SUBJECT: Ultraviolet radiation experiment
DATE: 5/95
I am trying to recreate an experiment for which I have lost the
original source. The basic idea is to expose bacteria on petri plates to
uv radiation sufficient to prevent most growth. One half of the plate is
covered with sunscreen to see if it provides any protection from uv damage.
Students can compare different brands and different spf's. Unfortunately,
the petri dishes that we are using reflect uv radiation. We get growth on
our controls even at 30 minute exposures. Glass should also block uv
wavelengths. Does anyone know what we can use to support the sunscreen
between the light source and the bacteria? Can I get a sheet of quartz big
enough to cover a petri dish? Should we expose bacterial suspensions in
quartz cuvettes and then plate them out? Is there some other (affordable)
material that we can use? I'd appreciate hearing from anyone who has done
this experiment or who knows where I can find the source.
Thanks --cmw
Dr. Charlene Waggoner cwaggon@andy.bgsu.edu
Bowling Green State University
Bowling Green, OH 43403
Charlene,
A cheap and readily available UV-B transmitter is Saran wrap. I do not know
if it will work in your experimental design but it might.
Best wishes,
John Hoddinott.
Instead of using bacteria in class experiments concerning uv
light, sunscreen, etc. we use a strain of uv-sensitive yeast that
has very little tolerance to uv light. It is available from Wards
Biology. Other suppliers may carry these creatures, but I have
not looked. Usually we expose the petri plates for 30 minutes to
a uv light that is used to sterilize a lab bench in a cell biology
research lab. The instructor does this, not the students, for
safety reasons.
We have also let students expose plates to sunlight, but this has
the added variables of weather and time of day. We anticipated
the variation due to weather, but the effect of time of day was a
surprise. It shouldn't have been, though, because all of us have
been told that morning and evening are safer times to sunbathe.
The morning lab sections saw much less mortality in untreated
plates than did the afternoon sections! I guess that warning
about sunbathing in the heat of the day has some truth to it.
One last point: instead of simply having students test sunscreen,
we ask them to develop an experiment, with controls, to test some
hypothesis about uv absorbance, filtering, reflectance, etc. The
less exciting experiments compare suncreens with different spf
levels, or use baby oil, etc. More interesting ones test
different kinds of sunglasses, colored filters, chapstick, plastic
and glass from windows, the little goggles one wears in a tanning
salon, etc. My favorite student-developed experiment dealt with
bathing suit material. The students wanted to see what their
all-over-tanning bathing suits were doing with uv light, but
unfortunately the results were ambiguous. Maybe 30 minutes is not
enough time, given that colored cloth will shade out all
wavelengths to some extent. Sigh.
Chester Wilson (c9wilson@stthomas.edu)
Biology Department
University of St. Thomas
St. Paul, MN 55105
Thanks for all of the suggestions and advice about what to use as a support
between the sunscreen and bacteria while exposing the plate to uv
radiation. Saran wrap works! We are now in the process of refining and
writing up the protocol. We will test it in a non-majors lab later this
month. I'll keep you posted. Thanks again. -cmw
Dr. Charlene Waggoner cwaggon@andy.bgsu.edu
Bowling Green State University
Bowling Green, OH 43403
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