Microscope: a device for magnifying objects that
are too small to be seen with the naked eye.
- Simple microscope: single lens magnifier
- Compound microscope:
employs two or more lenses
Parfocal: the objective lenses are mounted on the
microscope so that they can be interchanged without having to appreciably
vary the focus.
Resolving power or resolution: the ability to distinguish
objects that are close together. The better the resolving power
of the microscope, the closer together two objects can be and still
be seen as separate.
Magnification: the process of enlarging the size
of an object, as an optical image.
Total magnification: In a compound microscope the
total magnification is the product of the objective and ocular lenses
(see figure below). The magnification of the ocular lenses
on your scope is 10X.
| Objective lens X Ocular lens = |
Total magnification |
| For example: |
low power: |
(10X)(10X) = 100X |
| |
high dry: |
(40X)(10X) = 400X |
| |
oil immersion: |
(100X)(10X) = 1000X |
Immersion Oil: Clear, finely detailed images
are achieved by contrasting the specimen with their medium. Changing
the refractive index of the specimens from their medium attains this
contrast. The refractive index is a measure
of the relative velocity at which light passes through a material. When
light rays pass through the two materials (specimen and medium) that
have different refractive indices, the rays change direction from
a straight path by bending (refracting) at the boundary between the
specimen and the medium. Thus, this increases the image’s
contrast between the specimen and the medium.
One way to change the refractive index is by staining the specimen. Another
is to use immersion oil. While we want light to refract differently
between the specimen and the medium, we do not want to lose any light
rays, as this would decrease the resolution of the image. By
placing immersion oil between the glass slide and the oil immersion
lens (100X), the light rays at the highest magnification can be retained. Immersion
oil has the same refractive index as glass so the oil becomes part
of the optics of the microscope. Without the oil the light
rays are refracted as they enter the air between the slide and the
lens and the objective lens would have to be increased in diameter
in order to capture them. Using oil has the same effect as
increasing the objective diameter therefore improving the resolving
power of the lens.
