Ever wondered why we can't see small things?

Why we don’t see bacterias every day? We know microorganisms are everywhere, its in the air we breathe and the water we drink. Its in-front of our eyes. Then why can’t we see them? Why can't we see small things? To understand that, first we have to know how we SEE things.

Everyone knows, we see things when light from an object falls on our eyes. But how that image reach us? Does light carry information of that image? Yes, in the sense, each photon in the light ray have a different wavelength that matches the colour of the object where it is emitted or reflected. Each photon acts like a pixel of an image. When this light ray reaches our eyes, the signals are detected by the cones and rods in the retina.

Cones detect the colour and the rods detect the brightness. The brain will then formulate the image using this information. Now lets see, what’s the problem while viewing smaller objects. 

The photons that can be detected by our eyes has a limit. A typical human eye is capable of detecting the visible light with wavelength ranging, 390nm to 700nm. We are not adapted to see infrared, ultraviolet, radio waves etc. 

The reason we cannot see too smaller objects is that the wavelength of visible light is much bigger than the size of that object. The photons of visible range (390nm to 700nm) when reflected from smaller objects, like an atom, is too weak. Since an atom is very small compared to the wavelength of visible light, the wave equation requires that the light bounce off with a very broad spread of directions, not at all like reflection from a surface. Thus it makes hard to detect an atom.

Even using a conventional microscope, its hard to see objects below the length of the minimum wavelength of visible light, because a conventional microscope uses visible light to detect objects. It simply magnifies the things it detects.

So what should we do to see an atom? We makes use of an Electron microscope. In an electron microscope, the light source is replaced by a beam of fast moving electrons. As the electron's size is too small, it can successfully create image of smaller objects.

There are several types of electron microscopes for various uses. You can find the details and differences from here : http://www.explainthatstuff.com/electronmicroscopes.html

If you are wondering, the tiniest objects we can see is about 0.1mm, it is the size of a hair, lice etc.