Visible Spectroscopy Visible Spectroscopy Electromagnetic Radiation: Light & Color Electromagnetic Radiation: Light & Color
Jan 17, 2016
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