Electromagnetic Radiation

radio television radar wavelength 100 metres — 10 metres — 1 metres 100 mm 10 mm — 1 mm light

X-rays gamma rays

10" |im -f \ 10-5 |xm --10"6 pirn 10"7 |xm -cosmic rays 10"8 ^m —

LIGHT

infra-red red orange yellow green blue violet wavelength 0.8 um

ultra-violet

0.45

Figure 2.1 The spectrum of electromagnetic radiation.

emission spectrum peaks in the visible range (Figure 2.1), i.e. we can see much of the radiation from the Sun.

Solar radiation has wavelengths mostly within 0.1-3.5 pm and is called shortwave radiation (SW) (Note 2.D). The part which is visible is a mixture which can be separated into the colours of the rainbow by a prism, from violet to red. Violet light has a wavelength about

0.40 pm, while red light has wavelengths up to 0.76 pm. In addition, about 9 per cent of the Sun's radiated energy is invisible because it has wavelengths less than 0.4 pm, shorter than that of violet light, so it is called ultra-violet radiation (UV) (Sections 1.4 and 2.6). At the other extreme, about 50 per cent of the radiation has wavelengths beyond the red end of the visible range (i.e. beyond 0.7 pm) and is there-fore

Figure 2.2 Indicative graphs of the energy radiated by the Sun and the Earth at various wavelengths. The spectrum of the radiant energy from unit area of the Sun is scaled down. In reality the area under the Sun's curve should be about 160,000 times the area under the Earth's curve (i.e. approximately 6,0004/3004, from the Stefan-Boltzmann equation, see Note 2.C). Wavelengths absorbed by various gases are shown as dark bands.

Figure 2.2 Indicative graphs of the energy radiated by the Sun and the Earth at various wavelengths. The spectrum of the radiant energy from unit area of the Sun is scaled down. In reality the area under the Sun's curve should be about 160,000 times the area under the Earth's curve (i.e. approximately 6,0004/3004, from the Stefan-Boltzmann equation, see Note 2.C). Wavelengths absorbed by various gases are shown as dark bands.

called infra-red radiation (IR). This includes what is called longwave radiation (LW), which is the radiation whose wavelengths exceed 3.5 pm (Section 2.7).

The Earth emits only longwave radiation, which can be felt but not seen. A graph of this peaks around 10 pm (Figure 2.2). It is discussed in Section 2.7.

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