PASCO OS-8520 User Manual

Instruction Sheet
for the PASCO
Model OS-8520

012-05631A

3/95

$1.00

PHOTOMETER

Eyepiece

Neutral Density

Filter

Polarizer

Introduction

The PASCO OS-8520 Photometer is a non-electric com­parative instrument which was designed as an accessory
for the OS-8515 Basic Optics System. The Photometer
base can stand on any flat surface for use with other optics
systems or for experiments not requiring an optics bench.

Equipment

The OS-8520 Photometer includes:

–Photometer with Base
– Set of Polarizers and Accessory Holder
– Neutral Density Filter Set which allows 25%,

50%, 75% and 100% transmittance.

Photometer

The PASCO OS-8520 Photometer operates in a similar
fashion to a wax photometer except the wax is replaced
by two high brightness fluorescent acrylic disks. By look­ing through the eyepiece, students can see if the disks,
each receiving light from an opposite side of the photom­eter, are equal in intensity. Although it is difficult for the
eye to determine relative intensities, it can detect equal
intensities quite accurately.

A set of neutral density transmission filters (25, 50, 75,
and 100%) are included to be used on the Photometer to
block out a known amount of light so quantitative mea­surements may be made. Note that because the Photom­eter uses a fluorescent material, it will not respond
equally in the red and violet parts of the spectrum. This
Photometer was designed to be used with white light,
such as the PASCO Light Source (OS-8517). To de­crease the intensity of the light that is emitted by the light
source, use a mask rather than decreasing the voltage to
the bulb. The use of a mask preserves the white color of
the light whereas a decrease in the voltage would change
the color of the emitted light to red.

A set of two mounted polarizers is also included. The
polarizer perimeters are marked in degrees and the
polarizers can be rotated to any angle to show the amount
of extinction of the transmitted light.

All the filters snap into the ends of the photometer and
can be rotated once they are snapped into place. The
polarizers are designed to fit on their own separate holder.
One polarizer can be snapped into each side of the holder.

How to Use the Photometer

To determine if the two sides of the photometer are illu­minated by light of equal intensity, look down into the
conical eyepiece of the photometer. The cone is designed
to cast a shadow on the inner parts of the photometer to
allow a better view. Do not put your eye directly on the
eyepiece: keep your head at a distance which allows you
to comfortably focus on the orange indicator. If the light
is the same brightness on each side, the color and bright­ness of the two sides of the orange indicator will appear to
be the same.

© 1995 PASCO scientific
This instruction sheet written/edited by: Jon Hanks

10101 Foothills Blvd. • P.O. Box 619011 • Roseville, CA 95678-9011 USA

Phone [phone] • FAX [phone] • email: [e-mail]

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ways to

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Photometer 012-05631A

Limited Warranty

PASCO scientific warrants this product to be free from de­fects 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 workman­ship. This warranty does not cover damage to the product
caused by abuse or improper use. Determination of

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

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 equip­ment 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 cov­ered by the warranty.) Shipping costs for returning the equip­ment, after repair, will be paid by PASCO scientific.

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/r2. In general, the intensity of the
point light source at any distance, r, is given by

Thus, the ratio of the intensity (I) of the light at a position (r) as compared to the reference intensity
(Io) measured at a position (ro) is given by

2

r

I

o

=

2

I

r

o

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.

I =

constant

2

r

Point Source

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

Eyepiece

Photometer

Filter

Second Light

Source

2