Grades: 6 to 12
Duration: 1/2 Hour – 1 Hour
Supplies: Green Laser, syringe, stand, screen, pond (or lake) water
Background: the van Leeuwenhoek Microscope
Leeuwenhoek created a single lens microscope using a sall glass sphere. In this experiment, we will build a single lens microscope using a drop of water rather than a glass lens, and a green LASER beam to view microorganisms and learn the geometric optics that make this home made microscope possible.
To set up for this simple yet highly engaging demonstration, fill a syringe with water from a pond or river. If you live near a coast, get some sea water. If you are not near a pond or river or ocean, collect water from a puddle or some other standing body of water that is likely to have some tiny (0.2 – 0.5mm) organisms living in it.
Fill a syringe with the collected water. Fix the syringe on a lab stand so that a drop of water hangs from the tip. Position the drop of water to line up with the beam of a green LASER Blox so that the beam passes through the center of the drop, perpendicular to the wall.
Your set up should be about two meters from a screen or plain white wall where a bright green spot will display an impressive array of single celled animals, larva, fleas, floating and swimming.
This is generally engaging enough to interest students in the powerful optics that makes this impressive sight possible.
Let us know how it went or if you found a better set up for your classroom.
The Geometric Optics
For a drop of water, suspended from a syringe, as we have here, the figure to the right traces the path into and then out of the lens. The ray through the center does not deviate – all other rays, however, do bend towards the normal as they pass from air, index = n1, to water, index = n2.
For small angels, such as we have here, we use Snell’s law – n1sin⊖1 = n2sin⊖2
This is a general equation used for very small angles, and is useful for tracing rays through complicated systems, which, despite our simple set up, is what we have here.
Now, when you take at how this general phenomenon of light passing through a spherical lens creates the magnificent magnification before you, things get significantly more complicated and may be beyond the scope of high school students. The second figure below is a simple ray diagram of the situation.
If you would like to explore the math behind this, please refer to the excellent paper by Gorazd Planinsic on which this project is based