THE PHYSICS TEACHER ◆ Vol. 39, February 2001
18
Gorazd Planinsic, Department of Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana,
Slovenia; [email protected]
Due to a growing interest in biology
among science students, using demon-
stration experiments in physics classes that in-
volve observations of biological material or that
relate to plant, animal, or human physiology
can often raise student motivation. The experi-
ment described below involves observation of
microorganisms in a drop of pond water. An
explanation of the image formation in the
paraxial region is then given using geometrical
optics.
Beginning with the invention of the first mi-
croscope, humans have been fascinated by the
observation of microscopic life. In his spare
time, Dutch draper Antoni van Leeuwenhoek
(1632-1723) made his own single-lens micro-
scope using a small glass sphere. Keeping de-
Water-Drop Projector
tailed records of his observations, he made the
first drawing of a bacterium in 1683. Articles
describing construction of a simple variation of
Leeuwenhoek’s microscope (glass tubing micro-
scope) have appeared previously in TPT.1
However, it is easier still to build a single-lens
microscope by using a drop of clear water in-
stead of a glass lens. The small water drop
works as a spherical lens with a large magnifying
power.
Experiment
To collect your sample, fill a syringe with
water from a pond or large puddle with a lot of
decaying plants. If you live at the coast, use sea-
water. Try to catch some very small animals (0.2
mm to 0.5 mm) that move around in the water.
Many of them are found close to the bottom of
the pond.
Fix the syringe on a holder (Lego blocks or a
piece of wood works well) using adhesive tape as
shown in Fig. 1.
For a light source, fix a laser pointer on a
vertical positioner. An empty stick-deodorant
container will do, as shown in Fig. 1. To create
your lens, carefully push the syringe piston until
a water drop (about 2 mm in diameter) is
formed at the end of the nozzle.
Observation
Place the syringe holder on a table about two
meters away from a screen or white wall. Switch
on the laser pointer and adjust the beam to
point exactly through the middle of the water
drop and perpendicular to the screen. For fine
horizontal adjustment of the laser pointer, move
the deodorant container, and for vertical adjust-
ment use the built-in turn screw. With the right
adjustment of the laser, a bright spot extends in-
to a large round image on the screen about 2 m
Fig. 1. Water-drop projector: experiment setup.
