Light microscopes, also known as optical microscopes, are typically used in the classroom. In a light microscope, a series of lenses are used to produce a magnified image of an object.
They are useful for observing the overall structure of cells. However, they are not good enough to see the details of subcellular structures (e.g. mitochondria).
In the 20th century, the first electron microscope was developed. Electron microscopes have allowed us to get a better understanding of sub-cellular structures, as they have a greater magnification capacity, a higher resolution and greater resolving power compared to light microscopes.
The image below is formed by an electron microscope.
Electrons do not have a colour spectrum like the visible light used to illuminate a light microscope. They can only be ‘viewed’ in black and white. In the diagram above, false colours have been added.
The diagram below shows an image taken with an electron microscope. Due to the higher resolution, we can see the nucleus in more detail, which isn’t possible with a light microscope.
Instead of light rays, the transmission electron microscope (TEM) uses an electron beam. The level of magnification and detail with the electron microscope is far superior to that of traditional light microscopes.
There are two types of electron microscopes:
1. Transmission electron microscopes (TEM)
2. Scanning electron microscopes (SEM)
The transmission electron microscope (TEM) fires a beam of electrons, which is transmitted through extremely thin sections of cells, to produce a highly magnified image. They are typically used to provide detailed images of the internal structures of cells.
The highest magnification that can be obtained from a transmission electron microscope is around ×1,000,000. However, the images can also be enlarged photographically. The limit of resolution of the TEM is now less than 1 nanometre (nm), which has revealed structures in cells that are not visible with the light microscope.
The scanning electron microscope (SEM) works by bouncing electrons off the surface of an object that has had an ultrathin coating of a heavy metal, usually gold. A narrow electron beam scans the object and images are formed by these scattered electrons.
The image below shows a scanning electron microscope
SEMs are typically used to provide a detailed 3D image of the surface of cells or tissue. Scientists generally use an SEM to analyse a large area of a sample. Because of this, the resolution is lower and the magnifications used are often lower than for TEM.
The electron microscopes reveal tiny sub-cellular structures that are not visible with the light microscope. It also shows structures in more detail.
The diagram below shows the limited visible range of electron microscopes and light microscopes.
Mitochondria are visible with the light microscope but cannot be seen in detail. Ribosomes, on the other hand, are only visible with the electron microscope.