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Chemistry LibreTexts

7.2: Electromagnetic Radiation

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  • Page ID
    98625
  • Skills to Develop

    • To learn about the characteristics of electromagnetic waves. Light, X-Rays, infrared and microwaves are among the types of electromagnetic waves.

    Scientists discovered much of what we know about the structure of the atom by observing the interaction of atoms with various forms of radiant, or transmitted, energy, such as the energy associated with the visible light we detect with our eyes, the infrared radiation we feel as heat, the ultraviolet light that causes sunburn, and the x-rays that produce images of our teeth or bones. All these forms of radiant energy should be familiar to you. We begin our discussion of the development of our current atomic model by describing the properties of waves and the various forms of electromagnetic radiation.

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    Figure \(\PageIndex{1}\): A Wave in Water. When a drop of water falls onto a smooth water surface, it generates a set of waves that travel outward in a circular direction.

    Properties of Waves

    A wave is a periodic oscillation that transmits energy through space. Anyone who has visited a beach or dropped a stone into a puddle has observed waves traveling through water (Figure \(\PageIndex{1}\)). These waves are produced when wind, a stone, or some other disturbance, such as a passing boat, transfers energy to the water, causing the surface to oscillate up and down as the energy travels outward from its point of origin. As a wave passes a particular point on the surface of the water, anything floating there moves up and down.

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    Figure \(\PageIndex{2}\): Important Properties of Waves (a) Wavelength (λ in meters), frequency (ν, in Hz), and amplitude are indicated on this drawing of a wave. (b) The wave with the shortest wavelength has the greatest number of wavelengths per unit time (i.e., the highest frequency). If two waves have the same frequency and speed, the one with the greater amplitude has the higher energy.

    Waves have characteristic properties (Figure \(\PageIndex{2}\)). As you may have noticed in Figure \(\PageIndex{1}\), waves are periodic, that is, they repeat regularly in both space and time. The distance between two corresponding points in a wave—between the midpoints of two peaks, for example, or two troughs—is the wavelength (\(λ\), lowercase Greek lambda). Wavelengths are described by a unit of distance, typically meters. The frequency (\(\nu\), lowercase Greek nu) of a wave is the number of oscillations that pass a particular point in a given period of time. The usual units are oscillations per second (1/s = s−1), which in the SI system is called the hertz (Hz). It is named after German physicist Heinrich Hertz (1857–1894), a pioneer in the field of electromagnetic radiation.

    The amplitude, or vertical height, of a wave is defined as half the peak-to-trough height; as the amplitude of a wave with a given frequency increases, so does its energy. As you can see in Figure \(\PageIndex{2}\), two waves can have the same amplitude but different wavelengths and vice versa. The distance traveled by a wave per unit time is its speed (\(v\)), which is typically measured in meters per second (m/s). The speed of a wave is equal to the product of its wavelength and frequency:

    \[\begin{align} (\text{wavelength})(\text{frequency}) &= \text{speed} \nonumber \\[5pt] \lambda \nu &=v \label{6.1.1a} \\[5pt] \left ( \dfrac{meters}{\cancel{wave}} \right )\left ( \dfrac{\cancel{\text{wave}}}{\text{second}} \right ) &=\dfrac{\text{meters}}{\text{second}} \label{6.1.1b} \end{align}\]

    Be careful not to confuse the symbols for the speed, \(v\), with the frequency, \(\nu\).

    Different types of waves may have vastly different possible speeds and frequencies. Water waves are slow compared to sound waves, which can travel through solids, liquids, and gases. Whereas water waves may travel a few meters per second, the speed of sound in dry air at 20°C is 343.5 m/s. Ultrasonic waves, which travel at an even higher speed (>1500 m/s) and have a greater frequency, are used in such diverse applications as locating underwater objects and the medical imaging of internal organs.

    Electromagnetic Radiation

    Water waves transmit energy through space by the periodic oscillation of matter (the water). In contrast, energy that is transmitted, or radiated, through space in the form of periodic oscillations of electric and magnetic fields is known as electromagnetic radiation. (Figure \(\PageIndex{3}\)). Some forms of electromagnetic radiation are shown in Figure \(\PageIndex{4}\). In a vacuum, all forms of electromagnetic radiation—whether microwaves, visible light, or gamma rays—travel at the speed of light (c), which turns out to be a fundamental physical constant with a value of 2.99792458 × 108 m/s (about 3.00 ×108 m/s or 1.86 × 105 mi/s). This is about a million times faster than the speed of sound.

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    Figure \(\PageIndex{3}\): The Nature of Electromagnetic Radiation. All forms of electromagnetic radiation consist of perpendicular oscillating electric and magnetic fields.

