The Microscope - Microspy: Microscope science. Two factors play an important role in microspy: magnification and resolving power. Magnification is the comparison of the real size of a specimen with that of the one viewed under the microscope. Resolution refers to the clarity of the specimen viewed under the scope.
- Light microscopes have a maximum resolving power of 1500X . The wave length of light is 0.2um. This property of light makes it impossible to go beyond this magnification. Electron microscopes have shorter wave lengths 0.2nm and can magnify an object several 100,000 times.
Compound Light Microscope - Electron Microscopes:
The electron microscope was first developed in the 1950’s. Unlike optical microscopes which rely on light to provide the resolving power, they use an electron beam. An image is formed by the electrons being scattered to different extremes by the biological material.
The advantage of using electrons
In order to be able to see two objects separately, there must be a minimum distance between them. If they are closer together than this minimum, then the images will merge. This distance is known as the microscope’s limit of resolution. The smaller the limit of resolution, the greater the resolving power. For example, your eyes can only see the letters on this page, not the individual pixels. This is because the gap between them has a smaller limit of resolution than the resolving power of your eyes.
Resolution differs from magnification by the fact that magnification simply refers to the amount an object has increased in size.
The limit of resolution depends on the wavelength of the radiation passing through it. This means that no matter how good a light microscope is, it will always be limited by the wavelength of light.
Electrons behave like light waves , but have a much smaller wavelength. Resolving power is increased as the wavelength gets smaller, and as a result the electron microscope can resolve detail many thousand times better than the light microscope.
The 2 types of electron microscopes are the Transmission electron microscope or TEM and the Scanning electron microscope or SEM. - 1. TEM: Thin sections of a specimen are coated with a heavy metal. A beam of electrons are shot at the specimen. This beam is guided by electromagnets. A negative picture is produced. This microscope is used to detail the inner ultra structures of a cell.
- 2. SEM: Surface of a specimen is coated with gold. SEM’s again rely on electrons to create its image. However, instead of the electrons passing through the slide as in a TEM, a SEM bounces electrons of the surface of the object. This allows incredible 3 dimensional images to be created as seen below.
Limitations and benefits of electron microscopes
As can be seen from the picture above, the amount of detail produced on electron micrographs (EM) is immense. This is the main advantage they have over optical microscopes. In fact, it was only the advent of the EM which allowed many biological structures to be discovered.
There are disadvantages to the EM. Due to the fact that air in the analysis chamber would distort the electron beam, experiments have to be performed in a vacuum. This prevents any living material to be studied. This also then leads to the question, How realistic a picture can be gained by such a processed tissue sample? This point is open to debate. The other problem with EM’s is that they are very expensive, and are mainly confined to research laboratories.