# Understanding spectra with graphs.

We denote the energy content of light (or other electromagnetic radiation) with intensity, I. Precisely, the intensity is the amount of energy per unit time per unit area. Energy per unit time is measured in watts, so intensity is measured in watts per square meter.

Our book sometimes calls this energy flux, denoted with an F. But why use a fancy word when a more well known word is available?

Example: The intensity of sunlight is roughly 1000 watts/m2.

If we want to display information about how much energy is carried at each wavelength, we can make a graph of intensity vs. wavelength. (What we display is intensity per unit wavelength, with units watts/m2/nm. The typical graph label is just I.

• The horizontal axis is wavelength.
• For a wavelength of, say 508 nm, the height of the graph gives the intensity of just that part of the light that has a wavelength between 507.5 nm and 508.5 nm.

Here are graphs for some dim red light and some bright red light.

Here are graphs for some dim blue light and some bright blue light.

Updated 1 October 2007

Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA