The specimen is thin and flat so that only a single layer of cells is present. A compound light microscope shines a light through the specimen so if the specimen is thick light won't be able to pass through.\
Of all the techniques used in biology microscopy is probably the most important. The vast majority of living organisms are too small to be seen in any detail with the human eye, and cells and their organelles can only be seen with the aid of a microscope.
There are many different forms of microscopy but the one most commonly employed is “brightfield” microscopy where the specimen is illuminated with a beam of light that passes through it (as opposed to a beam of electrons as in electron microscopy). The general requirements for a specimen to be successfully examined using brightfield microscopy are:
That the cells and other elements in the specimen are preserved in a “life-like” state (this process is called “fixation”)
That the specimen is transparent rather than opaque, so that light can pass through it
That the specimen is thin and flat so that only a single layer of cells is present
That some components have been differentially coloured (stained) so that they can be clearly distinguished.
Because of the microscopy requirements, options for preparing specimens are limited to:
Whole-mounts, where an entire organism or structure is small enough or thin enough to be placed directly onto a microscope slide (e.g. a small unicellular or multicellular organism or a membrane that can be stretched thinly on to a slide)
“Squash” preparations, where cells are intentionally squashed or crushed onto a slide to reveal their contents (e.g. botanical specimens where cells are disrupted to reveal chromosomes)
Smears, where the specimen consists of cells suspended in a fluid (e.g. blood, semen, cerebro-spinal fluid, or a culture of microorganisms), or where individual cells have been scraped brushed or aspirated (sucked) from a surface or from within an organ (exfoliative cytology). Smears are the basis of the well-known “Pap test” that is used to screen for cervical cancer in women.
Sections, where specimens are supported in some way so that very thin slices can be cut from them, mounted on slides, and stained. Sections are prepared using an instrument called a “microtome”.
Get more helpful info about microscopy from sources below:
www.microscopyu.com
biologymad.com
www.leicabiosystems.com