Color

1
Chapter 28

• Color

2
The Color Spectrum

• Newton was the first to study color by using a
  prism
• He called the spread of colors a spectrum and
  noted the order
• Red, orange, yellow, green, blue, and violet (ROY
  G BV)
• Are white and black colors?
• White is a combination of all colors of light and
  black is the absence of light

3
The Color Spectrum

• Sunlight is an example of white light
• Under white lightwhite objects appear white and
  colored objects appear their color
• Newton showed that the color spectrum of the
  prism was actually a property of white lightnot
  the colors of the spectrum
• He did this by recombining the colors in a second
  prism to make white light again
• All the colors (on top of each other) combine to
  produce white light
• Objects appear black when they absorb all visible
  light

4
Light Colors

• When a light wave with a single frequency strikes
  an object, a number of things could happen.
• The light wave could be absorbed by the object,
  in which case its energy is converted to heat
• the light wave could be reflected by the object
• the light wave could be transmitted by the
  object.
• Rarely however does just a single frequency of
  light strike an object.
• While it does happen, it is more usual that light
  of many frequencies or even all frequencies are
  incident towards the surface of objects.
• When this occurs, objects have a tendency to
  selectively absorb, reflect or transmit light of
  certain frequencies.

5
Light Colors

• Reflection and transmission of light waves occur
  because the frequencies of the light waves do not
  match the natural frequencies of vibration of the
  objects.
• When light of these frequencies strike an object,
  the electrons in the atoms of the object begin
  vibrating.
• But instead of vibrating in resonance at a large
  amplitude, the electrons vibrate for brief
  periods of time with small amplitudes of
  vibration then the energy is reemitted as a
  light wave.
• If the object is transparent, then the vibrations
  of the electrons are passed on to neighboring
  atoms through the bulk of the material and
  reemitted on the opposite side of the object.
  Such frequencies of light waves are said to be
  transmitted.
• If the object is opaque, then the vibrations of
  the electrons are not passed from atom to atom
  through the bulk of the material rather the
  electrons vibrate for short periods of time and
  then reemit the energy as a reflected light wave.
  Such frequencies of light are said to be
  reflected

6
Color by Selective Reflection

• Color comes from frequencies that are reflected
• 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.
• If an object absorbs all of the frequencies of
  visible light except for the frequency associated
  with green light, then the object will appear
  green in the presence of ROYGBIV. Reflected light
  (color) is not at the natural (resonant)
  frequency of the material
• Most materials absorb light of some frequencies
  and reflect the rest
• A red rose
• Petals appear red because it absorbs all
  frequencies except redreflects the red
• Shine a green light on a rose and the petals will
  appear black because the green light is
  resonantit is being absorbed
• Apparent color depends on the objects natural
  frequency AND what is illuminating it

7
Color by Selective Reflection

• An object can only reflect light of frequencies
  present in its illumination
• A candle emits a yellowish light making things
  appear yellowish
• Incandescent (non fluorescent light) emits all
  visible frequencies but is more rich in lower
  frequencies enhancing reds
• Fluorescent light is richer in higher frequencies
  enhancing blues
• Perceived color depends on the light source
• Color differences are most easily detected in
  direct sunlight

8
Color by Reflection

• Which color(s) of light are reflected by the
  paper and what color the paper will appear to an
  observer?

9
Color by Selective Transmission

• The color of a transparent object depends on the
  color of light it transmitsdepends on the
  frequencies of color that are non resonant
• A blue piece of glass appears blue because it
  transmits primarily blue and absorbs all the
  colors illuminating it
• The glass warms due to the absorbed frequencies
• The material in the glass that selectively
  absorbs colored light is pigmentfine particles
  that selectively certain frequencies and transmit
  others

10
Color by Transmission

• What color will the glass appear?
• When red light shines on a red rose, why do the
  leaves become warmer than the petals?
• What color does a ripe (yellow) banana appear
  when illuminated with red light? Yellow light?
  Green light? Blue light?

