http://www.ck12.org Chapter 23. Quantum Physics
The electron microscope
Proof of De Broglie’s hypothesis meant that wavelengths much smaller than light could be used to probe ever smaller
and smaller structures. Using wavelengths larger or comparable to the size of an object you wish to see produces
blurry, out-of-focus images. For example, the wavelengths of visible light are in the range 400 nmto750 nm. Trying
to observe objects smaller than the size of these wavelengths is not possible. The limit to which an optical instrument
can focus properly is called the resolving power of the instrument. An analogy may be helpful to understand the
role of wavelength and resolving power. Imagine closing your eyes and using your palm to “recognize” a chair, by
touch. It will probably take only a moment to realize the object is a chair of some sort. The overall pattern will
be recognizable to you. However, if the chair has some intricate patterns carved into the wood, your palm will not
provide a distinct “image” of the pattern. Should you be permitted to use your finger rather than only your palm,
you will be able to perceive the finer features of the chair. In the first instance, your palm was much smaller than
the overall pattern, and so you were able to resolve the image (in your mind) of a chair. However, the finer details,
being smaller than your palm, were lost or at best blurry to you. Perhaps your palm felt a slight indentation. By
using the smaller probe of your finger, smaller details of the chair could be recognized. So, analogously, the smaller
the wavelength, the better the resolution.
Since matter waves of electrons have such small wavelengths, they can be used to resolve much finer images than
visible light. It was not long after the first successful experiments confirming the existence of matter waves that
scientists realized that better microscopes could be built, which used electron matter waves rather than light waves.
The first electron microscope was built in 1931.
Figure23.9 shows an image of an ant taken with an electron microscope. Electron microscopes can produce clear
images with magnifications millions of times greater than visible light.
FIGURE 23.9
An ant seen through an electron micro-
scope.Electron microscopes have many uses. They are an important diagnostic tool in medicine, a research tool in the field
of biomedicine, and play a vital role in the analysis of materials, where defects and strengths of materials can be
found and analyzed. The role of the electron microscope in science and industry cannot be overstated.
- The Planck equation relates the energyEof a photon of light to its frequencyf
E=h f
where the constanth= 6. 626 × 10 −^34 J−sis known asPlanck’s constant.