More Bio notes from next lesson on Cells and Microscopes.

Before Robert Hooke first glimpsed cells of cork in 1665, scientists knew they needed more than the human eye to study objects and living things. Microscopes provide a passport to the unseen world of how cells work, and how the whole organism functions.

Magnification is the ability of a microscope to make an image appear bigger. Resolution is the ability to distinguish small, close objects. The resolution of a microscope refers to its ability to show details clearly. If it weren't for the resolution of a microscope, to the viewer, everything would be a blur, and not at all useful.

Light microscopes use a beam of light passing through one or more lenses to produce an enlarged image of the object or specimen being viewed. Microscopes that use sets of lenses are called compound microscopes. The ocular lens is placed near to the viewer's eye. The objective lens is positioned near the specimen being viewed.

Cells appear to be transparent, although there are some variations in thickness and density.

Microscopes using electrons instead of light to form images can magnify images at least 100 times as much as the light microscope. Because electrons would bounce off the gas molecules in air, the stream of electrons and the specimen to be viewed must be placed in a vacuum cleaner. Therefore, living cells cannot be viewed in an electron microscope.

The transmission electron microscope produces a stream of electrons that passes through a specimen and strikes a fluorescent screen. By replacing the fluorescent screen with a piece of photographic film, a photograph called a transmission can be made.

The scanning electron microscope enables biologists to see detailed three-dimensional images of cell surfaces. Specimens are not sliced but are placed on a small metal cylinder and coated with a very thin layer of...