CHAPTER -3

CAUSES OF COLOR AND THEIR MECHANISM

By Granch Berhe

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.)

Fifteen causes of color and their mechanism

I. Vibrations and simple excitations

1) IncandescenceHot objects, the sun, flames, filament lamps, carbon arcs, limelight,

pyrotechnics*2) Gas excitationsVapor lamps, neon signs, corona discharge, auroras, lightning*, lasers*3) Vibrations and rotationsBlue water and ice, iodine, bromine, chlorine gas, blue gas flame

II. Transitions involving ligand field effects

4)Transition metal compoundsTurquoise, 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*

Fifteen causes of color and their mechanismiii. Transition between molecular orbital

6)Organic compoundsDyes*, biological colorations*, fluorescence*, phosphorescence*, lasers*7) Charge transferBlue sapphire, magnetite, lapis lazuli, ultramarine, chromates, Painted Desert,

Prussian blue

iv. Transitions involving energy bands

8) MetalsCopper, silver, gold, iron, brass, ‘ruby’ glass9) Pure semiconductorsSilicon, galena, cinnabar, vermillion, cadmium orange and yellow, diamond10) Doped semiconductorsBlue and yellow diamonds, light-emitting diodes, lasers*, phosphors*11) Color centersAmethyst, smoky quartz, desert ‘amethyst’ glass, fluorescence*, phosphorescence*,

lasers*

Fifteen causes of color

v. Geometrical and physical optics12) Dispersive refraction, polarization, etc.Rainbows, halos, sun dogs, photo elastic stress analysis, ‘fire’ in

gemstones, prism spectrum13) ScatteringBlue sky, red sunset, blue moon, moonstone, Raman scattering,

blue eyes, skin, butterflies, bird feathers*, other biological colors*

14) Interference without diffractionOil slick on water, soap bubbles, coatings on camera lenses,

biological colors*15) DiffractionAureole, glory, diffraction grating spectrum, opal, liquid crystals

biological colors*

Summary of Five main causes of color

Cause of color

Geometrical and Physical optics

Transitions Involving

Energy BandsTransition between Molecular orbital

Transitions involvingLigand field effects

Vibrations and simpleExcitations

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.

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.

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

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.

Sodium Vapor lamp

Lightning

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.

COLOR FROM VIBATIONS AND ROTATIONS

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.

How metal complexes are colored?

Effect of Ligands

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)

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.

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.

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=h

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

COLOR FROM MOLECULAR ORBITALSColor from charge transfer

• A crystal of sapphire Al203 containing 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,

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).

•

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

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).

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)

COLOR FROM DISPERSIONPrism: 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

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

Raleigh scattering

COLOR OF SKY AND SUN SET

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.

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.

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.