11
Sunlight

• The brightness of solar frequencies is
  unevensunlight is made up of a wide range of
  frequencies
• Red and purple regions are not as bright as those
  in the middle regions (yellow and green)

Radiation Curve of Sunlight
Human eyes are most sensitive to
yellow-green. Newer fire engines are being
painted yellow-green and sodium vapor street
lights are being used to illuminate the night.
12
Mixing Colored Light

• Light of all frequencies mixed together is white
• White also results from mixing red, green and
  blue light (only 3 colors)
• This is because there are 3 types of cones (color
  receptors) in the human eye

• Stimulated by lowest 1/3 of spectrum - light of
  these frequencies appears red
• middle
  green
• highest
  blue

(i) (ii) (iii)
When all 3 types of cones are stimulated equally,
we perceive white.
13
Mixing Colored Light

• In fact, any color in the spectrum can be
  produced by various amounts of red, green and
  blue these are called the additive primary
  colors
• Red Green Yellow
• Red Blue Magenta
• Blue Green Cyan
• Red Green Blue White

14
Complementary Colors

• Complementary Colors are two that are added
  together to form white light

Magenta Green White This works because
magenta is a combination of red and blue Yellow
Blue White (yellow is a combination of red
and green) Red Cyan White (cyan is a
combination of green and blue)
Complementary Colors Magenta Green Yellow
Blue Red Cyan
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Complementary Colors

• What color does red light plus green light
  appear?
• yellow
• What color does white light minus red light
  appear?
• cyan
• What color does white light minus magenta light
  appear?
• green

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Complementary Colors

• If magenta light and yellow light are added
  together will white light appear?
• What color will the paper appear?

17
Mixing Colored Pigments

• If R B G white, then why does mixing red,
  blue, and green paints make a brownish-black?
• Its because youre mixing pigments, rather than
  mixing lights directly pigments absorb specific
  colors,
• A red paint pigment absorbs the complementary
  color (cyan), and reflects red.
• This is color subtraction pigment takes away
  the complementary color (cyan) from the impinging
  light.
• So if you mix red, blue and green pigments, then
  you are absorbing cyan, magenta, and yellow,
  everything. So you get black ( absence of
  color).

18
Mixing Colored Pigments

• Color subtraction--in this process, the ultimate
  color appearance of an object is determined by
  beginning with a single color or mixture of
  colors and identifying what color or colors of
  light are subtracted from the original set.
• The subtractive primary colors (magenta, cyan,
  and yellow) are used in printers to produce full
  color images
• Example Magenta light shines on a sheet of paper
  containing a yellow pigment. Determine the
  appearance of the paper.
• M R B Y absorbs B
• Color (R B) B R

http//www.control.co.kr/java1/colors/TabbedcolorB
ox.html
19
Mixing Colored Pigments

• Yellow light shines on a sheet of paper
  containing a red pigment. Determine the
  appearance of the paper.
• Y G R R absorbs B and G
• (G R) G R
• Yellow light shines on a sheet of paper
  containing a blue pigment. Determine the
  appearance of the paper.
• Y G R B absorbs R and G
• (G R) (G R) Absence of color--black

20
Mixing Colored Pigments

• What primary paint colors (CMY) or combination of
  paint colors would you use to paint the boy
  below? He has pink (magenta) skin, blue jeans, a
  yellow sweater, a black baseball cap, red
  sneakers and aqua-colored socks. Indicate the
  primary colors of paint to be used on the diagram
  below.
• Skin ______________
• Jeans ______________
• Sweater ______________
• Cap ______________
• Sneakers ______________
• Socks ______________

magenta
cyan magenta
yellow
cyan, magenta yellow
magenta yellow
cyan
21
Why is the Sky Blue?

• The sky is blue because of scattering!
• light striking atoms and molecules in a gas (far
  apart from each other) gets re-emitted in many
  directions
• The tinier the particle, the more
  higher-frequency light it will re-emit
• sound vibrations smaller bells have higher
  pitch than larger bells
• N2 and O2 in atmosphere are like such tiny
  bells.
• So, as light moves through atmosphere, most goes
  in straight lines, but when it hits a molecule,
  the higher frequencies (violet, blue) especially
  get scattered in all directions.
• blue reaches you in any direction you look - you
  see blue light from everywhere overhead a blue
  sky !

