Electron microscopes

The limits of the light microscope

The magnification of a microscope is not the only factor that is important when viewing cells. The detail that can be seen, or resolution, is also important.

The ability to see greater detail in an image depends on the resolution or resolving power. This is the ability to see two points as two points, rather than merged into one. Think about a digital photo. It can be enlarged, but over a certain size, you won’t be able to see any more detail.

The resolution of a light microscope is around 0.2 μm, or 200 nm. This means that it cannot distinguish two points closer than 200 nm. One nm, or nanometre, is one billionth of a metre. This is written as \text{1/1 000 000 000} or in standard form as 1 × 10−9 m.

The electron microscope

Electron microscopes use a beam of electrons instead of beams or rays of light.

Living cells cannot be observed using an electron microscope because samples are placed in a vacuum.

There are two types of electron microscope:

  • the transmission electron microscope (TEM) is used to examine thin slices or sections of cells or tissues
  • the scanning electron microscope (SEM) has a large depth of field so can be used to examine the surface structure of specimens

TEMs have a maximum magnification of around ×1,000,000, but images can be enlarged beyond that photographically. The limit of resolution of a TEM is now less than 1 nm. The TEM has revealed structures in cells that are not visible with the light microscope.

SEMs are often used at lower magnifications (up to ×30,000). The limit of resolution of a SEM is lower than that of a TEM (approximately 50 nm).

A picture of a white blood cell taken with an electron microscope
A human lymphocyte white blood cell as seen with a transmission electron microscope