causes of color and their mechanism

1. CHAPTER -3
CAUSES OF COLOR AND THEIR MECHANISM
By Granch BerheBy Granch Berhe

2. What is color and light?
• Color is perceived when the wavelengths constituting white light
are absorbed, reflected, refracted, scattered, or diffracted by matter
on their way to our eyes.
• The interaction between light (photon) and matter due to change in
photons(v,f,λ, energy,θ ) can cause color.
• Electromagnetic radiation, or light, is a form of energy with dual
nature.
• Particle: interactions between electromagnetic radiation and
matter, such as absorption and emission of photon.
• Wave l properties example : Diffraction
(velocity, amplitude, frequency, wave length, wave number, phase
angle, polarization,etc.)

3. Fifteen causes of color and their mechanism
I. Vibrations and simple excitations
1) Incandescence
Hot objects, the sun, flames, filament lamps, carbon arcs, limelight,
pyrotechnics*
2) Gas excitations
Vapor lamps, neon signs, corona discharge, auroras, lightning*, lasers*
3) Vibrations and rotations
Blue water and ice, iodine, bromine, chlorine gas, blue gas flame
II. Transitions involving ligand field effects
4)Transition metal compounds
Turquoise, malachite, chrome green, rhodochrosite, smalt, copper patina,
fluorescence*, phosphorescence*, lasers*, phosphors*
5) Transition metal impurities Ruby, emerald, alexandrite, aquamarine, citrine,
red iron ore, jade*, glasses*, dyes*, fluorescence*,
phosphorescence*,lasers*

4. Fifteen causes of color and their mechanism
iii. Transition between molecular orbital
6)Organic compounds
Dyes*, biological colorations*, fluorescence*, phosphorescence*, lasers*
7) Charge transfer
Blue sapphire, magnetite, lapis lazuli, ultramarine, chromates, Painted Desert,
Prussian blue
iv. Transitions involving energy bands
8) Metals
Copper, silver, gold, iron, brass, ‘ruby’ glass
9) Pure semiconductors
Silicon, galena, cinnabar, vermillion, cadmium orange and yellow, diamond
10) Doped semiconductors
Blue and yellow diamonds, light-emitting diodes, lasers*, phosphors*
11) Color centers
Amethyst, smoky quartz, desert ‘amethyst’ glass, fluorescence*, phosphorescence*,
lasers*

5. Fifteen causes of color
v. Geometrical and physical optics
12) Dispersive refraction, polarization, etc.
Rainbows, halos, sun dogs, photo elastic stress analysis, ‘fire’ in
gemstones, prism spectrum
13) Scattering
Blue sky, red sunset, blue moon, moonstone, Raman scattering,
blue eyes, skin, butterflies, bird feathers*, other biological
colors*
14) Interference without diffraction
Oil slick on water, soap bubbles, coatings on camera lenses,
biological colors*
15) Diffraction
Aureole, glory, diffraction grating spectrum, opal, liquid crystals
biological colors*

6. Summary of Five main causes of color
Cause of
color
Geometrical and
Physical optics
Transitions
Involving
Energy Bands
Transition
between
Molecular orbital
Transitions
involving
Ligand field effects
Vibrations and simple
Excitations

7. 1) Vibrations and simple excitations
• Colors are seen when an object is heated to successively
higher temperatures. The light produced consists of photons
given off by electrons, atoms, and molecules when part of
their thermal vibration energy is emitted as radiation.

8. COLOR FROM GAS EXCITATION
 The Sun has has a surface temperature of approximately
5,780 K, giving it a white color which, because of atmospheric
scattering, appears yellow as seen from the surface of the
Earth.

9. COLOR FROM INCANDESCENCE
• Color is induced when an object is heated to sucessively higher tempratures.(B,
R,YW,BW)
• Incandescence is the release of electromagnetic radiation, usually visible radiation,
from a body due to its temperature. Black body radiation is the incandescence of a
theoretically perfectly black object

10. COLOR FROM GAS EXCITATION
• The incandescence Mechanism applies to the color of
any substance when heated.
• Specific chemical elements, present as a vapor or a gas
have their electrons excited into higher energy levels in
gas excitations. Light is emitted when the excess energy
is released as photons.

11. Sodium Vapor lamp

12. Lightning

13. COLOR FROM VIBRATIONS AND ROTATIONS
• The isolated water molecule is bent
and has three fundamental
vibrations,
• All of these vibrations result in
absorptions in the ultra-violet
region as shown.
• In liquid water or solid ice, the
hydrogen bonding between
adjacent molecules raises thee
energies of these vibrations
overtones) and leads to very weak
combination absorptions at the
long wavelength end of the visible
spectrum. As a result, pure water
and ice have a complementary very
pale blue color.

14. COLOR FROM VIBATIONS AND ROTATIONS

15. 2) Transitions involving ligand field effects
• Ligand-field-effect colors are seen in transition-metal compounds (turquoise,
chrome-oxide green) and impurities (ruby, emerald).
• Unpaired electrons are present in transition metal compounds, usually in d or f
orbital, as in salts of the d transition elements such as Cr,Fe,Co,Ni,Cu.
In some Metal complexes Light absorption can occur at lower energies in the
visible region of the spectrum. This leads to the ligand field colors of many
minerals and paints.
• A metal complex can absorb light by undergoing an electronic transition from
its lowest (ground) energy state to a higher (excited) energy state. That is due
to the effect of ligands on d and f orbital splitting energy.(Crystal Field Theory)
• In general the crystal field splitting energy corresponds to wave lengths of light
in the visible region of spectrum.The color of metal complexes can be
attributed to electronic transitions of lower and higher energy d and f orbitals.

16. How metal complexes are colored?

17. Effect of Ligands

18. COLOR OF MINERALS AND GEMSTONES
• This same explanation applies to Mechanism 5, where the
transition metal is only an impurity, typically present at about the
one percent level in an otherwise colorless substance.This
provides some of the colors in minerals, gemstones, ceramics,
glass, glazes, and enamels.
• Idiochromatic (Self-colored)
• Allochromatic (other colored)

19. 3)Transition between molecular orbital
• Molecular orbital explain the colors of organic compounds (indigo,
chlorophyll) and charge-transfer compounds (blue sapphire, lapis
lazuli).
• Here color derives from organic compounds involving electrons
belonging to several atoms within a molecule (Resonance).
• Molecules that have many resonance structures tend to absorb
and emit photons.

20. Conjugated organic compounds
• A ‘conjugated’ organic compound is one that contains alternating
single and double bonds in chains and/or rings of (mostly) carbon
atoms.
• Such an arrangement contains ‘pi-bonded’ electrons located in
molecular orbitals which belong to the whole chain and/or ring
system.
• If such systems are large enough, the excited states of these
electrons occur at energies similar to those of the unpaired
electrons in transition metal compounds and can therefore absorb
and emit photons.

21. Molecular orbital
• Light absorbed – electron excited to higher molecular orbital
and can emit photon in certain energy.
• Transitions occur from HOMO to LUMO
- Highest Occupied Molecular Orbital
- Lowest Unoccupied Molecular Orbital
∆E=hν

22. Molecular orbital colors
Just as with ligand field energy levels, some of the absorbed energy may be re-
emitted in the form of fluorescence.
• If the conjugated aspect of the framework of an organic colorant molecule is
destroyed, then the color will be lost.
• Chemical energy can also excite such a system and lead to fluorescence (or to a
much slower phosphorescence) as in the bioluminescence of fireflies and angler
fishes and in the chemoluminescent ‘lightsticks

23. COLOR FROM MOLECULAR ORBITALS
Color from charge transfer
• A crystal of sapphire Al203containing a few hundredths of one
percent of titanium is colorless.
• If,instead, it contains a similar amount of iron, a pale yellow color
is seen. However, when both impurities are present together they
produce a magnificent deep blue color,

24. 4)Transitions involving energy bands
• Transitions between Energy bands are
involved in the colors of metals and alloys
(gold, brass), of semiconductors (cadmium
yellow, vermilion), doped semiconductors
(blue and yellow diamond), and color centers
(amethyst, topaz).
•

25. Metallic and semi conductor colors from band
theory
• When light falls onto a metal, the electrons below the Fermi surface
can become excited into higher energy levels in the empty part of
the band by absorbing the energy from the light, producing
electron-hole pairs
• In some band theory materials it is possible for a gap, the ‘band
gap,’ to occur within the band, with important consequences for
color.
• A smaller or narrow band gap permits absorption of the medium
to large energy wavelengths of the visible spectrum and produces
color.
• Large band gap is almost insulator and colorless

26. 5)Geometrical and physical optics
• Geometrical and physical optics are involved in the
colors derived from dispersive refraction (rainbow,
green flash), scattering (blue sky, blue eyes, red
sunset), interference (soap bubbles, iridescent
beetles), and diffraction (the corona aureole, opal).

27. COLOR FROM DISPERSION
Dispersion is the phenomenon that the phase
velocity of a wave depends on its frequency.
Dispersion is also known as dispersive refraction.
• In a prism(color less glass cut), dispersion causes
the spatial separation of a white light into
spectral components of different wavelengths
(color discrimination)

28. COLOR FROM DISPERSION
Prism: based on refraction of light and fact that different
wavelengths have different values of refractive index in a
medium.
• Thus, blue light, with a higher refractive index, will be bent more
strongly than red light, resulting in the well-known rainbow
pattern

29. COLOR FROM SCATTERING
Rayleigh Scattering:
 The probability that a single
photon of sunlight will be scattered
from its original direction by an air
molecule is inversely proportional
to the fourth power of the
wavelength.
I = 1 / (wavelength)4

30. Raleigh scattering

31. COLOR OF SKY AND SUN SET

32. BLUE SKY
Since blue light is scattered much more frequently than
red light, when you look at the sky (excluding the sun)
you are more likely to see a blue photon of scattered
sunlight rather than a red one.
The color of our sky is caused by the interplay of blue-
light-scattering by air molecules, and white-light-
scattering by water drops and dust
Blue wavelengths are generally scattered down toward
the earth. This makes the sky appear blue wherever it
is daytime (and the sun is high in the sky). At sunset,
however, the opposite occurs.

33. RED SUNSET
• When the air is clear the sunset will appear
yellow, because the light from the sun has
passed a long distance through air and some
of the blue light has been scattered away.
• If the air is polluted with small particles,
natural or otherwise, the sunset will be more
red.

34. Chlorophyll
• Molecular orbital dye colors occur widely in the plant and animal kingdoms
as well as in the products of the modern synthetic dye and pigment
industry.