Chapter 28 Color

Separation of visible light into its different colors Newton used a prism to show how light is separated into its component parts When all the wavelengths of the visible light spectrum strike your eye at the same time, white is perceived Dispersion

Prism optical device having a triangular shape, made of glass or quartz; used to deviate a beam of light or invert an image A Prism can be used to separate light into the visible spectrum of colors

So 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

The Color Spectrum Spectrum the spread of colors seen when white light is passed through a prism the spread of radiation by frequency, so that each frequency appears at a different position

The Color Spectrum Spectrum Visible spectra occurs at Wavelengths  700 nanometers (nm) to 400 (nm)

Each frequency & wavelength corresponds to a specific color

The Color Spectrum Mixing all of the colors of light in 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.

Visible Spectra

The eye

luminance response of the eye The eye does not have an equal number of each of the three types of cones About half of all of the cones are red cones Most of the rest are green cones Only a few of the cones are blue cones What the three primary colors of light? Red Blue Green

Color Responses of the Cones

Luminance can be defined as how bright a light appears to a viewer Day time Night time

3 – 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? Homework Read and notes 28.4 – 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? Homework Read and notes 28.4 – 28.7

Three factors determine the colors we see 1. the color of the light that shines on the object 2. the reflective properties of the object 3. the biology of the eye that sees the object.

White light composed of all wavelengths of visible light is incident on a pure blue object Only blue light is reflected from the surface

When visible light strikes an object and a specific frequency becomes absorbed, that frequency of light will never make it to our eyes

Any visible light which strikes the object and becomes reflected or transmitted to our eyes will contribute to the color appearance of that object

So the color is not in the object itself, but in the light which strikes the object.

The only role that the object plays is that it might contain atoms capable of absorbing one or more frequencies of the visible light which shine upon it

What 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

Color by Reflection Light- is a type of electromagnetic wave which stimulates the retina of our eyes Light is reflected from objects in a similar way to how sound is reflected.

Color by Reflection At 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.

Color by Reflection If opaque, light passes back from where it came (reflection) Materials absorb some light & reflect the rest If a material absorbs all the light, it is black An object can only reflect light of frequencies present in the illuminating light

Color by Transmission The color of a transparent object depends on the color of the light it transmits Pigment- A material that selectively absorbs colored light

Color by Transmission Electrons in the pigment atoms absorb light of certain frequencies from the illuminating light Light of the other frequencies are reemitted

Sunlight Yellow-green light is the brightest part of sunlight. The violet portion of sunlight is the least bright. This is why sunlight appears yellowish

Mixing Colored Light Additive primary colors-Red, green, and blue. Complementary Colors- when two colors are added together to produce white. Red + Green = yellow Red + Blue = magenta Blue + Green = cyan Yellow + Blue = white Magenta + Green = white Cyan + Red = white

Mixing Colored Light

Mixing Light

Complementary Colors If you begin with white light and subtract some color from it, the resulting color will appear as the complement of the one subtracted. Color subtraction Color addition

Light Quiz R + G = YR + B = MB + G = C

Pigments 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.

Mixing Colored Pigments Paints and dyes contain pigments They produce colors by absorbing light of certain frequencies and reflecting light of others. Pigments use color mixing by subtraction Mixing colored light is color mixing by addition

Mixing 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.

Mixing Pigments Yellow plus Cyan yields Green Yellow plus Magenta yields Red Cyan plus Magenta yields Blue Equal quantities of Yellow, Cyan and Magenta yields Black

Color 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

Color 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).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

How we see color

The color of an object is not actually within the object itself The color is in the light which shines upon it that ultimately becomes reflected or transmitted to our eyes

Quiz 1. 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.

Why is the Sky Blue? The high frequencies (BIV) are more easily scattered by atmospheric particles While the lower frequencies (ROY) are most likely to pass through the atmosphere without a significant alteration in their direction

Why is the Sky Blue? The interaction of sunlight with matter can result in one of three wave behaviors: absorption, transmission, and reflectionabsorption, transmission, and reflection This scattering process involves the absorption of a light wave by an atom followed by reemission of a light wave in a variety of directions The amount of multidirectional scattering which occurs is dependent upon the frequency of the light

Why is the Sky Blue? The lower frequencies of sunlight (ROY) tend to reach our eyes as we sight directly at the sun during midday not all frequencies are equally intense the sun appears ………………….. during midday due to the ……………………….. of dominant amounts of ………………….. frequencies …………………. our atmosphere and to our eyes

Why are clouds white? Different size water droplets result in a variety of frequencies for scattered light Low frequencies from larger droplets and high frequencies from tinier droplets Therefore mixing all colors of light resulting in a (white cloud).

Why Sunsets are Red? As the sun approaches the horizon line, sunlight must traverse a greater distance through our atmosphere As 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

Why Sunsets are Red The 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 sunsets and some very serious environmental problems

Why Sunsets are Red When light passes through a thick atmosphere, light of the lower frequencies is transmitted while light of the high frequencies is scattered At noon, sunlight travels through the least amount of atmosphere As the day goes on the path through the atmosphere is longer and more blue is scattered from sunlight Less blue remains in the sunlight that reaches the earth and the sun appears redder

Why Water is Greenish- Blue Water is transparent to almost all visible frequencies of light Water molecules resonate to the visible-red frequencies This causes a gradual absorption of red light by water Water is greenish-blue because red is absorbed by molecules in the water

The Atomic Color Code-Atomic Spectra Every element has its own characteristic color when made to emit light Spectroscope-An instrument that analyzes the light from glowing elements The arrangement of thin slit, lenses, and a prism is the basis for the spectroscope

Can you see the baby?

CAN YOU SEE THE DOG?

CAN YOU SEE 10 FACES IN THE TREE

THERE'S A FACE IN HERE. CAN YOU SEE IT?

CAN YOU SEE THREE WOMEN IN EINSTEIN'S FACE?

Study for your test