22
Why is the Sky Blue?

• But violet is higher frequency than blue why
  dont we see a violet sky?
• We would if our eyes were equally sensitive to
  violet and blue, but the cones in our eyes sense
  blue much more, so we sense a blue sky.
• Other creatures with better violet vision like
  bumble bees, do see a violet sky!
• The lower frequencies pass more or less in a
  straight line
• red scatters a tenth as much as violet
• DEMO shine flashlight through water with a few
  drops of milk in it.
• View from the side, see bluish haze.
• If look straight through, see reddish-orange as
  this doesnt get scattered so much.
• Why are some skies bluer than others?
• Depends on whats in the atmospherehow much
  water, dust, etc
• these particles are larger than O2, N2, so
  scatter lower frequencies strongly as well,
  making sky less blue and more whitish.
• Drier atmospheres (in Greece and Italy) have
  deeper blue skies than more humid ones
• Polluted cities even grayish skies, because of
  exhaust gases absorbing rather than scattering

23
Why are Sunsets Red?

• Light thats not scattered is light that is
  transmitted i.e. reds-oranges especially.
• Previous DEMO front view of flashlight through
  milky-water.
• The thicker the atmosphere the light passes
  through, the more of the higher-frequencies that
  will be re-scattered away from the transmitted
  beam
• the transmitted beam is redder.
• This is what happens at sunsets (or sunrises)

Red-orange sunset, as almost all higher
frequencies scattered away.
As day progresses, more blue/violet scattered, as
sunlight travels greater distance through
atmosphere, so the sun looks increasingly orange.
24
Why are Clouds White?

• First, what are clouds?
• A mass of water droplets of various sizes
  suspended in the atmosphere.
• Different sizes of droplets scatter different
  frequencies
• smallest ones scatter high frequencies like
  blue/violet
• slightly bigger ones scatter medium frequencies
  like green
• bigger ones scatter reds
• net scattered is WHITE!
• If droplets and clouds get too large, get
  appreciable absorption, so they look darker
  (grayer).

25
Why is Water Greenish Blue?

• If look at surface of lake or ocean, it may
  appear deep blue but thats because it reflects
  the skys blue.
• However, if put white piece of paper under water,
  it is a pale greenish-blue.
• Why?
• Although water is transparent in the
  visible-light regime, it does absorb some red
  red light is reduced to ¼ of original brightness
  in 15 m of water.
• Since complementary color to red is cyan, this
  means water is cyan.
• Explains why many red crabs look black in deep
  water.
• Water also absorbs strongly in the infrared,
  which is why sunlight warms water

26
Questions?

• If molecules in the sky scattered low-frequency
  light more than high-frequency light, what color
  would the sky be? What color would sunsets be?
• The day-time sky would have low-frequencies,
  reddish-orange. The sunsets would be
  white-minus-these, blue!
• Why does smoke from a campfire look blue against
  trees near the ground but yellow against the sky?
• Particles in the smoke scatter mostly blue
  light, so against a dark background like the
  trees near the ground, we see the smoke as blue
  (analogous to sky, with dark beyond earth). we
  see the scattered light.
• But, against the bright sky, you see mostly the
  sky-minus-light-scattered from the smoke, white
  - blue yellow.
• This is also why the sun looks yellow in the
  day time!

27
The Atomic Color Code

• Each element has its own characteristic color
  when it emits light
• If the atoms are far apartyou can see the true
  color of the element
• Neon glows red
• Mercury glows bluish violet
• Helium glows pink
• If the atoms are close together the
  characteristic colors smudge to produce a
  continuous spectrum
• Light from glowing elements is analyzed using a
  spectroscope (an instrument used to separate
  light from a hot gas or other light source into
  its constituent frequencies)

28
The Atomic Color Code

• Looking at light from a glowing element through a
  spectroscope shows the colors are a made up of a
  variety of different light frequencies
• The spectrum of an element appears as a series of
  lines with each line corresponding to a distinct
  frequency of light
• Each element produces its own pattern of
  linesbecause each element has its own electron
  configuration that emits distinct frequencies
• Physicists use atomic spectra to find the atomic
  composition of common materials
• Helium was discovered using a spectroscope

The atomic spectrum for helium.