    Because the various kinds of electromagnetic radiation all have the same speed (c), they differ in only wavelength and frequency. As shown in Figure \(\PageIndex{4}\) and Table \(\PageIndex{1}\), the wavelengths of familiar electromagnetic radiation range from 101 m for radio waves to 10−12 m for gamma rays, which are emitted by nuclear reactions. By replacing \(v\) with \(c\) in Equation \(\ref{6.1.1a}\), we can show that the frequency of electromagnetic radiation is inversely proportional to its wavelength:

    \[ \begin{align} c&=\lambda \nu \\[5pt] \nu &=\dfrac{c}{\lambda } \label{6.1.2} \end{align} \]

    For example, the frequency of radio waves is about 108 Hz, whereas the frequency of gamma rays is about 1020 Hz. Visible light, which is electromagnetic radiation that can be detected by the human eye, has wavelengths between about 7 × 10−7 m (700 nm, or 4.3 × 1014 Hz) and 4 × 10−7 m (400 nm, or 7.5 × 1014 Hz). Note that when frequency increases, wavelength decreases; c being a constant stays the same. Similarly, when frequency decreases, the wavelength increases.

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    Figure \(\PageIndex{4}\): The Electromagnetic Spectrum. (a) This diagram shows the wavelength and frequency ranges of electromagnetic radiation. The visible portion of the electromagnetic spectrum is the narrow region with wavelengths between about 400 and 700 nm. (b) When white light is passed through a prism, it is split into light of different wavelengths, whose colors correspond to the visible spectrum.

    Within the visible range our eyes perceive radiation of different wavelengths (or frequencies) as light of different colors, ranging from red to violet in order of decreasing wavelength. The components of white light—a mixture of all the frequencies of visible light—can be separated by a prism, as shown in part (b) in Figure \(\PageIndex{4}\). A similar phenomenon creates a rainbow, where water droplets suspended in the air act as tiny prisms.

    Table \(\PageIndex{1}\): Common Wavelength Units for Electromagnetic Radiation
    Unit Symbol Wavelength (m) Type of Radiation
    picometer pm 10−12 gamma ray
    angstrom Å 10−10 x-ray
    nanometer nm 10−9 UV, visible
    micrometer μm 10−6 infrared
    millimeter mm 10−3 infrared
    centimeter cm 10−2 microwave
    meter m 100 radio

    As you will soon see, the energy of electromagnetic radiation is directly proportional to its frequency and inversely proportional to its wavelength:

    \[ \begin{align} E\; &\propto\; \nu \label{6.1.3} \\[5pt] & \propto\; \dfrac{1}{\lambda } \label{6.1.4} \end{align}\]

    Whereas visible light is essentially harmless to our skin, ultraviolet light, with wavelengths of ≤ 400 nm, has enough energy to cause severe damage to our skin in the form of sunburn. Because the ozone layer of the atmosphere absorbs sunlight with wavelengths less than 350 nm, it protects us from the damaging effects of highly energetic ultraviolet radiation.

    The energy of electromagnetic radiation increases with increasing frequency and decreasing wavelength.

    Example \(\PageIndex{1}\): Wavelength of Radiowaves

    Your favorite FM radio station, WXYZ, broadcasts at a frequency of 101.1 MHz. What is the wavelength of this radiation?

    Given: frequency

    Asked for: wavelength

    Strategy:

    Substitute the value for the speed of light in meters per second into Equation \(\ref{6.1.2}\) to calculate the wavelength in meters.

    Solution:

    From Equation \(\ref{6.1.2}\), we know that the product of the wavelength and the frequency is the speed of the wave, which for electromagnetic radiation is 2.998 × 108 m/s:

    \[ \begin{align*}  λν &= c \\[5pt] &= 2.998 \times 10^8 m/s \end{align*} \]

    Thus the wavelength \(λ\) is given by

    \[ \begin{align*} \lambda &=\dfrac{c}{\nu } \\[5pt] &=\left ( \dfrac{2.988\times 10^{8}\; m/\cancel{s}}{101.1\; \cancel{MHz}} \right )\left ( \dfrac{1\; \cancel{MHz}}{10^{6}\; \cancel{s^{-1}}} \right ) \\[5pt] &=2.965\; m \end{align*} \]

    Exercise \(\PageIndex{1}\)

    As the police officer was writing up your speeding ticket, she mentioned that she was using a state-of-the-art radar gun operating at 35.5 GHz. What is the wavelength of the radiation emitted by the radar gun?

    Answer

    8.45 mm

    Summary

    Understanding the electronic structure of atoms requires an understanding of the properties of waves and electromagnetic radiation. A wave is a periodic oscillation by which energy is transmitted through space. All waves are periodic, repeating regularly in both space and time. Waves are characterized by several interrelated properties: wavelength (\(λ\)), the distance between successive waves; frequency (\(\nu\)), the number of waves that pass a fixed point per unit time; speed (\(v\)), the rate at which the wave propagates through space; and amplitude, the magnitude of the oscillation about the mean position. The speed of a wave is equal to the product of its wavelength and frequency. Electromagnetic radiation consists of two perpendicular waves, one electric and one magnetic, propagating at the speed of light (\(c\)). Electromagnetic radiation is radiant energy that includes radio waves, microwaves, visible light, x-rays, and gamma rays, which differ in their frequencies and wavelengths.

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