Electron microscopes

How have light microscopes developed?

Replica of Robert Hooke's compound microscope, 17th century
A replica of Robert Hooke's compound microscope, 17th century

Romans in the First Century were the first to make glass. Since then, scientists have been trying to magnify objects.

No-one knows who first invented the microscope but there have been key stages as it developed:

  • 1590s - Dutch spectacle makers, Janssen, experimented by placing lenses in tubes. They made the first compound microscope. None of these microscopes survived, but it is thought that they magnified from x3 to x9.
  • 1650 - British scientist, Robert Hooke, observed and drew 'cells' using a compound microscope. Though he used the term cell he did not know they were cells.
  • Late 1600s - Dutch scientist Antonie van Leeuwenhoek constructed a microscope with a single spherical lens. It magnified up to x275 and allowed van Leeuwenhoek to be the first person to describe bacterial cells.
  • 1800s - the optical quality of lenses increased and the microscopes were similar to ones used today. This allowed the scientists Schleiden and Schwann to further study cells and propose cell theory.
Antonie van Leeuwenhoeks first microscope, 17th century
Antonie van Leeuwenhoek's first microscope, 17th century

Throughout their development, the magnification of light microscopes has increased but very high magnifications are not possible. The maximum magnification with a light microscope is around x1500.

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 is also important.

The ability to see an image in greater detail depends on the resolution or 'resolving power' of the microscope. Resolution is the ability to see two points as separate points, rather than seeing them as a single point.

Think about a digital photo. It can be enlarged but over a certain size, you won't be able to see any more detail. It will just become blurry.

The maximum 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  \frac{1}{1~000~000~000} or in standard form as 1 × 10-9 m.

The resolution of the light microscope is limited by the wavelength of light.

Light microscopes also have a shallow depth of field. When the light microscope is focused on the structure to be examined, the objects above and below it appear blurred.

The electron microscope

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

There are two types of electron microscope:

  1. The scanning electron microscope (SEM) has a large depth of field so can be used to examine the surface structure of specimens. SEMs are often used at lower magnifications than transmission electron microscopes (TEMs).
  2. The TEM is used to examine thin slices or sections of cells or tissues.

TEMs have a maximum magnification of around ×1 000 000, but images can be enlarged beyond that photographically. The limit of resolution of the TEM is now less than 1 nm.

The TEM has revealed structures in cells that are not visible with the light microscope but an electron microscope can only examine dead tissue.

A human lymphocyte white blood cell seen through a microscope
A human lymphocyte white blood cell as seen under a transmission electron microscope

Cell Theory

The scientists Schleiden and Schwann observed plant and animal tissues under the microscope and both described similar cellular structures. They developed 'cell theory' as a result of their studies and proposed that cells are the basic unit of life. However, they were unaware of cell division and thought that new cells formed in a similar way to crystals.

Further advancements of the microscope allowed scientists to study cells in detail and build on the work of Schleiden and Schwann. Cell theory now states:

  • all living things are made of cells, they are the basic units of life
  • new cells are formed from cells which already exist by a process of cell division and hereditary information is passed on from cell to cell during this process
  • the chemical reactions that create life occur within cells and they all have the same basic chemical composition