Defining Nanometre: One Billionth of a Meter

The question asks to identify the unit that represents one billionth of a metre. This requires understanding metric prefixes used for length measurements.

What is a Nanometre?

A nanometre (symbol: nm) is a standard unit of length within the International System of Units (SI). It is defined as one billionth ($10^{-9}$) of a metre.

Mathematically, this can be represented as: $1 \text{ nm} = \frac{1}{1,000,000,000} \text{ m} = 10^{-9} \text{ m}$

This unit is commonly used in fields like nanotechnology, semiconductor manufacturing, and materials science, where extremely small dimensions are measured.

Analyzing Unit Options

Let's examine the provided options to see which one matches the definition of one billionth of a metre:

• Parsec: This is a very large unit of astronomical distance, approximately $3.086 \times 10^{16}$ metres. It is not related to a billionth of a metre.
• Micrometre: Also known as a micron, a micrometre (symbol: $\mu$m) is one millionth ($10^{-6}$) of a metre. ($1 \mu\text{m} = 10^{-6} \text{ m}$). This is smaller than a nanometre.
• Angstrom: An angstrom (symbol: Å) is equal to one ten-billionth ($10^{-10}$) of a metre. ($1 \text{ Å} = 10^{-10} \text{ m}$). This is smaller than a nanometre.
• Nanometre: As defined above, a nanometre (nm) is exactly one billionth ($10^{-9}$) of a metre. ($1 \text{ nm} = 10^{-9} \text{ m}$).

Based on these definitions, the nanometre is the unit that is one billionth of a metre.