# Can We See A Single Atom?

Matter consists of molecules and molecules are build up by atoms. It consists subatomic particles; protons, neutrons and electrons. An atom is very small, the average radius of an atom is about $0.1$ $\text{nm}$.

Obviously, we cannot see atoms with our naked eyes. We cannot see atoms even with a very powerful microscope. To see something, we need light, an electromagnetic wave. This electromagnetic wave hits the surface of an object and gets reflected back from it into our eyes. On the basis of the information carried by that wave, our brain creates an image of that object.

Hence, to see an object, the visible light wave must get reflected from it. But atoms are even smaller than the wavelength of the visible light. They are so small that the visible light wave cannot touch them, the wave simply passes through them.

This problem can be solved by using electromagnetic wave of shorter wavelength like X-ray. X-rays can bounce off from things like crystals. Crystals are solids having layers of atoms. When an X-ray hits an atom in the crystal, it gets reflected back. If an X-ray does not hit the atom from the first layer, then it gets reflected back by the atom from the second layer or the third layer and so on.

After getting reflected, the X-rays strike the detector screen. And, from the detection of the pattern of the X-rays, scientists can figure out the three dimensional arrangement of atoms in the crystal. In this way, X-ray diffraction allows us to visualise the structure of atoms.

## Scanning Tunneling Microscope (STM)

In 1981, Scanning Tunneling Microscope (STM) was developed which revealed the atoms themselves. STM consists of a very small, sharp tip that conducts electricity. The tip is mounted by a rapid piezoelectric scanning device.

A current is supplied to the tip and the tip is moved rapidly by the scanner across the surface of an object. When the tip encounters an atom, the flow of electrons between the atom and the tip changes. This change in current and the x-y position of the atom are detected by a computer. The computer registers the current for each x-y point on the surface and then it collects the data and plots a map of current over the surface which results to a map of the atomic positions. From this map, we can finally visualise the individual atoms and it also tells us that atoms have spherical shape of different sizes.

But these all give us the picture of group of atoms rather than a single atom. The question is can we ever see a single atom? And, the answer is yes. Actually, a photograph of single strontium atom was captured in 2018 by David Nadlinger in University of Oxford.

## Photograph of Single Strontium Atom Captured by David Nadlinger

David Nadlinger placed a positively charged strontium atom between two metal electrodes. The electric field between the electrodes held the atom nearly motionless. Then, he illuminated the atom by using high powered blue-violet light laser. The laser energized the strontium electrons and made them glow brighter. Strontium atoms are relatively large. It absorbed and re-emitted the laser light which was captured in the photograph.

The idea of being able to see a single atom with the naked eye had struck me as a wonderfully direct and visceral bridge between the miniscule quantum world and our macroscopic reality,” He said. “When I set off to the lab with camera and tripods one quiet Sunday afternoon, I was rewarded with the particular picture of a small, pale blue dot.

The photograph was entitled as “Single Atom in an Ion Trap“. It won the 2018 Engineering and Physical Sciences Research Council‘s science photography prize.

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