bei48482_FM

128 Chapter Four

gratings. Figure 4.8 shows the emission line spectraof several elements. Every element

displays a unique line spectrum when a sample of it in the vapor phase is excited. Spec-

troscopy is therefore a useful tool for analyzing the composition of an unknown substance.

When white light is passed through a gas, the gas is found to absorb light of cer-

tain of the wavelengths present in its emission spectrum. The resulting absorption line

spectrum consists of a bright background crossed by dark lines that correspond to the

missing wavelengths (Fig. 4.9); emission spectra consist of bright lines on a dark back-

ground. The spectrum of sunlight has dark lines in it because the luminous part of the

Figure 4.8Some of the principal lines in the emission spectra of hydrogen, helium, and mercury.

700 nm

Red

600 nm 500 nm 400 nm

Orange Yellow Green Blue Violet

Mercury

Helium

Hydrogen

Figure 4.9The dark lines in the absorption spectrum of an element correspond to bright lines in its

emission spectrum.

Absorption spectrum

of sodium vapor

Emission spectrum

of sodium vapor

Gas atoms excited by electric currents in these tubes radiate light

of wavelengths characteristic of the gas used.

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