Light Microscope vs Electron Microscope

Both light microscopes and electron microscopes use radiation (light or electron beams) to form larger and more detailed images of objects (e.g. biological specimens, materials, crystal structures, etc.) than the human eye can produce unaided. (See also: What is eyesight ?)

An electron microscope is a microscope that uses beams of electrons instead of rays of visible light to form highly magnified images of tiny areas materials or biological specimens. Comparing light vs electron microscopes is made more complicated by the fact that there are different types of electron microscopes. The two main types of electron microscope are the Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM). Other types of electron microscope include the Scanning Tunneling Microscope (STM) and Field Emission Transmission Microscope (FE-TEM).

Brief notes, see the tables lower down this page for figures, explanations and comments:

Similarities:

  • Form larger (magnified) and more detailed (highly resolved) images of small objects or small areas of larger objects e.g. a leaf, part of a bone, etc. than can be formed by the human eye.
  • Used in study and research in biology and medical sciences (more about histology), material sciences e.g. metallurgy and other aspects of science.
  • Specimens must be carefully prepared using techniques appropriate for both the equipment and the sample e.g. slicing, staining, mounting, etc. (e.g. how to prepare histology slides).

Differences:

  • Size: Light microscopes are smaller and lighter, so are easier to move and set-up.
  • Cost / Availability: Light microscopes are less expensive than electron microscopes.
  • Radiation Type: Light microscopes use light (approx wavelength 400-700 nm), electron microscopes use beams of electrons (approx equivalent wavelength 1 nm).
  • Control of image formation : Light via glass lenses, beams of electrons can be focused using electromagnets due to negative charge on electrons.
  • Resolution*: Electron microscopes have much higher resolution than light microscopes
  • Magnification*: Electron microscopes have much higher magnification than light microscopes
  • Colour Images: Light microscopes form images including the range of wavelengths (colours) provided by the light source - but remember that the colours seen are often due to stains rather than the actual colours present in nature). Electron microscopes produce greyscale (sometimes called "black and white") images. However, "false-colour" electron micrographs are common - and can be very beautiful!
  • Preparation of specimens : Generally involves harsher processes, e.g. using corrosive chemicals, for viewing via electron microscope than preparation of slides for viewing using a light microscope. Therefore more skill required - both to prepare specimens and to interpret EM images (due to "artefacts" in images, artefacts = features in images that are not present in the specimen but rather are due to the processes used to prepare the specimen and produce the image)
  • Image Formation : Light microscope images can be viewed directly. Electron microscopes require use of a fluorescent screen, photographic plate or electronic display because electrons cannot be observed directly by the human eye.
  • Usage Limitations : Living specimens cannot be viewed using electron microscopes because electron microscopes require there to be a vacuum in the tube - otherwise the electrons would be absorbed by air molecules.

* Differences due to the shorter equivalent wavelength of electron beams compared with visible light.

See the table at the bottom of this page for the advantages and disadvantages of light vs electron microscopes.

The following simple block diagram shows some of the basic similarities between light microscopes and electron microscopes (in general) by comparing the radiation pathways for a light microscope with a general electron microscope.

Very simple block diagrams of Electron vs Light Microscopes

Diagram comparing a light microscope with an electron microscope - simple block representation rather than full ray diagram

Compare Electron Microscopes vs Light Microscopes (Physical Aspects)

As there are different types of electron microscopes and capabilities vary the following comparison of electron microscopes with light microscopes is very general (including 'typical' rather than exact figures).

 

Light Microscope

Electron Microscope

Radiation Type

Visible light

Beams of Electrons

Approx. wavelength of radiation

400–700 nm

effective wavelength < 1 nm

Radiation focussed by ...

Lenses - usually glass lenses

electromagnets

Image formed by ...

Light (of all colours whose wavelengths are supplied by the source of illumination) scatters from the various parts of the specimen and some of that scattered light reaches the objective lens (above the specimen) and is then re-directed through the objective and eyepiece lenses to form a focussed image.

Re. TEM: Selective absorption of electrons by the specimen. (There must be a vacuum inside an operating electron microscope because otherwise the electrons would be absorbed by air molecules before they could reach the specimen.) Parts of the specimen absorb electrons and therefore appear dark on the micrograph, while other areas of the specimen allow electrons to pass through - causing those areas to appear bright on the micrograph.

Image formed on ...

The human eye can view a real image directly by looking into the eyepiece of the microscope. An image is then projected onto the retina of the eye.

An image is formed by projecting a focussed image of the specimen onto a surface coated with electron-sensitive compounds. It may be labelled "fluorescent screen" on simple diagrams.

Typical resolution

 

 

200 nm

0.5 nm is the figure cited in AS Biology books

1 nanometre is 10-9m so
200 nm = 200 x 10-9m = 2 x 10-7m

1 nm (1 nanometre) = 1 x 10-9m, so
0.5 nm = 0.5 x 10-9m = 5 x 10-8m

Compare the resolution of an electron microscope with that of a light microscope:


resolution of electron microscope


=

0.5 nm


=

1

resolution of light microscope

200 nm

400

Typical useful magnification

less than x2000 usable image quality
"at best" x1500 useful image quality
typically up to x1000

approx x100,000 in SEM
approx x250,000 in TEM

Compare the magnification of an electron microscope with that of a light microscope:


magnification of electron microscope

=

100,000x

magnification of light microscope

1000x

Above: Table comparing the physics of light vs electron microscopes

Compare the Advantages and Disadvantages (i.e. limitations of use) of Electron Microscopes vs Light Microscopes

Due to constraints imposed by the ways in which different types of microscopes produce images, some microscopes can be used in certain ways that others cannot. For example, it is not possible to observe individual molecules using a light microscope or to watch living processes happen using an electron microscope.

 

 

Light Microscope (LM)

Electron Microscope (EM)

1.

Living cells & tissues

Can watch living processes take place e.g. microscopic pond life in action, and even cell division.

Not possible to view any living material due to vacuum inside EM

2.

Thickness of specimen

Specimen must be thin but can adjust focus to different positions (heights) within thin specimen on glass slide

Very thin sections only in TEM
Images surfaces (only) in SEM

3.

Depth: 2D or 3D ?

Image plane approx "flat" (2D) but, as above, can adjust focus through specimen

2D only in TEM ;
SEM images surfaces - hence shows depth info that seems like 3D

4.

Specimen preparation
/ artefacts

Simpler preparation (staining still required)

Harsher preparation procedures incl. use of corrosive chemicals that may cause "artefacts" in the resulting micrographs

5.

Magnification

Lower Magnification

Higher Magnification - so several micrographs may be needed per specimen

6.

Resolution

Lower Resolution

Higher Resolution - good for measuring sizes of smaller features

Above: Table comparing use of light vs electron microscopes

See also ... Light Microscope, Transmission Electron Microscope, Scanning Electron Microscope.

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