The quantum revolution The nature and characteristics of radiant energy

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Figure 4. The idealised drawing of a wave

uch of what is known about the structure of the electrons in an atom has been obtained by studying the interaction between matter and different forms of electromagnetic radiation. Electromagnetic radiation has some of the properties of both a particle and a wave.

Particles have a definite mass and they occupy space. Waves have no mass and yet they carry energy as they travel through space. In addition to their ability to carry energy, waves have four other characteristic properties: speed, frequency, wavelength, and amplitude. The frequency (v) is the number of waves (or cycles) per unit of time. The frequency of a wave is reported in units of cycles per second (s^-1) or hertz (Hz).

The idealised drawing of a wave in the figure below illustrates the definitions of amplitude and wavelength. The wavelength (l) is the smallest distance between repeating points on the wave. The amplitude of the wave is the distance between the highest (or lowest) point on the wave and the center of gravity of the wave.

Diffraction is a property of any wave motion and is one of the basic proofs of the wave nature of lightⁱ.

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Figure 5.

ight is a wave with both electric and magnetic components. It is therefore a form of electromagnetic radiation. Visible light contains the narrow band of frequenies and wavelengths in the portion of the electromagnetic spectrum that our eyes can detect, Figure 5.

The radiation emitted by solids or liquids always produces a continuous spectrum. The radiation emitted by glowing gases and vapours, unlike that emitted by solids and liquids, contains only definite wavelengths. Consequently, instead of a continuous band on the screen, we get a number of individual coloured lines separated by dark spaces. For instance, potassium vapour produces a spectrum consisting of three lines—two red and one violet. Such spectra are known as line spectra, Figure 6.

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Figure 6. A schematic view of the atomic spectrum of hydrogen in the visible and near ultraviolet region

he circumstance that the atoms of every element produce a quite definite spectrum characteristic only of this element is widely used for determining the qualitative and quantitative composition of substances and materials. This method of investigation is called spectral analysis.

Emission Spectrum of Hydrogen

When an electric current is passed through a glass tube that contains hydrogen gas at low pressure the tube gives off blue light. When this light is passed through a prism, four narrow bands of bright light are observed against a black background. These narrow bands have the characteristic wavelengths and colours shown in the figure 6.

F our more series of lines were discovered in the emission spectrum of hydrogen by searching the infrared spectrum at longer wave-lengths and the ultraviolet spectrum at shorter wavelengths. Each of these lines fits the same general equation, where n₁ and n₂ are integers and R_H is 1.09678 x 10^-2 nm^-1.