a very crude attempt at physics behind this year’s chemistry Nobel Prize.
Kick a football very hard. Lets say we kick it with a machine which is 54 Horsepower. Lets say this football hits a pole on the ground and uproots it. (or breaks the upper part of the wooden pole) Then this broken part moves and hits something else. This now does not have the energy of 54 HP. Lets say it has the energy of only 2 HP. Thats what happens in fluorescence.
The most powerful UV light has in its limit, ~27 times higher energy than the most powerful visible light. Anything in UV range is hardly seen by human beings*. But we can be hit by UV light. Technically most powerful UV is filtered due to atmosphere.
* Birds, Insects and Fishes are capable of seeing in some Ultra Violet range. Some human beings can see some very low energy UV and eye defects (such as missing lens) makes some capable of seeing some certain low energy UV light.
(Sanscritize; Ati maha-lohita, or uttara bahulohita, see that Ultra goes to uttara and violet goes to bahulohita, where lohita is RED, bahulohita therefore Violet, also see red and lohit both come from let of violet, evidently vio is bahu or maha, so ultra violet means extreme-greatly-red = violet)
What happens when some UV falls on biological molecules? (or any kind) some of these molecules like the wooden pole absorb the energy and there still remains energy which they can emit as visible light. So the UV light kicks the molecules and the molecules in turn emit visible light. This implies that the UV wavelength that the molecules absorbed energy at are very small compared to the wavelength at which they emit visible light. This is always the case.
When we say some materials are fluorescent it means they absorbed higher energy and emitted lower energy radiations of colorful light.
But this has a great deal of application apart from its theoretical interest.
Let me give another example. I give you a scale with marks on it at equal spacing. Lets say the minimum spacing is 1 mm. That means anything whose size is more than 1 mm (yes, milli meter) you can put this scale on it and measure some length on the object. In principle even if the object is far bigger than the scale, you can measure lengths on the object. But what if something is less than 1 mm, you can’t measure lengths (and therefore any kind of details) using this scale. This 1 mm, the least amount of measurable quantity is called “resolution” in experimental terminology. eg If I say my glucometer has a minimum energy measurement of 0.5 Calorie, I would say the resolution is only 0.5 calorie. That would also be the least-error due to the glucometer. It will read “you melted 20 calories in last half an hour”, it means at-least there is a 0.5 calorie error on the glucometer. There will be other reasons why this error will be blown up. But this is the minimum. Its also called sometimes as a systematic error or detector error or resolution. Its an energy resolution here in this context of the example of glucometer.
Now lets go back to the example of length or distance (also called space or spatial) resolution. The 1 mm is the resolution of the scale. So we need other instruments that would measure sub-mm details. In Physics labs you come across screw gauge, vernier calipers etc, that are capable of such glory. They are accurate toward sub-millimeter details.
Now a microscope is called a microscope because its a scope with a micrometer distance or spatial resolution. Microscopes are optical instruments typically showing details at the micrometer (also called micron) level. With such bacterium and other living micro-sters are visible.
With advance of science and technological knowledge and requirements we have been necessitated to observe things whose details are far more smaller than the micron. Lets say the details of a large molecule. A large molecule is sub-micron. That is its size and details are in the smaller than micron size of matter. Evidently microscopes are not capable of such glory.
Why not? When I gave the example of the scale, I had exactly this in mind. I think ahead. (pun)
A microscope uses the visible light as its torch. It irradiates the object to be imaged with visible light. Visible light typically have a 400 to 800 nano-meters wavelength. One nano-meter (nm) is 1000th part of one micron. And we are asking the microscope to achieve that job for us. Like we are asking our father to have for us a very expensive gift. He does not sleep.
The microscope makers didn’t sleep either until someone suggested the makers why they can’t make microscopes that can measure the nano meter details and why its a cool thing. Its called an Abbe’s Limit on resolution, which was figured out more than 120 years ago.
Just like the marks on the scale are 1 mm apart the wavelength are but a mark on the light wave. (an imaginary mark if you will) Depending on therefore what exact wavelength one uses for a particular study one has chosen the spacing between these marks. Lets say I chose a visible light of wave-length 500 nm. That would mean the spatial resolution of visible light of that wavelength would be 500 nm. Anything smaller than that would not be good enough to be measured. Your wife has a 200 nm wide mole on her cheek and you wanted to gift her a optical microscope ? Forget it.
But there is a slight glitch here. The actual resolution (minimum error of distance) of this particular wave-length is not 500 nm, but 250 nm.Thats because there is certain symmetry of the wavelength. Its the same after 250 nm toward 500 nm, just flipped upside down. Still it does not measure the mole on your wife’s cheek, bad news.
But the bad news doesn’t stop there.
We can’t know anything smaller than 200 nm using an optical microscope because the most powerful (and hence the smallest wavelength) light is at 400 nm. Smaller than that wavelength or larger than that energy are what are called UV (ultra violet) radiation. They happen from 400 nm down to 13 or something nm. But these are highly energetic.
So technically UV would give us the ability to glorify anything from 200 nm to say 6 nm in size. Molecules are though of even smaller size than that. But we are talking about large sized molecules.
Now there are biological molecules that are what are called fluorescent which I explained above. These molecules take in the energy of the UV and like the wooden pole hit by the football give out a fraction of the energy. These emissions from molecules are thus in the colorful oasis of visible light. So technically even if we would not see in UV light the molecules are visible toward instrument that operate due to visible light principles.
This year’s Nobel prize in Chemistry is based on the applicability of this fluorescent emissions by molecules which in turn makes the actual resolution achievable debunk the (still valid) Abbe’s Limits. We can go below micron level details and study the molecules in far better resolution. There have been two different methods developed independently that makes this possible. In each case Fluorescence is used. When the molecules are hit by UV light they absorb such and emit visible colors. So they can be activated and deactivated this way. In addition different colors can be used so that molecules can be tagged differently and the Abbe’s distance of 0.2 micron, fact that 200 nano-meter between molecules is a must, to see details, is avoided. Because different images at different colors can be superimposed. Its like long exposure photography in Astronomy are superimposed to cover more details.
So the molecules each of which emitted fluorescence, at different colors, are superposed and apart from each being seen clearly, the details are now smaller than micrometer. These are now therefore called nano-scopes instead of just microscopes.
So its like not acting our torch or flash on an object to produce an image we activated the molecules themselves to act like light sources and make their presence registered with accuracy. This is an innovative way of imaging far smaller details than the micron and it does not invalidate the Abbe’s Rule of 0.2 Micron, it goes around it.
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