EM Waves: Uses and Dangers

Different EM waves are used for different applications, which you are expected to be able to recall and explain. We will go through some of these uses in this article, which is unavoidably long as there is a lot of information to cover. Your exam preparation should therefore involve making summary notes from the wealth of information presented here, starting with a few words about the dangers of EM waves.

In general, the shorter the wavelength of an EM wave, the more dangerous that type of wave will be. Taking visible light as being “in the middle” of the electromagnetic spectrum, and accepting that light is not dangerous except at very high intensity, we can say that wavelengths shorter than visible light (UV, X-rays and gamma waves) carry a significant level of danger whereas wavelengths that are longer than visible light (IR, microwaves and radio waves) are generally less hazardous.

This is because EM waves with a shorter wavelength (higher frequency) carry more energy and when high levels of energy are absorbed by living organisms, damage can occur to their cells. In extreme cases, the organism can be killed. Therefore, gamma waves are likely to be the most dangerous whereas radio waves are likely to be the safest. In terms of danger, the correlation between high energy (short wavelength) and greater biological damage is a key point to bear in mind as we go through the uses of EM waves.

Gamma waves (highest frequency): As indicated above, gamma waves can kill things. For example, they can kill bacteria on food without affecting the taste or nutrients (which are chemicals rather than organisms). Gamma irradiation can therefore make food last longer. Note that this does not make the food radioactive! Gamma waves can also be used to kill cancer cells in radiotherapy, where small beams of gamma waves are aimed into the body from multiple directions. Each beam is weak enough not to damage the healthy tissue but where the beams meet, at the cancer, there is a higher intensity that can kill the cancerous cells. Radioactive sources, such as cobalt-60, are used to produce the gamma waves. Great care must be taken to protect medical staff as they will be handling the source repeatedly, whereas the patients will each receive only their own dosage. The key rules of radiation protection are; limit exposure time, maintain the greatest distance possible, install shielding and wear monitoring equipment (such as an electronic alarm or a film-based badge).

X-rays: These have a slightly lower frequency than gamma waves so they carry slightly less energy and are slightly less harmful. “Hard” X-rays are close to gamma waves and can be used for cancer treatment, as above, but most X-rays are “soft” and are used to look inside objects. This can mean checking metal objects, such as high-pressure pipes that will be used to carry superheated steam, which might burst open if there are any internal cracks. More obviously, X-rays are used for looking inside people, to check for broken bones. The image obtained is a shadow pattern that is produced because soft tissue lets X-rays pass through whereas denser bone absorbs X-rays: cracks and breaks in bones can therefore be seen as areas of different lightness in the picture. Additional information can be obtained if the patient consumes an X-ray absorbing substance, such as a “barium meal” that allows the digestive tract to be imaged even though it is made entirely of soft tissue. Importantly, X-rays are produced electrically, when high-speed electrons are fired at a special target: X-rays are therefore inherently safer than gamma waves because they can be switched-on very briefly, only at the moment when they are required.

Ultra-Violet (UV): Some substances emit visible colours when they absorb invisible UV in a process known as fluorescence. This makes UV ink ideal for putting invisible markings on valuable objects and it is also the method by which UV lamps work to detect forged banknotes. Wristbands that change colour to warn about the risk of sunburn also work in a similar way. The fact that UV causes sunburn is a clear danger of this sort of electromagnetic radiation: over-exposure to strong sunlight can lead to skin cancer and eye damage.

Visible light: We often take visible light for granted and it can sometimes be hard to think of applications other than the biological function of sight. Remember that visible light is used for photography and in photoelectronic systems, such as televisions and smartphone screens. It is also used for warning systems, such as lighthouses and flashing lights on emergency service vehicles. Don’t forget that visible light is actually its own spectrum of colours, from red to blue. Different colours can be generated artificially by combining varying ratios of red, green and blue light. The illustration below shows that the yellow you see on a computer screen is actually a combination of red and green light.

Infra-Red (IR): This type of electromagnetic radiation has an extremely wide range of uses. IR beams are used in TV remote-controls; IR lamps are used in heating (such as keeping restaurant food warm before it is taken to the tables); IR cameras can be used to detect temperature variations, which in turn can be used to visualise poor insulation in buildings. Other IR cameras are used in surveillance to locate criminals at night or to penetrate through smoke or fog. IR is also used for optical fibre communications (because it is absorbed less than visible light) and IR reflections from the Earth’s surface are used in satellite imaging.

Microwaves: Sitting between IR and radio waves, it is perhaps unsurprising that microwaves are used for both thermal and communication purposes. Communications can be short-range (such remote controls for garage doors, when IR would not work because the signal has to travel through the materials of the car), medium-range (such as Wi-Fi networks, which need to be able to penetrate nearby walls but don’t have to cover extended distances), intermediate-range (such as cellphone networks, which rely on transmitters placed at kilometre-scale distances) and long-range (such as satellite communications, including the signals that are picked-up by satellite TV dishes). The best-known heating application is, of course, the microwave oven. It is worth saying that some people express concern about the safety of microwaves, especially the transmitters that provide smartphone networks. There is no firm evidence for adverse biological effects but if you are worried about this possibility then don’t buy a smartphone and don’t have a Wi-Fi network at home: that will remove the closest and most persistent (therefore most hazardous) microwave sources.

Radio (lowest frequency): There is a huge range of wavelengths (frequencies) in the radio part of the EM spectrum, extending from extremely-low frequencies where the wavelength is greater than the diameter of the Earth (used for communicating with submarines) to extremely-high frequencies that are part of the microwave range (so include Wi-Fi networks as well as smartphone and satellite signals). Common uses include radio and TV channels, commercial and private radio (including aircraft and emergency services) as well as radio-astronomy and amateur radio applications. Although radio waves are generally the least harmful, because their signals carry the lowest amount of energy, it is still important to keep a sensible distance from high-power transmitters.