Electromagnetic Radiation: A Comprehensive Guide
Electromagnetic Radiation
What is Electromagnetic Radiation?
Electromagnetic radiation is a type of energy transmitted through space at enormous speeds, containing electric and magnetic components.
Current Theory
The current theory used to explain the behavior of electromagnetic radiation is the dual theory (wave-particle duality).
Wavelength
The wavelength of electromagnetic radiation is the distance between two successive maximum or minimum points in the wave, measured in meters or centimeters.
Frequency
The frequency of electromagnetic radiation is the number of waves generated in one second, measured in Hertz (Hz).
Theories of Electromagnetic Radiation
- Maxwell’s Wave Theory
- Wave-Particle Duality Theory
- Planck’s Quantum Theory
Classical Wave Model
The classical wave model of electromagnetic radiation uses wavelength, frequency, and speed to describe its behavior.
Wave Phenomena
- Diffraction
- Polarization
- Refraction
Quantum Phenomena
- Absorption
- Emission
Photons
The flow of energy is seen as discrete particles or packets of energy called photons or quanta.
Frequency
Frequency is the number of oscillations passing a point in one second. It is the reciprocal of the wavelength (in cm).
Wavelength
Wavelength is the linear distance between two successive points of a wave peak or valley.
Instrumentation
Absorption instruments include colorimeters for molecular VIS, spectrometers, and UV-VIS spectrophotometers. They differ in the accuracy with which the wavelength is selected.
Radiation Sources
- Hydrogen or deuterium lamps (UV)
- Tungsten lamps (visible)
Continuous Sources
Produce wide ranges of wavelengths (e.g., 400-700 nm).
Line Sources
Produce a single spectral line (e.g., 458 nm for atomic absorption).
Wavelength Selectors
Isolate or separate wavelengths produced by the radiation source. Examples include absorption filters, interference filters, and monochromators (optical arrangements). Grating monochromators are preferred (e.g., 2000 lines/mm).
Sample Cell
Contains the sample in dilute solution (analyte + reagent + solvent). Pure solvent (without analyte) is used as a reference. Quartz glass or plastic is used for UV-VIS.
Detector
Transforms the light signal into an electrical current. Phototubes, for example, function by having radiation strike the cathode, causing the emission of electrons from a photoemissive substance. These electrons travel to the anode, producing an electric current.
Signal Processor
Modifies the signal transduced by the detector for submission to a readable device.
Beer’s Law
A = abc
Electromagnetic Spectrum
The electromagnetic spectrum is a presentation of all known radiation by their wave and quantum parameters.
- Gamma rays (nuclear)
- X-rays (inner electrons)
- UV radiation (outer electrons)
- Visible radiation (outer electrons)
- Infrared radiation (molecular vibrations and rotations)
- Microwaves (molecular rotation)
- Radio frequency (induced magnetic spin)
Absorption
The transfer of energy to atoms and molecules in the sample, causing transitions in energy states.
Dispersion
Radiation projected at all angles from the original direction, caused by suspended particles in the sample.
Spectrophotometry
Measures absorption.
Turbidimetry
Measures dispersion.
Transmission
The passage of radiation without permanent energy transfer, giving rise to refraction and polarization.
Refractometry and Polarimetry
Measure transmission.
Optical Methods: Absorption Spectra
Graphical representations of the reduced radiant power (absorbance) as a function of wavelength. Two types of spectra exist:
- Line spectra: Absorption at specific frequencies due to a small number of possible energy states (atomic absorption).
- Band spectra: Due to more complex processes and a larger number of energy states (molecular absorption).
Bouguer-Lambert Law
Relates absorption to the path length (b) through the absorbent medium: -dP/db = log(Po/P) = K2b
Beer’s Law
Relates absorption to the concentration (c) of the absorbent: -dP/dC = k3P …. log(Po/P) = K4c. This law applies when the incident radiation is monochromatic (single wavelength) and the absorbing species (atoms, ions, molecules) act independently. Absorbance is limited to a uniform cross-section.
Least-Squares Method
A mathematical method used to fit a line to a set of points or values when deviations exist. It is based on the straight-line equation: Y = mX + b.