College Physics II Inverse square law EPCC
Objective: Understand the inverse square law.
Part 1: Inverse square law
Refer to figure 1: Acquire a light source for this experiment. Select the light function from
PhyPhox®. Select the “simple” function for measuring the illuminance. Next place the cellphone
as close to the light source as
possible. Use a ruler to measure
out a 25.0 cm path away from the
light source. Initiate the “light”
application and record the light
intensity at this point. Make sure
you align your light detector to the
light source to gain the maximum
intensity as well as maintaining it
level. Record this value in table 1.
Continue to take measurements
at every 5.0 cm till the end of the
path, careful to align properly the
light detector on your phone with
the light source.
Next plot your data points along
an intensity versus distance graph
using excel. Then fit a power
curve to your data points and
determine the pattern you
ecorded.
Table 1.
Distance (m) Illuminance (Lux)
0.01
0.05
0.10
0.15
0.20
0.25
Questions – answer each question within 3 sentences :
1. Refer to your graph. Is the plot curved or straight? What does this shape resemble and is
it consistent with the inverse square law?
2. How would you expect any other form of electromagnetic radiation to behave when you
move away or move close to the source?
5.0 cm
10.0 cm
15.0 cm
(a) (b)
Figure 1: An example set up for measuring light intensity. Place
the cellphone as close as possible to the light and record the
intensity. Take measurements every 5.0 cm (a). Make sure you
select the 'simple' option in the 'light' recording option in
PhyPhox®.