Topic 1 of 3
Spectrum order and shared properties
Radio waves, visible light and gamma rays are parts of one electromagnetic spectrum. They differ in wavelength and frequency, while sharing the same speed in vacuum.
Wavelength is a distance between equivalent adjacent points of a wave. Frequency is the number of complete cycles per second, in hertz. A wave transfers energy as it propagates.
What all electromagnetic waves share
All electromagnetic waves are transverse: their oscillations are perpendicular to the direction of propagation. The oscillating quantities are electric and magnetic fields, rather than air particles moving up and down.
They can travel through a vacuum, without a material medium. All travel at the same vacuum speed. For the calculations here, use the supplied value c = 3.0 x 108 m/s.
A sinusoidal drawing can represent how a field varies. It is not a wavy path followed by a photon or a row of air particles carrying the radiation. The vacuum-speed statement also does not say that every real communication system has the same total delay: material paths and equipment can introduce other effects.
Order wavelength and frequency in opposite directions
Read the same order in two directions
Moving down this list, frequency increases and wavelength decreases. All electromagnetic waves have the same speed in vacuum.
Visible light, enlarged
The separate region names are useful categories. Microwaves are also commonly described as part of the wider radio-frequency range.
Increasing frequency: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays.
Increasing wavelength: gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves, radio waves.
Within visible light, red has a longer wavelength and lower frequency than violet. Both are electromagnetic waves. The named regions are convenient divisions of a continuous spectrum, not physical gaps; microwaves are also often included within the wider radio-frequency range.
Worked comparison
Two frequencies, one vacuum speed
A 100 MHz signal has f = 100 x 106 = 1.0 x 108 Hz. Its wavelength is λ = (3.0 x 108)/(1.0 x 108) = 3.0 m.
A 2.5 GHz signal has f = 2.5 x 109 Hz. Its wavelength is λ = (3.0 x 108)/(2.5 x 109) = 0.12 m.
The second signal repeats more often and has more closely spaced wave cycles. Both still travel at c in vacuum.
Work from wavelength when that is supplied
One nanometre is 10-9 m. For a supplied wavelength of 600 nm, λ = 6.00 x 10-7 m, so f = c/λ = 5.0 x 1014 Hz.
At 1200 nm, the wavelength is twice as large and the frequency is half as large: 2.5 x 1014 Hz. Wavelength and frequency cannot both increase while their product remains the same speed.
Radio reception ends with a different wave
A radio transmitter sends an electromagnetic signal to a receiver. The receiver processes the information and drives a loudspeaker. The loudspeaker then makes sound waves in the surrounding air.
The radio signal can cross a vacuum; the sound from the loudspeaker needs a material medium. In air, sound is longitudinal, while the radio wave is transverse. Hearing a radio programme does not mean that the broadcast crossed the space between transmitter and receiver as sound.
Travel-time comparison
Equal paths through vacuum
Two electromagnetic signals are emitted together and each follows a 900 km vacuum path. Convert the distance to 900000 m.
t = distance/speed = 900000/(3.0 x 108) = 0.0030 s = 3.0 ms.
Both arrive together in this ideal comparison, regardless of their different frequencies. This is the propagation time for the stated paths, not the complete delay of a working network.
Keep frequency separate from speed. More cycles per second does not mean faster propagation when the vacuum speed is fixed.