Question 12: Why we cannot see an atom?
We cannot see an atom because it is too small to be seen by visible light. Principally, we can see an object whose size has to be bigger than half the wavelength of the light used for viewing it. Now the wavelength of visible light is in the range 400-700 nm, while the size of an atom is 0.1-0.5 nm (one-thousandth of the wavelength of light), therefore, we cannot see an atom. The reflected photon from the atom interfere with one another and nothing distinguishable can be seen. Thus atom can never be seen in visible light.