Re: What is the smallest living thing that you can see with the human eye?

Dear Chelsea,
Your question is tricky to answer for two reasons. 

First, to be able to "see" something small means that you can distinguish it 
from another thing. For example, imagine you are at the beach.  You pick up 
grains of sand, and you can easily see each grain, even though they are small.  
This is called "resolution" - you can "resolve" the grains of sand, or see each 
one.  But, if you look down the beach, you can no longer see each grain of sand, 
or resolve the grains. Here is another example: when you look up at a star, you 
might actually be seeing two stars.  The light looks like it is coming from one 
star, because you can not "resolve" the two stars.  Small living things, like 
yeast, can be very difficult to resolve (they grow close together), even with a 
magnifying glass- you need a microscope.  According to physics, the smallest 
thing the human eye can resolve at a meter distant is about 1/10 of a 
millimeter. If you hold it close up to your eye, you may be able to see 
something as small as 4/100 of a millimter, or 40 microns.

The second problem is that many small living things, like protozoa, are also 
transparent.  Maybe someone you know has dropped a contact lens and tried to 
find it.  Even though contact lenses are big (about a centimeter) they are very 
hard to see, because they are clear.  They do not reflect much light.  Small 
clear protozoans are very very hard to see. Protozoa range in diameter from a 
few thousandths of a millimetre to several millimetres.  But, if you stain them 
purple with a dye, you can easily see them.

See great pictures of protozoa:
http://www.jracademy.com/~mlechner/Protozoa.html

Hope this helps.
Erin