Color

Separation of visible light into its different colorsNewton 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

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

Each wavelength corresponds to a color

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

Visible spectra occurs at Wavelengths 700 nanometers (nm) to 400 (nm)

Visible Spectra

The Color Spectrum•Mixing 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.

The eye

Color Responses of the Cones

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 – 28.7

• 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 – 28.7

Dispersion

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

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

morefrequencies 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

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 subtractionColor addition

Light Quiz

R + G = Y R + B = M

B + 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).

• 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

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 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 RedWhen 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