GEK1532 Causes of Colour

8/6/2019 GEK1532 Causes of Colour

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GEK1532

Physical Causes of Color

Thorsten Wohland

Dep. Of ChemistryS8-03-06

Tel.: 6516 1248

E-mail: [email protected]

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?



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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



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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


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



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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)
http://www.michaelbach.de/ot/col_benham/index.htmlhttp://www.michaelbach.de/ot/col_benham/index.html


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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

GEK1532 Causes of Colour

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