Question

Astronomers and physicists need a unit to describe extremely small distances, such as the size of an atomic nucleus. Which unit is commonly used for such tiny lengths, and what is its value in metres?

Hint:

Think of the unit named after Enrico Fermi, used for nuclear scale distances.

Answer 1

The unit used for extremely small distances, such as nuclear sizes, is called the fermi.

Step 1: Definition.
One fermi is defined as \(10^{-15}\) metres. It is named after the physicist Enrico Fermi, who did important work in nuclear physics.

Step 2: Why a special unit is needed.
The metre is far too large a unit to conveniently describe distances inside an atom or a nucleus. A hydrogen nucleus has a radius of about 1 fermi, and heavier nuclei have radii of a few fermis. Using metres directly would mean writing very small decimal numbers every time, so scientists use the fermi as a convenient scale.

Step 3: Answer.
So the correct answer is the fermi, where 1 fermi \(= 10^{-15}\) m. This unit is also sometimes written as femtometre (fm), since femto is the SI prefix for \(10^{-15}\).