\(\def \u#1{\,\mathrm{#1}}\) \(\def \abs#1{\left|#1\right|}\) \(\def \ast{*}\) \(\def \deg{^{\circ}}\) \(\def \ten#1{\times 10^{#1}}\) \(\def \redcancel#1{{\color{red}\cancel{#1}}}\) \(\def \BLUE#1{{\color{blue} #1}}\) \(\def \RED#1{{\color{red} #1}}\) \(\def \PURPLE#1{{\color{purple} #1}}\) \(\def \th#1,#2{#1,\!#2}\) \(\def \lshift#1#2{\underset{\Leftarrow\atop{#2}}#1}}\) \(\def \rshift#1#2{\underset{\Rightarrow\atop{#2}}#1}}\) \(\def \dotspot{{\color{lightgray}{\circ}}}\)

Intensity

Suppose we have a solar panel pointed directly at the Sun. The amount of power it receives depends in part on how bright the Sun is, but it also depends on the size of the solar panel: a panel with four times the area will receive four times as much power. It is useful to be able to separate these two quantities. We thus define the intensity $I$ of the sunlight striking the solar panel as the total power received divided by the panel's area:

Intensity is measured in watts per square meter, and it is a property of the sunlight itself, rather than of the panel. The average intensity of sunlight on the Earth's surface at noon is about $1000\mathrm{W/m^2}$. It is smaller before and after noon (due to the atmosphere), so that the average intensity over the course of a day is $343\mathrm{W/m^2}$. Intensity can be generalized beyond sunlight, of course: we can talk about the intensity of light bulbs, the intensity of radiation, even the intensity of sound.