Microscopes are used to show the internal structure of materials. They also give biologists/ physicist the power to magnify and distinguish objects that are not visible by the human eye.

Electron microscope

When light microscopes were used anything smaller than 250nm could not be seen, so the only possible solution was to use radiation that has a shorter wavelength than light. Better solution was to use electrons. They are negatively charged particles, which orbit the nucleus of the atom. When metals become very hot they lose electrons, because the electrons gain alot of energy. These electrons behave like electromagnetic radiation. They have a very short wavelength compared to light wavelength, so a greater resolving power is possible.

Electron microscopes are scientific instruments that use a beam of highly energetic electrons to observe objects on a very fine scale. Electron microscopes have a focused beam of electrons instead of light to have a "image" of the object, and gain information of its structure and composition.

How they work:

. A stream of electrons is formed and accelerated towards the object using a positive electrical potential.

2. This stream is a thin focused monochromatic beam.

3. This beam is focused onto the sample using a magnetic lens.

4. The electron beam interacts with the sample, affecting its focus.

Grass Hopper, viewed using an electron microscope

It's not possible to see an electron beam, so the electron beam is projected on to a fluorescent screen so that the image can be viewed. The areas that are hit by electrons are bright, giving an overall black and white image. The stains that are used to improve the contrast of biological patterns for electron microscopy contain heavy metal atoms, which stops the electrons going through. The fluorescent screen, the beam and the specimen must be in vacuum, because if the electrons collided with air molecules they would scatter, making it impossible to achieve a clear image. Water boils at room temperature in vacuum, so dehydrated specimens must be used, there fore leaving only dead material to be examined.

Two types of electron microscope are now in common use. Scanning electron microscope, the electron beam is showered back and forth across the surface of the specimen, which is coated by a metal coating. Electrons are deflected by the metallic surface, producing a clear 3d image of the surface. The advantages of this microscope are that the structures of the surface can be seen. A great depth of information is achieved, even using the same magnification and resolution as the light microscope such detail cannot be achieved because you would constantly have to keep on focusing up and down. The disadvantage of this is that it cannot have the same resolution as the TEM.

The transmission electron microscope (TEM) works by passing a beam of electrons through the specimen before it was viewed. Electrons that passed through the specimen are seen. This means that thin sections of the specimen are seen, such as the insides if cells.

The advantage light microscope over electron microscopes is its ability to observe living organisms and tissues. Various types of electron microscopes require the specimen to be coated with gold and placed in a vacuum chamber. What ever form of life is present will not survive under these conditions. Where as a light microscope allows once to observe living organisms.