The Electromagnetic Spectrum

In some previous posts I have touched upon the concept of electromagnetic radiation but not yet fully explained what they are. Today I hope to shed some light (pardon the pun) on why they play an important role in daily life.

You see waves when you throw stones into a pond, or when you shake a slinky up and down. These are are a type of wave called mechanical waves. Mechanical waves require a medium to travel through; in the first example the medium would be water, and in the second example the medium would be the slinky itself. Other examples of mechanical waves include seismic waves, and sound. Consequently, Star Wars should be fighting its battles in complete silence, but of course that would make for a deeply depressing dogfight.

Electromagnetic waves are a type of wave that can travel in vacuum. This is because the energy of electromagnetic waves is carried by oscillating electric and magnetic fields (hence the name) and not matter. Visible light, ultraviolet light and microwaves are all examples of electromagnetic waves.

How do we categorise electromagnetic waves then? This is comes down to three important properties of a wave: its speed, its frequency, and its wavelength. For any given wave, these three properties are linked by the equation:

speed = frequency × wavelength

The speed of the wave is, of course, how fast it travels. The frequency is how many cycles pass in a second, and the wavelength is the distance between the start of one cycle and the start of the next cycle (or the distance between two adjacent crests or troughs). The speed of an electromagnetic wave is always the same in vacuum, 299,792,458 ms-1 (the speed of light). Therefore, if a wave has a high frequency, it has a low wavelength, and vice versa. A wave that has a high frequency also carries a lot of energy.

The electromagnetic spectrum displays the range of waves, ordering them by frequency and wavelength. There are no discrete boundaries as to where one type of wave ends and another begins, which is why it is referred to as a spectrum.

The Electromagnetic Spectrum
This excellent diagram is provided by chemwiki.ucdavis.edu.

The fact that different electromagnetic waves have different frequencies and wavelengths make them useful for different applications.

  • Radio waves lie at the low-frequency end of the spectrum. Due to its long wavelength, radio waves are used in long-distance communication systems such as radio communication, radar, broadcasting and communication satellites.
  • Microwaves are also used in communication systems due to its long wavelength. Microwaves are good at heating things, such as food. Microwave ovens work by causing food molecules to oscillate and therefore heat up.
  • Infrared radiation is the heat you feel from a hot object, and is in fact emitted by all objects (as aforementioned in Can’t Stand the Heat?). As a result infrared has many imaging applications, such as in night vision, meteorology and astronomy. Infrared is also emitted by the remote control you use to switch on your television.
  • Visible light is the range of waves that the retina in our eyes are able to detect, yet it represents a tiny section of the spectrum. This section, named the visible spectrum, can be further split into different colours.
  • Ultraviolet light is emitted by very hot objects like the Sun (Hello, Summer). Effects of overexposure include suntan and sunburn. UV light is used in fluorescent lights (Let There Be Fluorescent Light…) and in many analytical applications such as forensics.
  • X-rays carry more energy than UV light, so they are far more ionising so extreme care is taken during X-ray exposure. They are frequently used for medical imaging as they penetrate soft tissues but are absorbed by bones.
  • Gamma (γ) rays are extremely high-frequency waves which are released during radioactive decay. High doses of gamma radiation kills cells, so they are used to sterilise surgical instruments.

The electromagnetic spectrum is an example of how, although there is a lot that we can see, there is so much more that we cannot and have yet to discover.

Yanhao

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