Monochromatic light is light that has only one wavelength, and thus cannot be further divided into different components using a prism. But most light sources are not monochromatic - in other words, the light they radiate is a mixture of different wavelengths.
We are interested in the spectral power distribution of a given light source - that is, for every wavelength, how much of the light (or power) it radiates has that particular wavelength? In other words, how is its light distributed across the different wavelengths?
For example, mercury light is composed mainly of light with wavelengths 404.7, 407.8, 435.8, 546.1, 577.0, 579.0 nanometers. It is said to have a line spectrum, because its light has only particular discrete wavelengths.
Another example is incandescent electric light, which has all the wavelengths in the visible spectrum. It is said to have a continuous spectrum.
A device called a spectroscope can be used to measure spectral power distribution. Light enters the spectroscope and is refracted by a prism onto a screen so that the different wavelengths are spread out. Then a detector moves across the screen and, for each wavelength, measures the rate at which energy is received, i.e. the power at that wavelength. Put together, this data gives the spectral power distribution of the light source.
We can represent spectral power distribution by a bar graph such as Figures 2.1 and 2.2. In these graphs, the y-axis indicates power per wavelength, and the x-axis indicates wavelength. The area under a section of the graph gives the total power of the light in that range of wavelengths.
Spectral power distribution of mercury light
Spectral power distribution of incandescent electric light
Figure 2.2 has a stepped line even though it represents a continuous spectrum. This is because we can't actually measure the power at precisely one wavelength - the best we can do is measure the power for a small range of wavelengths. But we can connect these discrete measurements with a smooth curve, shown in Figure 2.3, called a spectral distribution curve.
Spectral distribution curve of radiant power of incandescent electric light