Microscopes

A level Biology – Microscopes

This article is about microscopes at A level.

At GCSE you would have studied cells using a light microscope. At A level you will learn that these microscopes have limitations. Although we can produce microscopes with more powerful lenses to enhance the current magnification, we cannot improve resolution. Resolution or resolving power is the ability to distinguish between two separate points that are close together. To solve this problem at A Level we use Electron microscopes. There are two main types scanning electron microscopes and transmission electron microscopes. Electron microscopes have a significantly higher resolution than a traditional light microscope. The reason for this is that they use an electron beam which has a shorter wavelength compared to a beam of light which has a longer wavelength. Electron microscopes use condensing electromagnets to direct the beam of electrons onto the sample. The electromagnet is under a vacuum, otherwise the air molecules would stop the passage of electrons. The consequences of this is that the sample needs to be dead rather than alive. Due to the sample prep procedure you can introduce “artefacts” into the slide which wouldn’t normally be present in the sample.

Electron microscopeA typical electron microscope which is normally linked to a computer and the image can be viewed on this.

© Franz Pfluegl | Dreamstime.com

Transmission Electron Microscopes

Transmission electron microscopes use an electron beam that passes through the sample. Therefore, the sample has to be very thin. It does have extremely high resolution about (0.2nm AQA and OCR) and (Edexcel 5nm). Atoms have a typical size of about 9-10nm. So this shows you how high the resolution is. As the beam passes through the sample there is a plate on the other side that will detect the electrons that strike it. Where the sample is less dense, more electron pass through and strike the plate in these regions of the plate it becomes brighter. Where the sample is denser, fewer electrons pass through and so these parts of the plate become darker. This will produce a photomicrograph. The magnification possible with a transmission electron microscope is around x500000.

Scanning Electron Microscopes

When using a scanning electron microscope the sample doesn’t need to be as thin as the electrons don’t pass through the sample, but instead they strike the surface. This causes the build up of a 3d structure. Often the images from a scanning electron microscope are coloured by false colouring. Scanning electron microscopes do not have as high a resolution as a transmission electron microscope. However, it is still higher than a light microscope. The resolution for a scanning electron microscope is about x100000

Magnification.

As well as knowing the different types of microscopes you will need to convert different types of measurements and be comfortable with the magnification formula.
I have included a you tube tutorial that I have wrote on magnification: