According to Wikipedia, “An electron microscope is a microscope that uses a beam of accelerated **electrons** **as a source of illumination**. **As the wavelength of an electron can be up to 100,000 times shorter than that of visible light** **photons**, **electron microscopes have a higher resolving power** **than** **light microscopes** **and can reveal the structure of smaller objects**”.
Can anyone explain the text in bold? I don’t understand how an electron can be a source of illumination.
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The electrons bounce off the object. The way they bounce reveals the structure of the object. This is exactly how light illuminates something, too: the photons bounce off it, and how they bounce and which photons bounce tells you about the object.
(Strictly speaking this is a *scanning* electron microscope. Others, like TEMs, work differently. But the basic principle – that you use how electrons interact with the material to probe its structure – remains.)
The reason you can’t use light at these scales is that (visible) light is “too big” to bounce off small details. If I put an object on a pool table and asked you to figure out its shape by bouncing marbles off it, you could do it. But if I asked you to figure out its shape using giant beach balls, you’d have a much harder time. In this analogy, electrons are like marbles, and photons of visible light are like the beach ball.
There is no light involved. Electron microscopy uses electric charge to measure distance from sensor to sample surface in micro millimeter increments. A computer uses this data to construct an image.
OK, I am going to really explain it like you are 5, but it will take a bit of time.
Imagine your light beam (you are used to thinking about microscopes using light) as filling a circle of maybe 1 inch in diameter. Think of that circle as being divided up into a grid. So there are maybe ten lines running top to bottom, vertically and ten lines left to right, horizontally. So now your circle has a grid super-imposed on top of it, right?
Think of each square in that grid as having a little wire attached to the underside of it. Each wire is connected to a photodetector-transducer off to the side or underneath it, depending on what kind of electron microscopy you are doing. These wires connect in a one to one fashion. So the photons that are collected from grid slot 5D, are now sent off as a relative signal to a grid space 5D on the photodetector/transducer. There it is tabulated into a digital number and sent to a computer that re-assembles the image based on the digital signal it has gotten.
Different kinds of transducers/detectors, different kinds of voltages or light and different kinds of samples can be used to generate a lot of different kinds of images. But they are all based on this technology, — on the idea that by maintaining a 1:1 spatial awareness of signal from a sample, you can achieve high magnification, resolution and level of information. If it is a light microscope, you are using light to generate signals. In that case the detector/transducer is collecting photons and converting those into a digital signal. If it is a laser, you are still collecting photons, all induced by a beam of photons that have the same wavelength and energy. If you are using a transmission electron microscope, you are putting a detector underneath your specimen and recording electrons as they pass through a sample or are deflected by it. And likewise, if you are using a scanning electron microscope, you are collecting and analyzing secondary electrons that come off the sample after being induced to do that by an electron beam.
There are other things you can do too. You can increase the voltage, so you are collecting back-scattered electrons (meaning ricocheted electrons which happens when they hit heavy metals) or you can even increase the voltage so much that yoou knock electrons loose in your sample, and you collect the x-rays that are emitted as result. Lots mroe things, but the are ALL dependent on the 1:1 –> 1:1 –> 1:1 connection of sample to detector to computer.
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