ABSTRACT
A narrow wavelength-band of electromagnetic radiation (from about 380 nm to 780 nm, as shown in Fig. 1.2) is perceived by our eyes as light.
As for any other electromagnetic radiation, the velocity of light ( c) is approximately 3 10 8 m/s (or 300 000 km/s). Its two main attributes are its quantity and its quality. Its quantitative aspects are discussed in Section 2.1.3 (photometry). Its quality is characterized by wavelength ( ) and its reciprocal, the frequency ( f ). The product of these two always gives the velocity:
c f (2.1)
so, if one is known, the other can be found by dividing the known one into the velocity
3 10 3 108 8
f for
2.1.1.1 Colour of light The colour of light is determined by its spectrum or spectral composition. Light of a particular wavelength, or a narrow band of wavelengths is referred to as monochromatic. The colour of broad-band light depends on the relative magnitude of its components, on its spectral composition. A continuous spectrum white light can be split by a prism into its components, which are perceived as colours, shown in Table 2.1 .
