Might seem obvious but I can’t wrap my head around why our eyes won’t let us see very small things.
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The farther away you are from something, the larger your field of view becomes, i.e., more things are in your field of vision. The more things are in your field of vision, any particular thing will take up a smaller and smaller proportion of your field of vision. Thus, the father away you are from something, the smaller it looks. This is called (or vaguely close to what is called) “visual angle”.
On top of that, your eyes don’t have infinitely high resolution. The retina has “pixels” so to speak – light-sensitive cells called cones and rods.
So the problem with trying to see, e.g. a single bacterium, is that 1) the bacterium is around* the same size (one cell) as one of your rod or cone cells, i.e. even at 100% zoom they’re only 1 pixel large, and 2) you’re never seeing them at 100% zoom anyway because they’re not right up against your retina. Some distance away from you they occupy only some fraction of a fraction of a single pixel when accounting for the smaller visual angle. The shape and color of that individual bacterium is going to blurred out and averaged with all the other information that lands on that one pixel.
*as in, I assume, the same order of magnitude. I have no idea how big rod or cone cells are and bacteria vary quite a bit in size anyway
Our eyes have a finite resolution. There’s a minimum size beyond which we can’t make out anything smaller. Similar idea to the resolution of a screen; your screen can’t make out anything smaller than the size of its pixels.
Microorganism are smaller than the resolution of our eyes, so we can’t distinguish them
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