How is the electron microscope different from a compound microscope?

Answer 1

Short Answer:

It is fair to say that a light microscope and an electron microscope use the same principles, but the technical details are enormously different.

The similar principles mean that both enlarge images of small object and have a lensing system to put an illuminating beam on a sample and another system to carry out the refocusing of the scattered beam into an enlarged image to be viewed.

Beam: An electron microscope uses an electron beam to illuminate a specimen and produce the image.

An optical or light microscope uses a light beam to illuminate a specimen and produce the image.

Lenses: Lenses in an optical microscope are glass, though special types of glass improve performance, it is fundamentally the same material as common glass. Electron microscopes employ magnetic fields and electron fields to guide and focus the electron beam. Electron microscope this do not have physical lenses.

Resolution: Electron microscopes have much better resolution and are capable of much higher magnification than light microscopes because the wavelength of the electrons is thousands of times smaller than the wavelength of light.

Light microscopes can typically resolve structures to a fraction of a micron compared to electron microscopes which in practice achieve resolutions of a few nanometers. Practically, electron microscope can have almost a thousand times greater resolution than an optical microscope.

Magnification: The useful magnification of an electron microscope is also in the range of a thousand times greater than an optical microscope.

Samples: Optical microscopes can view basically anything that one can put under the objective, though special dyes are often used to enhance features of the sample. Electron microscopes typically require the sample to be in a vacuum so the electrons are not scattered by air. Samples frequently require special treatment with a metal coating such as gold because the high energy electrons are not strongly scattered by small atoms such as carbon, oxygen and nitrogen that are common to biological samples.

The actual performance of any microscope depends on its design and lensing system and so significant variation exists in the above practical characteristics and performance of both types of microscopes can be enhanced in various ways.

More Specifics:

Electron microscopes use electrons to illuminate and form an image of a sample and light microscopes use light to illuminate and form an image of a sample.

A microscope of either type is characterized by its magnification and resolving power. The magnification depends on the lensing system and can be increased to any degree, but the maximum useful magnification is limited by the resolving power.

The resolving power of a microscope can not be better than the limits placed on it by the size of the wavelength of the illuminating beam. The smaller the wavelength, the smaller the structures that can be resolved in them image.

Visible wavelengths of light are a few hundred nanometers. An electron microscope operates with electrons accelerated to a few hundred thousand electron volts of energy and with a wavelength in the range of few hundredths of an Angstrom.

An electron microscope has a theoretical resolving power that is much greater than a light microscope and can reveal smaller structures because the electrons used have wavelengths (few hundredths of Angstroms) almost 100,000 times shorter than visible light (few hundred nanometers).

An optimized electron microscope can achieve a practical resolution of a few Angstroms and a useful magnifications in the millions of times.

A good light microscope can resolve structures smaller than a micron but is limited to about a few hundred nanometers resolution. The useful magnification of a light microscope is not much more than a thousand times.

The electron microscope uses electrostatic and electromagnetic fields to act as lenses to control and focus the electron beam and to form an image. An optical or light microscope employs glass lenses.