionThis scattering process involves the absorption of a light wave by an atom followed by reemission of a light wave in a variety of directionsThe amount of multidirectional scattering which occurs is dependent upon the frequency of the light
40Why is the Sky Blue?The lower frequencies of sunlight (ROY) tend to reach our eyes as we sight directly at the sun during middaynot all frequencies are equally intensethe sun appears ………………….. during midday due to the ……………………….. of dominant amounts of ………………….. frequencies …………………. our atmosphere and to our eyes
41Why are clouds white?Different size water droplets result in a variety of frequencies for scattered lightLow frequencies from larger droplets and high frequencies from tinier dropletsTherefore mixing all colors of light resulting in a (white cloud).
42Why Sunsets are Red?As the sun approaches the horizon line, sunlight must traverse a greater distance through our atmosphereAs the path which sunlight takes through our atmosphere increases in length, ROYGBIV encounters more and more atmospheric particles. This results in the scattering of greater and greater amounts of yellow light
43environmental problems Why Sunsets are RedThe effect of a red sunset becomes more pronounced if the atmosphere contains more and more particles.The presence of sulfur aerosols (emitted as an industrial pollutant) in our atmosphere contributes to some magnificent sunsetsand some very seriousenvironmental problems
44At noon, sunlight travels through the least amount of atmosphere Why Sunsets are RedWhen light passes through a thick atmosphere, light of the lower frequencies is transmitted while light of the high frequencies is scatteredAt noon, sunlight travels through the least amount of atmosphereAs the day goes on the path through the atmosphere is longer and more blue is scattered from sunlightLess blue remains in the sunlight that reaches the earth and the sun appears redder
45Why Water is Greenish-Blue Water is transparent to almost all visible frequencies of lightWater molecules resonate to the visible-red frequenciesThis causes a gradual absorption of red light by waterWater is greenish-blue because red is absorbed by molecules in the water
46The Atomic Color Code-Atomic Spectra Every element has its own characteristic color when made to emit lightSpectroscope-An instrument that analyzes the light from glowing elementsThe arrangement of thin slit, lenses, and a prism is the basis for the spectroscope