2.4: Electromagnetic waves

Waves

Electromagnetic waves are simply a type of wave. They can be described with some of the same words and variables as ocean waves. If you are a surfer, you probably check the swell forecast for wave height and wave period before you head out to the water. The best waves for surfing have a long period, which helps to separate and clearly define individual waves, like the one in Figure \(\PageIndex{1}\). Wave periods of 12, 14, or longer seconds are ideal. Depending on whether you are a beginner or an advanced rider, you might prefer a wave face height of 2 to 30 feet. The wave period is the time it takes for a wave to complete one cycle. Wave period is simply the inverse of wave frequency. Wave face height is related to both wave amplitude and frequency, but is more complex in surfing, as it is measured at wave break. In the case of electromagnetic energy, the waves do not break.

Figure \(\PageIndex{1}\): Waves at a beach are an example of a commonly experienced wave phenomenon. (Public Domain; NOAA via Wikimedia Commons).

Electromagnetic Radiation

Radiation can be considered in two ways: as electromagnetic waves or as photons. In atmospheric science, we generally consider radiation as a wave rather than a photon. Electromagnetic radiation is a form of energy produced by oscillating electric and magnetic fields and is known by various names depending on its wavelength. For example, you’ve probably heard of radio waves and X-rays, where the primary difference between the two forms of electromagnetic radiation is the wavelength of the waves in each. Figure \(\PageIndex{2}\) shows the relationship between wavelength, frequency, and temperature. Colder objects radiate at lower frequencies and longer wavelengths, and warmer objects radiate at higher frequencies and shorter wavelengths. This relationship holds across all scales, from atomic nuclei to planets, and across temperatures from near absolute zero to millions of Kelvins.

Figure \(\PageIndex{2}\): The electromagnetic energy spectrum, including whether it penetrates the Earth’s atmosphere, wavelength visual (red line), radiation type, wavelength value in m, wavelength scale comparison, frequency in Hz, and temperature of black body objects that emit at that wavelength of radiation. Table \(\PageIndex{1}\) provides the information in a tabular form. (CC BY-SA 3.0; Inductiveload via Wikimedia Commons).

Table \(\PageIndex{1}\): The relationship between types of electromagnetic waves, including their wavelength, frequency, temperature, ability to penetrate Earth's atmosphere, approximate size of wavelength, and equivalent temperature.

Region of the Spectrum	Approximate Wavelength (meters)	Frequency (Hz)	Penetrates the Earth's atmosphere?	Approximate size of wavelength	Temperature Equivalent (Kelvin)
Radio	> 1 meter	< 300 MHz	Yes	Radio towers, buildings	~ 1
Microwave	1 m to 1 mm	300 MHz – 300 GHz	No	Butterflies	~10
Infrared (IR)	1 mm to 700 nm	300 GHz – 400 THz	Partly	Needle point	~100
Visible Light	700 nm to 400 nm	400 THz – 750 THz	Yes	Protozoans	~10^3
Ultraviolet (UV)	400 nm to 10 nm	750 THz – 30 PHz	Mostly no	Molecules	~10^4
X-rays	10 nm to 0.01 nm	30 PHz – 30 EHz	No	Atoms	~10^7
Gamma rays	< 0.01 nm	> 30 EHz	No	Atomic nuclei	~10^8

Radio waves have the lowest energies, the longest wavelengths (100 km to 1 m), and the lowest frequencies of any type of electromagnetic radiation. Thus, they occupy the left-most position in Figure \(\PageIndex{2}\). Radio waves are essential for wireless technology, including AM/FM radio broadcasting, cellular telephone communications, television, radar, and satellite communication. As we move toward higher-energy waves, Microwaves are next, with slightly higher energy and shorter wavelengths (1 mm to 1 m) than radio waves. Certain microwave frequencies (such as those used in microwave ovens) are absorbed by water molecules, causing them to vibrate and generate heat. But the energy in these waves is still too low to cause bodily harm. Next is Infrared radiation, with wavelengths from 1 mm to 700 nm, emitted by ordinary objects such as rocks, the Earth, and even humans. These waves are often used in fibre optic communication and remote controls. Next up is visible light, with wavelengths ranging from 700 to 400 nm. It comprises only a tiny fraction of the EM spectrum, yet it accounts for everything we see and nearly 47% of the energy Earth receives from the Sun. Ultraviolet radiation (UV) is located next in the electromagnetic spectrum, with wavelengths shorter than those of visible light (400-100 nm). We have finally reached a region of the spectrum with sufficient energy to damage objects and human skin. The Sun emits UV radiation, most of which is blocked by the Ozone layer and is prevented from reaching the Earth's surface. Still, some UV radiation reaches the surface, which is why wearing sunscreen is important if you're going to be out in the Sun. Next up are X-rays with significantly higher energies and shorter wavelengths (10 to 0.1 nm). They possess significant penetrating power, enabling them to pass through soft tissues while being absorbed by denser tissues such as bone, making them essential for medical and dental imaging. However, prolonged or repeated exposure to X-rays can cause significant tissue and DNA damage, leading to cancer and other health risks. Gamma rays occupy the far right side of the EM spectrum and have the highest energies and the shortest wavelengths of any other EM waves. They are generated by atomic nuclei during radioactive decay, nuclear fission/fusion, or violent astronomical phenomena such as supernovae and black holes. These high-frequency, high-energy radiations are among the most dangerous to humans, and even mild exposure can cause significant cellular damage.