I know the machine fires electrons at a sample and the electrons are reflected back. How does that translate into a super detailed image? Why is it higher resolution than a light microscope? Why can it only use dead samples?
Edit: Also, why is it in black and white?
In: Technology
A picture can’t be more detailed than the size of the particles used to take the picture.
Electrons have shorter wavelength than photons in the visible light range. ( around 10**-12 vs 10**-7 meters )
IOW, the wavelength of an electron from an electron microscope is about 100 thousand times smaller. Smaller wavelengths mean more precise images.
You can take pictures of living things, but they won’t be very healthy afterwards. They are getting hit by very energetic particles which will kill most living things.
Also, even if the living thing can survive being zapped by a particle beam, it has to hold its breath for a long time. Electron microscopes need the sample in a vacuum to prevent arcing of the electron gun.
And, on top of all of this, most samples for electron microscopes are sliced into very thin sheets.
Electrons have extremely short wavelengths, much shorter than visible light. These very short wavelengths allow for much greater angular resolution from the light being reflected off of the object, which allows an immense amount of detail to be exposed when something is imaged with an electron microscope. The shorter the wavelength, the more instances of the light source of that wavelength can be captured in an image. It’s impractical (and unsafe) to do it on larger scales.
It’s almost like sampling with audio files. Visible light photons are like an mp3: the quality (detail) is lower because the format takes less samples when ripped from its source, because the samples are bigger (like a normal photograph taken in larger wavelength light). An electron microscope is like a lossless audio format, such as FLAC: it takes a ton of tiny samples of the source material (like an image taken using an electron microscope with smaller wavelength light), allowing for a higher quality representation of that source.
Did you ever s those toys with the pins, and you can push your hand into it and it makes an imprint of your hand?
It’s like that, except instead of many little pins it’s a beam of electrons, they “poke” into all the nooks and crannies of the surface and it forms an image. It’s not in colour because it’s basically just the “shape” of the object and the shape of it’s surface. Shapes don’t have colour.
The subject in the electron microscope is placed in a VACUUM, so any living subject would generally become not alive anymore. Soft animals like a worm have to be freeze-dried. Also the subject is coated in a metallic film of gold or platinum, again, not a process you can do with a living animal.
Electron microscopes operate under high vacuum. Electrons dont travel very far in air and even if they did, detail would be blurred because of the collisions with air molecules. Living things don’t survive very long in a vacuum.
There are two types, transmission (TEM) and scanning (SEM).
A TEM is rather similar to an optical microscope. A beam of electrons passes through a thin sample and is focussed by electrical coils, instead of glass lenses, to make the image.
An SEM scans a fine beam of electrons across the sample and records the amount reflected from each point to build up the image. It’s a bit like you sitting in a dark room with a laser pointer. You wave the dot across the far wall and the changes in brightness tell you about the objects there. Although SEMs generally have less magnification than TEMs, they can deal with 3D samples that need much less preparation.
>I know the machine fires electrons at a sample and the electrons are reflected back.
This is close enough for our purposes.
>How does that translate into a super detailed image?
The important item here is that the level of detail you can resolve depends on the wavelength of what you’re bouncing off the target. You can see finer details if you image in blue light (450nm) than Near IR (2000 nm).
Due to quantum physics being weird, electrons aren’t really balls but instead wiggly energy packets that have a wavelength. There are a bunch of different ways to calculate the wavelength of fundamental particles like electrons but in general its <1nm making it able to show you much much finer detail than light could because it can sneak into nooks and crannies.
If you take a microscope and just keep cranking up the zoom on an optical image, it will get bigger but it will also get blurrier because the little distortions from when the light clipped the edge of the lens and tiny abnormalities of the lenses matter when you’re at 1,000,000x zoom. This is the same reason you can’t just zoom wayyyyy in on an 8 inch telescope and see great detail on Pluto, you need a bigger lens and shorter wavelength to let you capture the detail so you can zoom in on it.
Since the electron wavelength is soo much smaller you can zoom in proportionally more before the edge effects matter and blur your image beyond use.
> Why can it only use dead samples?
Because it’d be dead by the time you’re sampling it
What happens when you shoot the electron beam? It smacks into atoms and sends other electrons shooting off to be detected. If you have air in the chamber that air would catch the electrons before they hit your sample so electron microscopes are all in vacuum chambers and suck all the air out before imaging.
The sample is also generally prepped in a fine layer of carbon or gold to give a nice conductive surface for good imaging. That wouldn’t be particularly great for anything that was alive.
>Also, why is it in black and white?
Because you’re better at detecting changes in black/white than in color
All the sensor is getting back is a location on it and how many electrons hit that spot, its effectively only able to judge brightness and can’t tell you anything about those electrons. When you look at a color image its because you want to know the count of how many photons would have triggered your red cones, your green cones, and your blue cones, but if you only have one set of data (photons that hit cones) then grayscale is the right way to represent that
If you need more information about the surface an electron microscope has tools *wayyyy* better than color. You can put down the little cursor and go “hey what’s this?” and the system will shoot electrons at it and plot you [the chemical analysis of that spot](https://www.researchgate.net/profile/Hanan-Youssef-7/publication/272408064/figure/fig3/AS:323999889412099@1454259001358/SEM-and-EDS-chemical-analysis-for-the-Ag-substituted-Ti-Z-A.png) which tells you what its made of and roughly how much of what which is *wayyyyy* better than trying to eyeball the color to tell if its copper or gold
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