Post on 07-Apr-2018
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GEK1532
Physical Causes of Color
Thorsten Wohland
Dep. Of ChemistryS8-03-06
Tel.: 6516 1248
E-mail: chmwt@nus.edu.sg
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
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Interaction of Light with matter:
Absorption
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A note on absorption: concentration
dependence
Transm
iss
ion
Transm
iss
ion
Ab
sorp
tion
-
-
No absorption
of red
Absorption of blue is
larger than absorption
of green
White Light
Inte
ns
ity
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Concentration dependence of color:
dichroic color
L.C. Thomas, Fig 1.4
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Reflections
Specular reflection
(mirror like)
Incidence angle is equal to
angle of reflection: =
Diffuse reflection
Does carry not much color
information More color information
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Why is the sky blue?
from HyperPhysics by Rod Nave
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Where does the rainbow come
from?
from HyperPhysics by Rod Nave
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Primary rainbow
SUN
Observer
This drop reflects
red light to your eye
This lower drop
reflects blue light to
your eye
from HyperPhysics by Rod Nave
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from HyperPhysics by Rod Nave
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The color of water
The color of water is due to some absorption in the red part of the
spectrum. This absorption is due to some vibrational transitions of the
molecule. This is probably the only case of color by vibrations one willusually encounter in nature.
http://webexhibits.org/causesofcolor/index.html
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Examples of blue water color
http://webexhibits.org/causesofcolor/index.html
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Color of the bottom (e.g. sea floor): If the water body is shallow and the sea
floor can be seen, its absorption and reflection of light will contribute to the
color seen.
Water is slightly blue. On many beaches with
white sand the blue color of the water can be
seen.
Particles in the water can give rise to
different colors. E.g. sediments in rivers,
cyanobacetria and algae in the sea.
Reflected color
from the sky
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Interactions of light with matter
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Total internal reflection:
Remember refraction
n1 n2
n1 < n2
n1 n2
n1 > n2
Snells law: The difference between and is the bigger, the bigger thedifference is between n1 and n2.
(n1sin = n2sin )
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Total internal reflection
n1 n2
n1 < n2
n1 n2
n1 > n2
Total internal reflection (TIR) can happen only when light propagates in a
denseer medium and comes to a interface with a less dense medium:
Example: TIR happens from glass to air or water to air, but not from air to
water or air to glass.
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Interactions of light with matter
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Interference describes the superposition of two or more em waves resulting
in an amplification (constructive interference) or an attenuation
(destructive interference) of the amplitudes of the em field and thus in
intensities.
Interference
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Interference
Example:
Interference of water waves
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Interference
a a
b
If light impinges on a thin film or glass slide, some of the light is reflected fromthe surface, some of the light is transmitted into the film and then reflected from
the back surface. Since the light in both cases has traveled different distances
(see the pieces labeled a and b), the light can interfere.
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Interference
a a
b
Imagine now a film on
which white light falls
onto at different angles.
The light will take a
different path and thedifference between the
light rays is as well
different.
a a
b
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Interference
a a
b
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a a
b
Interference
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Example: Soap Film
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
Since we see the soap film under different angles, at different
positions different parts of the light will be absorbed or reinforced and
we see colored stripes.
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Diffraction
Interference of water waveslight
screen
Interfe
rence
pattern
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Single Slit
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
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Summary
Physical causes of color: scattering,
refraction
Dichroic color (due to absorption)
Total Internal Reflection
Interference
Diffraction
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Colors in Biology
http://webexhibits.org/causesofcolor/1B.html
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Colors in Biology
Sometimes several of these mechanisms can work together to yield color.
Colors in nature can come from three different sources:
Structural colors: scattering, interference, refraction, diffraction,
reflections
Pigmentary colors: Chemical molecules absorb some light. The reflected
light exhibits then the complementary color.
Colors from luminescence
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Vegetable colors
Most plant colors come from colored substances, i.e. the colors areproduced by absorption. Structural colors are rare in plants.
Chlorophyll
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Carotenoids
Flavonoids
Flavonoids contain 15
carbon atoms and
have two phenol rings
Examples: Flavonol Flavone
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Autumn color changesPigment ClassCompound
TypeColors
Porphyrin chlorophyll green
Carotenoid carotene andlycopene
xanthophyll
yellow,orange,red
yellow
Flavonoid flavoneflavonolanthocyanin
yellowyellowred, blue,
purple,magenta
Decreasing light levels in autumn cause a slow down in chlorophyll
production.
The green color fades and depending of how much carotenoids or
flavonoids the leave contains it can look yellow, orange or red.
http://webexhibits.org/causesofcolor/1B.html
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Pigmentary colors in humans and
animals
Skin color: Melanin: phaeomelanin (red to yellow) and eumelanin (dark brown
to black)
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Changing color
Squid: Cells concentratingmelanin in small centers can
expand and distribute melanin
thus changing the color of the
squid
Lizzard: Expansion of cells thatcontain melanin can change the
color of a lizzard.
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Changing color
Grasshopper: Changes of the
location of colorants in the
skin can change color of the
grasshopper
Hercules Beetle: It changesdepending on humidity.
dry: yellow; humid: dark
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Scattering and Pigmentation
Rayleigh scattering,
stronger for blue light
(note: sometimesRayleigh scattering is
refered to as Tyndall
scattering)Retina
Iris
Absorption of red
and green light
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Yellow pigment (blueis absorbed)
Scattering Layer
(RayleighScattering)
Scattering and Pigmentation
Pictures from Andrew ParkersSeven Deadly Colors (Free Press)
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Whiteness or Silveriness
Snow, foam, chalk, paper all are examples of
materials that scatter and reflect light in some
way at many surfaces. The resulting color inall cases is white.
This white though can vanish when the air
spaces are filled with some liquid (see oily
paper, or wet chalk).
random structures -> matt appearance
Regular structures -> shiny appearance
(sometimes silverish)
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Interference colors
As in bubbles interference can as well work in feathers and wings.Insect wings shimmer in many colors due to interference. A special
example here is the peacock:
http://webexhibits.org/causesofcolor/15C.html
One can see very fine branches on the
feathers. These are responsible for the multiple
reflections and the interference effects of the
peacocks feathers.
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Is it really interference?
Seen under different
angles the color of thepeacock feather changes:
a characteristic of
interference colors
Albino peacocks do not possessany melanin thus most white light
is reflected instead of absorbed.
The white light is much stronger
than the interference color and the
peacock seems to be white.
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Violet as an interference color
Ridges are 100 nm in size
White bar is 1/10 of a millimeter
Structure in the wings responsible for the color
Pictures from Andrew Parkers SevenDeadly Colors (Free Press)
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Examples of bioluminescence
UV light can be absorbed will then be given off at a longer wavelength.
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Snake, Benhams disk
http://www.michaelbach.de/ot/col_benham/index.html
Picture from Andrew Parkers SevenDeadly Colors (Free Press)
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Summary
Colors in Biology
structural colors: interference, diffraction,and scattering
pigmentary colors: absoprtion Many biological are a combination of both
Interference and diffraction based colors
can be distinguished since they willchange their appearance depending onthe angle of viewing