Re: What is the greatest magnification that any microscope can give?

Greetings:

TYPES OF MICROSCOPES

Over the centuries many types of microscopes have been developed to aid 
human eyes to see very small objets. Since the device was first developed 
by Anton van Leeuwenhuek (1632-1723) many different types of OPTICAL 
MICROSCOPES have been invented, enabling magnifications more than one 
thousand times (1,000x). This was followed in 1931 by the first ELECTRON 
MICROSCOPE which has provided magnifications up to one million times 
(1,000,000x). Most recently, in 1981, the ATOMIC FORCE MICROSCOPE was 
invented which now provides magnifications up to one billion times 
(1,000,000,000x).

FUNDAMENTAL LIMIT OF RESOLUTION

The fundamental limit of optical resolution in conventional optical 
microscopes that we look through with human eyes is determined by the 
wavelength of electromagnetic energy that is used to illuminate the object. 
We cannot resolve objects or details that are smaller in dimensions than a 
light wavelength. Human vision spans from 720 nanometers (2.83 microinches) 
in the red wavelengths of light to 400 nanometers (1.57 microinches) in the 
blue violet wavelengths. Scientists typically use 560 nanometers (2.2 
microinches) as an average value for white light containing all colors of 
the rainbow. (NOTE: a microinch = one millionth of an inch, a nanometer = 
one billionth of a meter). 

HUMAN EYE RESOLUTION 

As a small object is moved closer to a human eye it appears larger with 
more detail because it is filling more of the light sensors in the eyes 
retina. The human eye has maximum resolution when an object is viewed as 
close to the eye as possible before it goes out of focus. This point is 
called the NEAR POINT or the POINT OF MOST DISTINCT VISION. This point is 
about 25 centimeters (10inches) from the typical unaided human eye and the 
angular resolution of the eye at this point is about 1/60 degree (.0167 
degree). This is equivalent to being able to resolve two fine human hairs 
spaced one hair width apart when placed at the point of most distinct 
vision . 

NOTE: a fine human hair is about 73 micrometers (29 microinches) in 
diameter. 

A fine hair is also about 130 wavelengths of light in diameter, so that 
human vision at it's best has an angular resolution 130 times less than the 
fundamental optical limit of resolution. This is why we use telescopes and 
microscopes to improve our ability to see more detail in objects located at 
distances farther and more close to the eye's near point and also to 
improve our ability to resolve images at the near point. The best designed 
optical instruments place their images at the eye's near point so that we 
can observe the greatest detail in these  telescopic or microscopic images, 
with the least eye strain, and improve our eye's resolution through the 
process of magnification. There is a beautifully illustrated web book on 
human visual perception located at the following URL: 

http://www.yorku.ca/research/vision/eye/

Also; in the Mad Science archives I have answered a question on resolution 
limits in which there are photographs that demonstrate the limits of 
resolution:

Subject: Re: image resolution limits
Date: Wed May 21 15:00:24 1997
Posted by Adrian Popa
Position: Staff Optical/Microwave Physics
Area of science: Physics 
ID: 863582649.Ph 

If the eye can only resolve dimensions of 130 optical wavelengths then a 
microscope with a magnification of 130x  would be required to see 
detail at fundamental limit of resolution (one wavelength) as 
described above. Of course trying to see hair thin objects spaced one hair 
with apart would cause an eye strain, so increasing the magnification up to 
about 1300x would make the objects 10 times larger, with less eye strain. 
However; at 1300x you will see no more detain in the image than what you 
would see at 130x. 

Thus in summary, optical microscopes operate with magnifications between 
10x and 1300x and magnifications greater than 130x  provide larger images 
but do not provide any more detail. I discuss optical microscopes with 
television displays in the answer referenced above; however, TV only makes 
the image larger and perhaps brighter, it will not increase the detail.

ELECTRON MICROSCOPES

The first electron microscope was built by Ernst Ruska and other German 
scientists 1931.  An electron microscope can resolve smaller features 
because the electrons in its beams have a much shorter wavelength than do 
beams of visible light. The wavelength of visible light is about 560 
nanometers (2.2 microinches) while the electron beams used in most electron 
microscopes have wavelengths of less than 0.1 nanometer and they can 
resolve objects about 1000 times smaller than an optical microscope, 
enabling magnifications of 1,000,000x without loss of detail.. However; 
human eyes cannot see at electron wavelengths so we need a television type 
screen or special photographic film to make electron microscope images 
visible to human eyes. You can see pictures of electron microscope 
equipment and images taken with them at the LEO Electron Microscopy Ltd. 
URL at:

http://www.mwrn.com/leo/sem.htm

There are two basic types of electron microscopes (EM), transmission (TEM) 
and scanning (SEM) which are discussed on the LEO web pages.

      
ATOMIC FORCE MICROSCOPES (AFM)

The AFM is latest and greatest type of microscope. This device was 
invented in 1981 by Gerd Benning and Heinrich Rohrer-who shared the Nobel 
prize with Ruska in 1986. New versions of the AFM are still being invented 
and the SCANNING TUNNELING MICROSCOPE  (STM) is the type that is the most 
developed to date. The STM can resolve images 1000 times smaller that 
electron microscopes giving magnifications greater than one billion times 
(1,000,000,000)! 

The following information is taken from the National Institute for 
Standards and Technology (NIST)  web site where the STM device is described 
and images of atoms can be seen:

http://physlab.nist.gov/GenInt/STM/stm.html

    
START QUOTE
"The scanning tunneling microscope (STM) is widely used in both
industrial and fundamental research to obtain atomic-scale images of    
metal surfaces. It provides a  three-dimensional profile of the
surface which is very useful for characterizing surface roughness,
observing surface defects, and determining the size and conformation of 
molecules and  aggregates on the surface. Several other recently developed 
scanning microscopies also use the scanning technology developed for the 
STM. The Scanning Tunneling Microscope depends on the quantum mechanical 
phenomenon of tunneling through a potential barrier. This device was 
invented in 1981 by Gerd Benning and Heinrich Rohrer-who shared the Nobel 
prize with Ruska in 1986. A tungsten probe with avery fine tip (even as 
small as one atom!) is held between 0.1nm and 1nm above a conducting 
surface. When a small potential difference is applied between the probe and 
the surface, an electron tunneling current flows through the vacuum between 
the tip and the surface. The position of the probe is controlled by a 
piezoelectric crystal (A piezoelectric crystal changes
its size when a potential difference is applied to it). As the probe slowly 
scans the surface, its vertical position is adjusted so that the tunneling 
current, and therefore the height above the surface, stays constant. The 
probe therefore traces the topography of the surface. The "image" is built 
up on a computer screen. -------

---------- Vertical resolution is an astonishing 0.001nm-much smaller than 
the size of a single atom! The best horizontal resolution archived so far 
is about 0.1nm."

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Best regards, your Mad Scientist
Adrian Popa