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Asked by sarahcreegan to Sinead, Eleanor, Adam on 11 Nov 2013.
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Eleanor Holmes answered on 11 Nov 2013:
The smallest distance that can be resolved by the human eye is about 0.1 millimeter.
With an optical microscope one might be able to see 100 or 200 times smaller than that. The real limiter is the fact that our eyes are only tuned to see well in the visible spectrum. That is light with wavelengths between 700 and 400 nanometers. In optics there is a concept called a diffraction limit and it describes how far apart two objects have to be so that we can see them as two distinct objects. Now this is the same as the smallest size of an object you can see can be because if you can’t even tell if it’s one object or two, well you can’t really see it, can you?
So the diffraction limit is dependent on the wavelength of light you are using to look at the object. In our case, for the eye, the smallest wavelength is around 400 nm. If we do the maths it works out that we can see, with the very best optical microscope lenses, things as small as 0.14 micrometers (0.14 millionths of a metre).
To get any smaller than that you need to start using special techniques and light with much shorter wavelengths than we can see. So it’s no longer a human seeing the object, but a computer.
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Sinead Cullen answered on 12 Nov 2013:
Hi Sarah,
The smallest thing a person can see is the human hair, or something that can be found in human hair, lice, also known as nits.
Anything smaller than this you need to use a microscope. So things like viruses that can cause things like the flu you can not see so you need to look at something that small with a microscope.Hope this helps 🙂
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Adam Murphy answered on 12 Nov 2013:
Eleanor and Sinead are both right, but there’s a little cheat you can do to see even smaller things in some cases.
It’s used to see bubbles in things that shouldn’t have bubbles in them. If you shine a laser into a liquid, the laser light hits the bubbles and bounces off in all directions, you can see the bounced light and it tells you there’s a bubble there.
People who grow crystals use it to see if their crystals are really pure by shining a laser pointer into it and looking for the bright bubbles! Doing this, they can see things that are 0.01 millimetres in size. All thanks to the awesomeness that is lasers!
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