The term intensity is used to describe the rate at which light spreads over a surface of a given area some distance from a source. The intensity varies with the distance from the source and the power of the source. Power is a property of the light source that describes the rate at which light energy is emitted by the source. Power is often expressed in units of watts. Table 1 shows the dependence of intensity (I) upon the distance (r) from a 60-watt light bulb. Numbers are rounded to the second decimal place.
Different light bulbs are rated to have different power values. Typically, the purpose of a light determines what power bulb will be used in the light. Obviously, a night light bulb will have a lower power rating than the light bulb used in a floor lamp. Table 2 shows the dependence of intensity (I) at a distance of 1.0 meter from various types of light bulbs.
Table 1
r (m) I (W/m2) 0.10 477.46 0.20 119.37 0.50 19.10 1.00 4.77 2.00 1.19 5.00 0.19 10.00 0.05
Table 2: Intensities at 1-m Distances