Light is a wave, and it belongs to a family of waves that all travel at the same enormous speed and differ only in wavelength and frequency. By the end of this lesson you should be able to explain what an electromagnetic wave is and why it needs no material to travel through, state the speed they all share, put the bands of the spectrum in order both by wavelength and by frequency, and use the wave equation on numbers written in the shorthand these wavelengths require.
A wave that needs no medium
Go back to the bell in the sealed jar from the sound lesson. As the air was pumped out the ringing faded to nothing, because sound is particles passing a push along and there were no longer any particles to do it. Throughout the whole demonstration, though, you could still see the bell — so whatever light is, it crossed the space that had defeated the sound. That is not a small detail. It rules out light being a mechanical wave of the kind this module has dealt with so far, and it is the reason sunlight reaches us across a hundred and fifty million kilometres of near-vacuum while the Sun, a spectacularly violent object, is completely silent.
What oscillates in a light wave is not matter but fields. A changing electric field creates a magnetic field, and a changing magnetic field creates an electric field, so once the pair has been set going by something that shakes electric charge — a hot filament, an aerial, an atom — each keeps regenerating the other and the disturbance propagates outwards on its own. That is why nothing material is needed: the two fields carry each other. Waves of this kind are called electromagnetic waves, and they are transverse, with the oscillations at right angles to the direction of travel. They are also perfectly happy to travel through some materials — air, glass, water — but they never require one.
Everything from the wave anatomy lesson applies to them unchanged. They have an amplitude, which sets how much energy they carry; a wavelength, measured from one crest to the next; a frequency in hertz; and a period which is 1 divided by the frequency. The wave equation applies too, which is the point of the rest of this lesson.
One speed for all of them
Every electromagnetic wave travels through a vacuum at exactly the same speed, and it is a large number: about three hundred million metres per second. That is written more compactly as 3 × 10⁸ metres per second, and since that shorthand is unavoidable here it is worth defining. The notation 10⁸ means 1 followed by eight zeros, so 3 × 10⁸ means 3 followed by eight zeros: 300 000 000. The speed is given its own symbol, a lower-case c, because it turns up throughout physics.
The speed of every electromagnetic wave in a vacuum
- the speed of light, and of every other electromagnetic wave, in a vacuum, in metres per second (m/s)
- 3 followed by eight zeros: 300 000 000
- metres per second — the unit of speed
The same value is used for travel through air, because air slows light by only a few parts in ten thousand. Two things about c are examined. The first is that it is the same for every band: radio waves are not slower than gamma rays, and red light is not slower than blue. The second is that it is fast enough to be effectively instantaneous over any distance you can walk — light crosses a room in about a hundred-millionth of a second — which is why a flash of lightning arrives long before the thunder it made, even though both left at the same moment.
Since all of them share one speed, the wave equation from earlier in this module locks their frequency and wavelength together. Speed is frequency multiplied by wavelength, and if the speed is fixed at 3 × 10⁸ metres per second then a long wavelength forces a low frequency and a short wavelength forces a high one. There is no freedom left: name either quantity and the other is determined. That is what makes the spectrum a single ordered line rather than a two-dimensional catalogue.
The spectrum in order
The whole family, arranged from the longest wavelength to the shortest, runs: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays. Read the same list backwards and you have them in order of increasing wavelength; read it forwards and, because of the locked relationship above, you also have them in order of increasing frequency. Radio waves have the longest wavelengths and the lowest frequencies, gamma rays the shortest wavelengths and the highest frequencies, and every band in between sits where its wavelength puts it. The ranges are continuous and the boundaries between the named bands are conventions rather than physical walls — there is no wavelength at which infrared abruptly becomes a different kind of thing.
Visible light is a very narrow slice of that strip, running from wavelengths of about 700 nanometres at the red end down to about 400 nanometres at the violet end, a nanometre being a thousand-millionth of a metre. Within it, red has the longest wavelength and the lowest frequency of the colours and violet the shortest and highest, with orange, yellow, green, blue and indigo in between. Everything outside that narrow band is invisible to the eye and identical in nature — the reason we notice visible light at all is a fact about human eyes, not a fact about the waves.
The prefixes come with the territory, so here they are in one place. A kilohertz is a thousand hertz, a megahertz is a million hertz, and a gigahertz is a thousand million hertz; each step up is a factor of a thousand. On the wavelength side, a millimetre is a thousandth of a metre, a micrometre a millionth, and a nanometre a thousand-millionth. Radio wavelengths are metres to kilometres, microwaves are centimetres to millimetres, infrared is micrometres, visible light is hundreds of nanometres, and X-rays and gamma rays are smaller still.
Why it matters in aviation
An aircraft in the cruise is using several widely separated parts of this one spectrum at the same time, and the only thing that differs between those uses is the frequency. Radio waves carry speech and navigation signals over long distances. Microwaves, with wavelengths of a few centimetres, are short enough to be reflected usefully by raindrops and by the ground, which is what makes weather radar possible and what allows a radio altimeter to measure height above the surface by timing a signal down and back. Infrared is the band warm objects radiate most strongly in, so it is what fire and overheat detection looks for. Visible light is the narrow slice the crew's own eyes work in, and the one that lights and markings are designed around.
The choice of band in each case follows from how waves of that length interact with matter. Long waves pass around obstacles and travel far, which suits communication; short ones reflect off small objects, which suits measuring what is nearby. Nothing changes across those applications except the frequency, and everything else changes because of it.
What the test asks
Two distractors dominate. The first treats radio waves as a kind of sound, usually in an option saying that radio waves cannot cross space, or that they need air to travel through, or that they travel at the speed of sound. Radio waves are electromagnetic, so they need no medium and travel at 3 × 10⁸ metres per second — the confusion arises only because radio is used to carry sound, and what the aerial radiates is not what the loudspeaker eventually produces. The second is the assumption that a shorter wavelength means slower travel, or that gamma rays somehow race ahead of radio waves. In a vacuum every band travels at exactly the same speed, and a change in wavelength is always paid for by the opposite change in frequency rather than by a change in speed. Beyond those, expect an ordering item where two neighbouring bands are swapped — ultraviolet and X-rays are the usual pair — and an item that asks which band has the highest frequency, where the answer is gamma rays and the trap is choosing the one with the largest wavelength number.