D2.1 Electromagnetic Radiation
The periodic table summarizes much information about chemical elements. That information can be better understood by assuming that physical and chemical properties of elements depend on the underlying structures of their atoms. An important way to learn about the structures of atoms is to study how energy, in the form of electromagnetic radiation, interacts with matter.
Electromagnetic radiation consists of oscillating, perpendicular electric and magnetic fields that travel through space and can transfer energy. The oscillating fields (waves) are characterized by wavelength (λ, measured in meters, m) and frequency (ν, measured in hertz, Hz or s−1). In a vacuum, electromagnetic radiation travels at the speed of light (c):
Electromagnetic radiation occurs in small, indivisible quantities of energy called photons. The energy of a photon, Ephoton, can be determined from either its frequency or its wavelength:
In this equation h represents Planck’s constant; h = 6.626 × 10−34 J s.
Your calculation in the above Exercise showed that the energy of a single photon is quite small. Most interactions of electromagnetic radiation and matter involve lots of photons and lots of atoms. The total energy transferred is proportional to the number of photons, N. If all photons have the same frequency:
Notice that electromagnetic radiation has been described as involving wave motion and also as a number of particles (photons). Originally, scientists thought that electromagnetic radiation could be described entirely by a wave model, but that model was unable to predict all experimental observations. Consequently, both wave and particle models need to be combined for full understanding of electromagnetic radiation.
The figure below shows the enormous range of all types of electromagnetic radiation: Frequencies of 105 Hz to 1020 Hz, that is, wavelengths of 103 m (km) to 10−12 m (pm) have been observed. What we can see, visible light, is only a tiny portion (380-740 nm) of that range.
Different parts of the electromagnetic spectrum typically use different units: Low-energy photons, such as microwaves and radio waves, are specified in frequencies (MHz or GHz); mid-energy photons, such as infrared and visible light, are specified in wavelengths (μm, nm, pm, or Å); high-energy photons, such as x-rays and gamma-rays, are specified in energies (keV or MeV; 1 eV = 96.5 kJ/mol). As the equation above shows, these units can readily be converted to one another.
Our eyes detect visible-range photons, allowing us to see the world around us. But scientific instruments allow us to “see” lots more by detecting photons over a much wider range of energies. For example, studies of atomic spectra via experiments involving interaction of gaseous matter with visible, ultraviolet, and infrared photons, led to a better understanding of the structure of atoms.
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