What does it mean to see an atom?

Suppose you tried to use the world’s strongest optical microscope to see an atom. What would happen? You would probably reflect light from the atoms into your microscope. Light has wave properties, so imagine waves of light shining on an atom. The wavelength of visible light is about ten thousand times the length of a typical atom.

To help think about this, let’s switch to thinking about water waves rolling in on a beach. If you stand in the water, the waves roll past you, unaffected. Hardly any wave energy is reflected. That’s because the size of your body is so much less than the wavelength of the water waves. The waves move by as if you were not there, so reflection of water waves will not reveal the presence of a person in the water.

The wavelength of visible light is about 10^-6 m (the same as 10³ nm), as shown in the drawing . The size of a typical atom is about 10^-10 m, which is 10,000 times smaller than the wavelength of light. Since an atom is so much smaller than the wavelength of visible light, it’s much to small to change the way light is reflected, so observing an atom with an optical microscope will not work.

How about radiation like light but with a shorter wavelength? X-ray wavelengths are about the same size as atoms, but reflecting x-rays from matter forms a complex pattern of spots that depends on the arrangement of the atoms. Analysis of these patterns reveals a lot of important information about crystals, but the x-ray images do not show individual atoms.