PASCO OS-8520 PHOTOMETER - Manual (Page 2)

2
Photometer 012-05631A
Limited Warranty
PASCO scientific warrants this product to be free from de-
fects in materials and workmanship for a period of one
year from the date of shipment to the customer. PASCO
will repair or replace, at its option, any part of the product
which is deemed to be defective in material or workman-
ship. This warranty does not cover damage to the product
caused by abuse or improper use. Determination of
whether a product failure is the result of a manufacturing
defect or improper use by the customer shall be made solely
by PASCO scientific. Responsibility for the return of equip-
ment for warranty repair belongs to the customer. Equipment
must be properly packed to prevent damage and shipped
postage or freight prepaid. (Damage caused by improper
packing of the equipment for return shipment will not be cov-
ered by the warranty.) Shipping costs for returning the equip-
ment, after repair, will be paid by PASCO scientific.
Experiment 1: Inverse Square Law
EQUIPMENT NEEDED
– Bench (OS-8518) – 2 Point light sources (OS-8517)
– Photometer with filter set (OS-8520)
Purpose
The purpose of this experiment is to show that light intensity is inversely proportional to the
square of the distance from a point light source.
Theory
The light from a point light source spreads out uniformly in all directions. The intensity at a
given distance, r, from the light will be equal to the power output of the light divided by the
surface area of the sphere through which the light has spread. Since the area of the sphere goes
as the square of its radius, r, the intensity will drop off as 1/r
2 . In general, the intensity of the
point light source at any distance, r, is given by
I = constant
r 2
Thus, the ratio of the intensity (I) of the light at a position (r) as compared to the reference intensity
(I
o
) measured at a position (r
o
) is given by
I
I
o
=
r
o
2
r 2
Set Up
➀ Place the photometer at the 70 cm mark on
the optics bench.
➁
Place a point light source at 40 cm. Put a
neutral density filter on the side of the
photometer that is opposite the point
source. See Figure 1.1. Place the other
light source on the same side of the bench
that has the neutral density filter.
➤ NOTE: This experiment can be done using
one point light source and a second light
source (used as a reference) that is not a point
source. If you are using only one point
source, put the point source on the side of the
photometer that does not have the filter.
➂
Adjust the neutral density filter for 100% transmittance.
Point Source
Second Light
Source
Eyepiece
Filter
Photometer
40 cm
70 cm
The bench and light source shown with the
Photometer are available as part of the
PASCO OS-8515, Basic Optics System
Figure 1.1: Experiment Set-Up