The Light Microscope

The Light microscope.

The following picture is a ‘light microscope’ common to all science laboratories.

Microscopes have allowed scientists an open window to the structure of cells and their function. The first microscopes were devised in the 17th century, and revolutionised scientific theory. They allowed scientist to put aside guess work and allowed delivery of evidence for the first time that large organisms are made of cells.

Cells were first viewed some 300 years back, by Robert Hooke, a famous scientist and nobleman of his generation. He assembled one of the first optical microscopes and his work examining cork samples spawned the use of the term cell. ‘Hooke saw that these sections were many up of many tiny regular compartments, which he called cells.’ (Fullick A 1994 p7).

(Source:http://www.tutorvista.com/light_microscopy_labels_365~97).

It wasn’t until the 1840’s that ‘Cell theory’ was proposed. Matthias Schleiden and Theodor Schwann considered cells to be the basic unit of life. They jointly proposed living organisms were made up of individual cellular units.

Microscopes work by magnifying images of made specimens to allow details to be viewed which ordinarily would be out of the range of the naked eye. Magnification is simply how much bigger an image is compared to the specimen and a simple formula to calculate this is:

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(Source:http://www.tutorvista.com/light_microscopy_labels_365~97).

Size of image

Magnification= Real size of image

For example look at the rectangle (a) drawn below and rectangle (b) besides it. Rectangle (b) is just the same rectangle but it has been magnified with the aid of a magnifying lens.

Actual size rectangle (a) Seen under a lens rectangle (b)

To find the magnification level shown in (b), first measure the length of the actual size of the rectangle (a), it measures 20mm. Then measure the enlarged image (b) it measures 80mm in length. If we transfer our number to the above equation we can work out a magnification of the rectangle in (b).

Size of image (b) = 80

Magnification= Real size of image (a) = 20 = 4 times magnification.

Key to clarity depends upon the resolution. It’s the resolution which allows us to distinguish between the different matter in a cell. It relates to how detailed an image is, a lower resolution may only allow single objects to be viewed, but distinction between objects can be made when clarity is enhanced by a higher resolution.

The light microscope as the name suggests uses light in connection with lenses to magnify objects. Through the process of refraction objects appear larger. ‘Lenses refract light rays and produce a magnified image of an object.’ (Jones, M 2008 p1).

Because of their size light microscopes are very portable and can be used anywhere. They are fairly straight forward to set up and use, and are found in most learning institutions. Preparation of material is relatively quick and a novice can function one with few instructions. Light microscopes are relatively inexpensive to purchase and operate too. One benefit of such a microscope is that it shows natural colours when images are viewed. Another advantage is they are unaffected by magnetic fields, which means there is no restriction in their use. Though there are some drawbacks we can identify, mainly the obvious one being they have a low resolution when compared to electron microscopes. ‘They have a maximum resolution of about 0.2 micrometres (μm).’ (Barnett, G et al 2006, p10).

The maximum magnification though is limited to x1500 magnification, which is quite useful but has limitations when looking to explore deeper.

The human eye cannot see objects smaller the 0.1mm in diameter. The resolution of light microscopes isn’t good enough to show objects smaller than 0.2μm. Because the resolution of the light microscope is limited it means objects smaller the 0.2μm cannot be viewed, in terms of cell structure it means under maximum magnification a light microscope will only manage to identify several of the cells organelles. By comparison an electron microscope can pinpoint all the organelles of a cells make up. There are however some cells very visible to the naked eye, take for example a simple birds egg, ‘unfertilised birds eggs are single cells’. (Fullick 1998 p7).

So therefore by definition the largest birds eggs are the largest cells visible, (an ostrich egg is vast). Another example is an amoeba, it can just about be made out with the naked eye when floating in liquid, Jones, M (2008) explains that amoeba cells are ‘unusually large for a cell’, though we know all the processes for its life are carried out within that one cell. It is a unicellular organism, therefore you are looking at a cell.