Microscopy

Types of microscope

There are three main types of microscope:

• The light microscope directs light waves through a sample (or reflects light off the surface of a live sample) into the eye of the observer. The benefit of this type of microscope is that in many instances a true colour image is produced and live organisms can be viewed. However, they have a relatively low magnification and resolution since the wavelength of the light is relatively long. To prepare a sample to be viewed with a light microscope a thin section should be cut. This is then placed on a microscope slide with a stain. A coverslip is lowered on the sample using a mounted needle, which helps to reduce the likelihood of artifacts being present.

A light microscope.

• The transmission electron microscope requires specimens to be fixed in wax and thinly sliced. A beam of electrons is passed through the sample, the level of electron disturbance is then detected and displayed on a screen. A false colour is finally added to highlight certain aspects of the sample. This method of microscopy has the highest level of magnification and resolution (again because of the very short wavelength of electrons) but requires the samples to be dead, cannot give a true representation of the surface or colour of the sample and can occasionally cause distortion as a result of the preparation process.

• Scanning electron microscopes bounce electrons off a sample that has been 'fixed' in a heavy metal, the electron scatter is detected and a detailed image showing the 3D structure of the sample is displayed on a screen. A false colour may be added to highlight certain aspects of the sample. This method has a higher magnification and resolution than a light microscope because electrons have a much shorter wavelength than light but requires the samples to be dead and can only show the surface of a sample, or the centre of a sample if it has been 'sectioned'.

Magnification calculation

Once a sample has been viewed, it is important that scientists can calculate the actual size of the specimen under study. To do this the following calculation can be used:

Actual size = Image size / Magnification

Often, millimetres need to be converted into micrometres (µm; 1/1000th of a millimetre) or nanometres (nm; 1/1000000th of a millimetre) to make the numbers more manageable.

Definitions