How many atoms does it take for an element to become visible to the naked eye?

Let’s jump right in on this and start with the diameter of an atom. Atoms measure from 0.1 to 0.5 nanometers (nm) across (0.1 nm written out fully is 0.0000000001 metres).

You won’t be surprised to hear that humans can’t see individual atoms – in fact, it takes a very expensive electron force microscope to investigate materials at this level of detail.

The eye’s actual resolution is dependent on the distance of the object from you. Think of two diagonal lines leaving your eye in opposite directions, creating a triangle, with the object forming the third side of the triangle. The further from your eye the object is, the wider it has to be to form that third side.

The eye’s resolution is the smallest angle at the triangle point by your eye at which you can ‘resolve’ (tell apart) two objects at the other two points of the triangle. For humans, this is 0.01 degrees.

In practice, humans can see objects about 0.04 millimetres (mm) in width at normal distances, which is roughly the width of a human hair.

So how many atoms fit into 0.04 mm? Say we have an atom that’s 0.4 nanometres across, to make the numbers easy.

That atom is 0.0000000004 metres across, while 0.04 mm is 0.00004 metres. So an element about 100,000 atoms in width would be just about big enough for a human eye to see it.

If there are 100,000 atoms in just the width of one hair, just imagine how many there are in the everyday objects around you!

[Sources: Diameter of atomsNaked eye visibility | Also posted on Ask a Scientist]

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