How a Compound Light Microscope Works

How a compound light Microscope works 

To comprehend how the compound light microscope (also called a "bright field" microscope) works we must first
understand that convex lenses bend light rays in a peculiar
manner  so that light hitting the center of the lens goes
straight through. But light hitting other areas is bent toward
a focal point. This bending allows the view at a specific
distance from an object to see the image as larger than it
would appear to the naked eye.
In the 1590s Zaccharia and Hans Janssen came up with the
idea of lining up two convex lenses. The image at left shows
this principle in action. Light is first emitted by the light
source and is directed by the condenser lens on to the
specimen, which might be a loose object, a prepared plate
or almost anything. A microscope can even be applied to
small parts of larger objects, though with a bit more
difficulty. (The light does not absolutely need to originate
below the specimen.)
The light from the specimen then passes through the
objective lens. This lens is often selected from among three
or four and is the main determinant for the level of
magnification. It bends the light rays and in the case of this
example sends them to a projector lens , which reverses
their direction so that when the image reaches the eye it
will not appear "upside-down". Not all microscopes have a
projector lens, so the viewer may be seeing a reverse
image. In these cases, when the slide is moved, it will
appear to be moving in the opposite direction to the viewer.
The light rays then travel to the oracular lens or "eye piece".
This is often a 10X magnification lens, meaning it magnifies
the magnified image an additional ten times. The image is
then projected into the eye. It is very seldom that a
specimen is in focus the moment it is placed beneath a
microscope. This means that some adjustment will have to
be made. Unlike in telescopes, the focal length between
lenses remains constant when adjusting the focus. The lens
apparatus is brought closer to or further from the object.
The focus adjustment is often along the neck of the tube
containing the lenses, but it might just as well move the
slide up and down. The best way to make this adjustment is
to make a course adjustment so that it is too close to the
object and then back off with the fine adjustment . This
helps to ensure that the specimen is not inadvertently
smashed by the lens.
A microscope may have either one or two oracular lenses, it
may even project the image on a computer screen. With
binocular microscopes, the eye pieces will often need to be
adjusted for each person who uses the device. The distance
between eyes will be compensated for by an adjustment that
moves the lenses closer together or farther apart. Difference
in the strength of vision between individual eyes may also
be adjusted for at the eye-pieces.