History of Imaging – Part 3
Let’s rewind a little bit and go back to the tricky nature of light. If we think of light as a wave then we know that it has a certain wavelength – microscopes are based off of the idea that light rays will bounce off of objects and that their image will be magnified; but what happens if you try to bounce light off of something that is smaller than a wavelength of light? It’s analogous to throwing a basketball (our wavelength) at a smaller baseball (our object) and expecting something basketball sized to bounce off the baseball. The microscopes at that time, then, couldn’t resolve objects smaller than the wavelength of light.
This is a rather unfortunate predicament to be in because, as said in the previous post, science is empirical and usually reductionist in method. We are able to observe matter to a certain size and then, suddenly, our observation stops. As the twentieth century rolled around – this problem was solved.
The 1901 Nobel Prize in Chemistry was awarded to Richard Zsigmonty for his invention of a microscope that could resolve objects that were smaller than the wavelength of light. He did this by employing principles of light scattering rather than principles of light reflection. Remember how we defined a ray of light? That definition was based off of light scattering so it makes sense that at some point our idealized light rays and their principle of reflection would not be sufficient and we would have to return to scattering.
We’ve been working with photons as opposed to other particles that we could hypothetically be working with. Why, might you ask, don’t we throw a little squash ball instead of the basketball? Then the squash ball will properly bounce off of the baseball. With the advent of electron microscopy, that was the basic idea. In this case, we are not considering particles bouncing though – that was simply an analogy for wavelengths. Electrons have shorter wavelengths and therefore can provide higher resolution images compared to photons.
The equation above describes the wavelength of a photon. For the electron, we use the deBroglie wavelength, the calculation of which uses relativistic mechanics giving a wavelength of about 1.25 nm which is much smaller than the wavelength of the photon.
The first prototypes for electron microscopes were made in the 1930s and in 1986, Ernst Ruska was awarded the Nobel Prize for his work on the electron microscope.
We’ve been using light as our primary tool in microscopes so far, but the technology is that we use in EyeWire is primarily based off of electrons. We are now up to the point where Danny’s blog comes in. Read it here!
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