CHAPTER 18

PHOTON

A "particle" of light with energy hv, where h is Planck's constant and v is the frequency of the light.

CUVET

A cell with transparent walls used to hold samples for spectrophotometric measurements.

SPECTROPHOTOMETER

A device used to measure absorption of light. It includes a source of light, a wavelength selector (monochromator), and an electrical means of detecting light.

ABSORPTION SPECTRUM

A graph of absorbance or transmittance of light versus wavelength, frequency, or wavenumber.

ABSORBANCE

A, Defined as A = log(P0/P), where P0 is the radiant power of light (power per unit area) striking the sample on one side and P is the radiant power emerging from the other side. Also called optical density.

SPECTROPHOTOMETRY

Any method using light to measure chemical concentrations.

EXCITED STATED

Any state of an atom or a molecule having more than the minimum possible energy.

WAVELENGTH

Distance between consecutive crests of a wave.

HERTZ

Hz, SI unit of frequency, s^-1; also called reciprocal seconds.

MONOCHROMATIC LIGHT

Light with a very narrow range of wavelengths ("one color").

WAVENUMBER

Reciprocal of the wavelength, 1/wavelength

BEER'S LAW

Relates absorbance (A) of a sample to concentration (c), pathlength (b), and molar absorptivity (ε): A = εbc. This equation is more correctly called the Beer-Lambert-Bouguer law.

TRANSMITTANCE

T, Defined as T = P/P0, where P0 is radiant power of light striking the sample on one side and P is radiant power emerging from the other side of the sample.

FREQUENCY

The number of cycles per unit time for a repetitive event. For a light wave, it is the number of complete oscillations of the electromagnetic field per second.

GROUND STATE

The state of an atom or a molecule with the minimum possible energy.

ELECTROMAGNETIC SPECTRUM

The whole range of electromagnetic radiation, including visible light, radio waves, X-rays, etc.

STANDARD CURVE

graph showing the response of an analytical technique to known quantities of analyte. Also called calibration curve.

MOLAR ABSORPTIVITY

ε Constant of proportionality in Beer's law: A = εbc, where A is absorbance, b is pathlength, and c is the molarity of the absorbing species. Molar absorptivity tells how much light is absorbed at a particular wavelength by a particular substance. Also called extinction coefficient.