The invention of the microscope played a vital step in the progression of science as a whole and has allowed scientists to see structures not visible to the naked eye. Cells were first seen in 1665 by Robert Hooke and scientists have since been able to discover the existence of microorganisms, study the structure of cells and see smaller structures of plants than ever before. With the improved electron microscope developed in the 1930’s, we can now see the structures inside cells, with some microscopes able to resolve objects to less than a nanometre apart. Several factors affect how clearly a specimen can be seen with a microscope.
Magnification is the ratio of an objects image to its real size. Magnification is dependent on the lens and eyepiece of a microscope.
Resolution is a measure of the clarity of an image. It is the minimum distance two points can be separated and still be distinguished clearly as two points i.e. the amount of detail. Resolution is limited by the wavelength of light or radiation used to view a sample.
Clarity is dependent on the focus, contrast & brightness of an image. Focus depends on the focal length of the lenses, contrast is the relative difference between the light and dark areas of an image and brightness is how light or dark the image is overall.
The optical (light) microscope was the first type of microscope invented and uses visible light to view a specimen. The specimen is usually stained with a coloured dye then mounted on to a slide. The light is directed through the specimen on to an objective lens, where it then travels to the eyepiece to be viewed. The focal length is the distance between the objective lens and the eyepiece. One advantage of the light microscope is that specimens can be living or dead.
A transmission electron microscope uses a beam of electrons to highlight the specimen, as electrons behave like waves and are easily focused and detected. The advantage of electron microscopes is that organelles inside specimens can be seen, however sample preparation is a lengthy task as the specimen must be dehydrated, stained and fixed to allow the electrons to pass through without any interference from water molecules, which decrease focus. CLARITY RESOLUTION MAGNIFICATION
ELECTRON MICROSCOPE Salts are generally used to highlight organelles of the specimens in electron microscopy which will help make areas lighter, improving contrast. An electron microscope has three separate lenses therefore the focal length is increased, meaning a greater level of focus. A scanning electron microscope has a resolution of around 10nm A transmission electron microscope can have a resolution of 0.2nm.
The resolving power of an electron microscope is higher than a light microscope as a beam of electrons has a wavelength of less than 1nm so can resolve smaller specimens and structures. Resolution is maintained with further magnification. An electron microscope