PASCO OS-8520 PHOTOMETER - Manual (Page 3)

3
012-05631A Photometer
Procedure
➤NOTE: You may want to cover the crossed-arrow object on each light source to reduce the
excess light in the room. The room lights must be off for this experiment.
➀
Turn off the room lights. The only sources of light should be the two point sources.
➂
Look into the photometer and move the light source on the filter side to a position that gives equal
intensities. The light source on the filter side will remain at this position for the rest of the experi-
ment. This light will act as the reference intensity I
o
. Record the positions of the photometer and
the light source that is opposite the filter side of the photometer in Table 1.1. The position of the
reference light (on the filter side) is not needed.
➂ Rotate the neutral density filter to 75% transmittance. Move the point light source (the one oppo-
site the filter side) to the position where the intensities are once again the same when viewed in the
photometer. Record this new position of the light source in Table 1.1.
➃
Repeat the last step for 50% and 25% transmittance.
Analysis
➀
Using the measured positions in Table 1.1, calculate the distances of the point source from the
photometer and record in Table 1.1.
➁
For each of the different positions, calculate the intensity using
I =
r
o
r
2
I
o
where r
o
is the initial distance of the point source (100%) and r is the distance at the given inten-
sity. Note that the intensity is calculated in terms of the initial intensity I
o
. Record your answers
in Table 1.1.
➂
Calculate the percent difference between the calculated intensities and their corresponding ex-
pected values. Record in Table 1.1.
Table 1.1: Positions
Photometer
Position
= _________
Average Position
of Point Source
Distance
from Photometer
Calculated
Intensity
%
diff
Light Sources Intensity
100% 75% 50% 25%
Position 3
Position 2
Position 1