Department of Physics and Astronomy
2004 AAPT Apparatus Competition, Entry Description
Low
Cost Category
Apparatus
Title: Observing photoelastic
stress without polarizers
Abstract
Optically anisotropic plastic looks colorful when placed
between two polarizers. Such colorful
display can be demonstrated without polarizers. In the experiment light is collimated to illuminate a
Plexiglas plate at BrewsterÕs angle.
Its reflection passes through a piece of plastic and is incident on
another Plexiglas plate at BrewsterÕ angle. The reflected light from the second plate shows the colorful
image of the plastic sample.
Equipment and costs required to construct apparatus
|
Item |
Source |
Part
number |
Cost |
|
GE
40-W (120 V) light bulb, clear |
Home
Depot |
|
$1.40 |
|
Hand-held magnifier: 6.35 cm in diameter , 2x
magification |
Potomac
Display and Manufacturing www.potomacdisplay.com |
5-ML2225 |
$3.95 |
|
Plexiglas:
18Ó x 24Ó x 0.25Ó for $11.57 (only
one fourth was used) |
Home
Depot |
|
$2.89 |
|
Whitewood: 1Ó x 4Ó x 12Õ for $2.65 (only one half was used) |
Home
Depot |
|
$1.33 |
|
BC Plywood: 2Õ x 4Õ x 0.5Ó for $10.56 (only half was used) |
Home
Depot |
|
$5.28 |
|
Tempered hardboard: 2Õ x 4Õ x 0.125Ó for $2.69. (only
one fourth was used) |
Home Depot |
|
$0.67 |
Total Cost |
$15.52 |
||
Description
A 40-W light bulb is mounted inside a homemade wooden box
and at a focal length of 16 cm from a magnifier. Light from the bulb exits the box through a 5-cm hole and
illuminates the magnifier.
Collimated by the lens, it then travels horizontally and illuminates a
Plexiglas plate with a refractive index of 1.51 at BrewsterÕs angle of 33.5o
(see Fig. 1 below). The back
of the plate is painted black to eliminate unwanted reflections. Light reflected from the front of
the Plexiglas plate is polarized.
It then passes through a plastic sample that sits on a horizontal
stage. Light transmitted through
the plastic sample is incident on another Plexiglas plate that is mobile on top
of the stage. An observer can
align the second Plexiglas plate to show the colorful image of the plastic
sample (see Fig. 2 below).
The experiment demonstrates that the colorful photoelastic stress
pattern of a piece of plastic can be produced without the use of polarizers.
Sketches
Figure 1. The Apparatus. The 40-W light bulb is the middle of the box with the side open. The black plate shown in the upper left is the second Plexiglas plate. It sits on a transparent stage.
Figure 2. The image of a circular piece of plastic. It is the reflection from the second Plexiglas plate that sits on the horizontal stage.
