Spectrophotometry and Light Absorption in Chemistry

Absorbance

A unit of measurement proportional to concentration of light-absorbing molecules in the sample.

Absorption Spectrum

Graph showing how absorbance or ε varies with wavelength for frequency for wavenumber.

Beer's Law

Absorbance = molar absorptivity × path length × concentration (A = Ebc).

Cell Length

The distance light travels through a sample in a cuvette within a spectrophotometer.

Chemiluminescence

Chemical reactions that release energy in the form of light.

cuvet

A cell with flat silica-fused faces containing the sample being tested.

Diffraction

Bending of light rays by grating.

Electromagnetic Spectrum

A scale ranging from gamma rays to radio waves used to identify substances by observing how they absorb, emit, or scatter different wavelengths of energy.

Excited State

When a molecule absorbs a photon, its energy increases.

Fluorescence

Emission of a photon from a transition between states with the same spin (S —> S0)

Frequency

Number of oscillations a wave makes each second.

Grating

Optical element with closely spaced lines.

Ground State

Lowest energy state of a molecule.

Hertz

One oscillation per second.

Luminescence

Any combination of electromagnetic radiation, including fluorescence, phosphorescence, and other processes.

Molar Absorptivity

Tells how much light is absorbed at a particular wavelength.

Monochromatic Light

Measures the interaction of a sample with a small wavelength of light.

Phosphorescence

Emission of a photon from a transition between states with different spins (T --> S).

Photochemistry

Chemical reaction initiated by absorption of light.

Photon

Particles of light.

Self-Absorption

Absorption of excitation energy by neighboring molecules of the same substance.

Spectrophotometer

Measures the amount of light absorbed by a sample to determine its property.

Spectrophotometric Titration

Changes in absorption or emission of electromagnetic radiation are monitored to detect the endpoint.

Spectrophotometry

The use of electromagnetic radiation to measure chemical concentrations.

Speed of Light

The product of frequency and wavelength (C = 2.998 × 10^5 m/s).

Standard Curve

Also known as calibration curve, absorbance (y) vs. concentration (x).

Transmittance

Fraction of incident light that passes through a sample (T = P/P₁).

Wavelength

Crest-to-crest distance between waves.

Wave Number

Where v = 1/λ.