1.3 Beyond visible light
During the twentieth century, astronomers extended their capabilities by developing telescopes and detectors that were sensitive to radio waves, microwaves, infrared and ultraviolet radiation, X-rays and gamma rays. All these forms of electromagnetic radiation, along with visible light, are emitted by the Sun.
This is a good place to examine the electromagnetic spectrum in more detail and to review some of its features. Figure 6 shows the wavelength values in metres but only some of the marks are labelled. The marks go up in 'times ten' steps so, to the right of the 1 metre mark, the first (unlabelled) mark indicates a wavelength of 10 metres (10 m), the next 100 m and the third 1000 m. Going to the left of the 1 metre mark, the next mark is 0.1 m, then 0.01 m (which is 1 centimetre or 1 cm for short), 0.001 m (1 millimetre or 1 mm), and so on. Going further to the left involves putting more and more zeroes after the decimal point, so the wavelengths are sometimes measured in micrometres - millionths of a metre. One micrometre is 0.000 001 metres.
What is the definition of the wavelength of an electromagnetic wave?
It is the distance between successive peaks of the wave.
Roughly, what is the wavelength of visible light?
It is about 0.000 001 metres (actually, 0.000 000 4 to 0.000 000 7 metres).
What type of electromagnetic radiation has the longest wavelength? What, approximately, is its wavelength range?
Radio waves are the longest: in Figure 6, the shortest radio waves lie about one step to the left of the 1 m mark, which denotes a wavelength of 0.1 m. The band representing radio waves extends to the far right of the chart. The furthest mark on the right represents a wavelength of 1000 m (three steps to the right of the 1 m mark), so the figure shows that radio waves can have wavelengths of over 1 kilometre. (Such wavelengths are typical of radio stations still broadcasting in the 'long wave' band.)