Visible SpectroscopyVisible Spectroscopy

Electromagnetic Radiation: Light & ColorElectromagnetic Radiation: Light & Color

•is propagated at the speed of lightis propagated at the speed of light

•has properties of particles and waveshas properties of particles and waves

•the energy of a photon is proportional the energy of a photon is proportional

to its frequencyto its frequency

Electromagnetic RadiationElectromagnetic Radiation

Electromagnetic Radiationhttp://chemistry.beloit.edu/Stars/EMSpectrum/index.html

Cosmic raysCosmic rays

g Raysg Rays

X-raysX-rays

Ultraviolet lightUltraviolet light

Visible lightVisible light

Infrared radiationInfrared radiation

MicrowavesMicrowaves

Radio wavesRadio waves

Cosmic raysCosmic rays

g Raysg Rays

X-raysX-rays

Ultraviolet lightUltraviolet light

Visible lightVisible light

Infrared radiationInfrared radiation

MicrowavesMicrowaves

Radio wavesRadio waves

EnergyEnergy

Electromagnetic RadiationElectromagnetic Radiation

Longer Wavelength (Longer Wavelength ())Shorter Wavelength (Shorter Wavelength ())

Higher Frequency (Higher Frequency ()) Lower Frequency (Lower Frequency ())

Higher Energy (Higher Energy (EE)) Lower Energy (Lower Energy (EE))

UltravioletUltraviolet InfraredInfrared

Electromagnetic RadiationElectromagnetic Radiation

400 nm400 nm 750 nm750 nm

Visible Light

Longer Wavelength (Longer Wavelength ())Shorter Wavelength (Shorter Wavelength ())

Higher Frequency (Higher Frequency ()) Lower Frequency (Lower Frequency ())

Higher Energy (Higher Energy (EE)) Lower Energy (Lower Energy (EE))

Electromagnetic RadiationElectromagnetic Radiation

Visible Light & Color

• Sir Isaac Newton (1704) used a prism to show that sunlight was composed of light with all colors in the rainbow. He defined it as the spectrum.

The Visible Spectrum and Color

Black and White vs. Color

• Light of all frequencies is white light, eg. sunlight

• Black is the absence of light, not color. • Color is light of one or more wave lengths but not all.• Candlelight lacks high frequencies. It emits yellowish light.• Incandescent light emits light at all visible frequencies, but is

richer towards the low frequencies and hence enhances the reds.• Fluorescent light is richer in high frequencies and enhances blues.

The Retina & Photoreceptive Cells

Perception of Color

• Humans can distinguish hundreds of thousands of different colors

• Humans have 3 types of receptors– red (peak response at wavelength = 580nm)– green (peak response at wavelength = 545nm)– blue (peak response at wavelength = 440nm)

• Red, green, and blue are the primary colors.• Light containing equal intensities off all three appears white.• Complementary colors are magenta (green), yellow (blue), and

cyan (red)

http://chemconnections.llnl.gov/organic/Chem227/227assign-06.html#vision

Perception of Color• Light containing equal intensities of red, green,

and blue appears white.• Red paint reflects red and absorbs the other

colors.• Paint and dyes contain tiny solid particles of

some pigments, they usually reflect a wide rage of frequencies (mixture of colors) and absorb the rest.– Cyan pigments absorb red– Blue paint reflects blue, violet, green It

absorbs red, orange, yellow.– Yellow paint reflects, red, orange, yellow,

green. It absorbs blue, violet– When blue and yellow are mixed, they reflect

only green

Color & Atoms

• When atoms are excited to higher energy levels from a ground state they emit the energy that was absorbed.– Neon gas produces a brilliant red– Mercury vapors produces violet– Helium produces pink

• The light emitted from each different element produces different light intensities versus the light frequency.

Gaps between electron energy Gaps between electron energy levels correspond to wavelengths;levels correspond to wavelengths;between 200 and 800 nm (Ultraviolet-between 200 and 800 nm (Ultraviolet-Visible)Visible)

Transitions between energy statesTransitions between energy states

EE = = hh

Absorption and Emission of LightE

nerg

y

Absorption Emission

(Excitation from ground stateor from an excited state to a higher state)

(Dropping from an excited stateto ground state or lower state)

Spectrophotometer

Attenuation of Light

Transmittance vs. Absorbance

Transmittance scale is linearAbsorbance scale is exponential

The Spectrum

• The spectrometer measures the intensity of a reference beam (Po=Ir) and the intensity of a beam through a sample (P=Is).

• Absorbance is the log of the ratio

• Graph is absorbance vs. wavelength.

II

s

r

Beer’s Law

• Graph is absorbance vs. concentration.• Beer’s Law:

A = cl; A =abc (a) is the molar absorptivity, c is the sample concentration in moles per liter, and l (b) is the length of the light path in centimeters.

•

C

Beer’s Law A = abc

Path Length Dependence, b

ReadoutAbsorbance

0.82

Source

Detector

Beer’s Law A = abc

Path Length Dependence, b

ReadoutAbsorbance

0.62

Source

Detector

b

Sample

Beer’s Law A = abc

Concentration Dependence, c

ReadoutAbsorbance

0.82

Source

Detector

Beer’s Law A = abc

Concentration Dependence, c

ReadoutAbsorbance

0.62

Source

Detector

b

Sample

Beer’s Law A = abc

Concentration Dependence, c

ReadoutAbsorbance

0.42

Source

Detector

b

Sample

Beer’s Law A = abc

Wavelength Dependence, a

ReadoutAbsorbance

0.82

Source

Detector

Beer’s Law A = abc

Wavelength Dependence, a

ReadoutAbsorbance

0.30

Source

Detector

b

Beer’s Law A = abc

Wavelength Dependence, a

ReadoutAbsorbance

0.80

Source

Detector

b

What is the manganese concentration in a sample that has an absorbance of 0.658, a path length of 1.50cm and a molar absorptivity, 5.85 x 103 L/mol*cm.

A = b c

Prelab Calculation

What is the manganese concentration in a sample that has an absorbance of 0.658, a path length of 1.50cm and a molar absorptivity, 5.85 x 103 L/mol*cm.

A = b c

7.50 x 10-5M MnO4

Prelab Calculation