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Resolution of Microscopes
Name: Paul T.
Status: student
Age: 16
Location: N/A
Country: N/A
Date: 1999-2001
Question:
I was wondering why the electron microscope has a much
higher magnification(500'000X) compared to a light microscope(1500X). The
LM uses lenses to increase magnification but the electron microscope does
not have lenses. My lecturer told me that it has something to do with
resolving power. She said that since the resolving power of an EM is
0.5nm while the resolving power of a LM is 0.2um, the EM will perceive 2
pts as further apart and so the whole image will be enlarged. As I said,
I thought magn was achieved with lenses?
I have learnt that the resolving power of a microscope is defined as the
minimum distance by which 2 points must be separated in order for them to
be perceived as 2 separate points rather than a fused image. From my
understanding, the resolving power is the sharpness of a TV. but no matter
how sharp a 21-inch TV is, 2 points on it, say, about 1 mm apart may not
be perceived as separate, while the same points on a projection TV with
the same sharpness may be seen as separate. Can anybody pls
explain/clarify? Thanks. Sorry for the lengthy question.
Replies:
Paul -
The short and simple answer to your long question is that the wavelength of
light is greater than the wavelength of an electron. The shorter wavelength
allows you to resolve points that are closer together.
Larry Krengel
Paul,
Besides the influence of lenses -- be they glass in an optical microscope or
magnetic like those in an electron microscope -- the resolving power of a
microscope is a function of the wavelength of the "light" used to illuminate
the specimen being examined. The shorter the wavelength, the better the
resolving power. The wavelength of blue light is shorter than red. Thus,
resolving power is improved on any optical microscope if the light is more
blue
than red. The wavelength of electrons are much shorter than that of light of
any color. For that reason and a few others, the electron microscope holds the
advantage over an ordinary light microscope. However, not all specimens can be
examined under an electron microscope. The why of this is the subject of
another discussion.
Regards,
ProfHoff
I have learnt that the resolving power of a microscope is defined as the
minimum distance by which 2 points must be separated in order for them to
be perceived as 2 separate points rather than a fused image. From my
understanding, the resolving power is the sharpness of a TV. but nomatter
how sharp a 21-inch TV is, 2 points on it, say, about 1 mm apart may not
be perceived as separate, while the same points on a projection TV with
the same sharpness may be seen as separate. Can anybody pls
explain/clarify? Thanks. Sorry for the lengthy question.
Vince Calder
The resolution limit depends on the wavelength and the quality of
lenses. With perfect lenses, the achievable resolution is roughly
equal to the wavelength. Visible light has a wavelength of around .5
micron; the electrons used in an electron microscope have such a small
wavelength that it is not the limiting factor.
Tim Mooney
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Update: June 2012
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