Microscopes give us a large image of a tiny object. The microscope works
a lot like a telescope except that the object is very close to the objective
lens.
A
microscope is basically a two-part magnifying glass. Most of the time, when we
refer to a microscope, we mean a compound microscope or an objective and an
eyepiece mounted in a tube. The objective is the part that you can think of as
a magnifying glass, or lens. The eyepiece is also kind of like a magnifying
glass, but smaller, and serves to direct the light straight into your eye. In a
typical compound microscope, these two lenses are mounted on either end of a
tube like this:
… where the
eyepiece is located on the "eye end" of the figure and the objective
is located at the "object end" (see the curved lens held in place by
two black holders towards the object end of the tube?). That's really all there
is to it! The objective lens focuses a magnified image of the object at exactly
the focal length of the eyepiece lens. The magnified image is then directed by
the eyepiece straight into your eye.
Let's put a tiny organism (a
"daphnia" or water flea) we found in pond water on a glass slide. The
clips on the microscope's flat stage hold the slide in place. Now take a look
through the microscope eyepiece. A mirror at the bottom of the microscope
reflects light rays up to the daphnia through a hole in the stage. Objective
lenses magnify the image which is made even larger when we see it through the
eyepiece lenses.
The number marked on the objective lens
tells us how many times the organism we have on the slide is being magnified.
It says 40x. Wow, that means the tiny daphnia is forty times larger than life.
To focus we can move the objective lens closer to or farther from the organism.
The objective lens is usually a compound
lens, a combination of two lenses made from different kinds of glass. When only
one lens is used, we often get distortion. This distortion (chromatic
aberration) is caused because the colours making up light are not refracted
(bent) the same amount when passing through a glass lens. When we use a
compound lens, any distortion from the first lens is corrected by the second
lens.
Electron
microscopes use beams of electrons instead of light. The electron beam is moved
around using magnets which act like the lenses in an ordinary microscope.
Electron microscopes can magnify objects over 200,000 times. Using a scanning
tunneling microscope (STM), scientists have actually been able to look at
atoms, minute units of matter. The computerized STM moves above a material.
Electrons jump from the tip of the STM to the surface of the material being
scanned. This creates an electric charge. The electric current changes as the
tip is moved across the material. By moving the tip back and forth across the
material, the computer can map the electron clouds (and the atoms).
Different
types of microscopes have been used to look at human cells, identify minerals,
solve crimes, see how freezing affects food, study metals, and find the causes
of crop diseases. Microscopes are an essential tool in medicine too. They have
been used to identify the causes of many deadly diseases like malaria and
tuberculosis. Microscopes can also help to find out why a person or animal
died.
Scientists can even use a microscope to
figure out where illegal drugs come from. For example, looking at opium
crystals through a microscope reveals different shapes depending on where the
poppies they came from were grown. This information can help pinpoint the
source of illegal drugs.
You can do all kinds of amazing things
with a microscope. The possibilities for what you can find out with a
microscope are endless. And who knows you might identify something new and it
may even get named after you!
http://www.yesmag.bc.ca/how_work/microscope.html, http://www.madsci.org/posts/archives/mar2001/984513997.Eg.r.html