A nanometer is $10^{-9}$ meters.  To get an idea of how incredibly small a nanometer is, we can take a look at the scale of various things we are familiar with.

At the meter scale, there are humans who are about 1 or 2 meters in height.

One thousand times smaller, at the millimeter scale are fleas or ants or the top of a pin. The diameter of hair is several hundred microns or tenths of a millimeter.

Another thousand times smaller, at the micron scale are cells in our body.

Another thousand times smaller, at the nanometer level, are the diameter of DNA.

An atom diameter is a few Angstroms, or a few tenths of a nanometer.

A good example (borrowed from Richard Feynman‘s Lecture on Physics, Volume 1 Chapter 1 Atoms in Motion) is that if you enlarge an apple to the size of the Earth, then the atoms in that apple are now the size of the original apple.

A nanometer is incredibly small!

According to Richard Feynman in his Lectures on Physics Vol. 1 Chapter 1 Atoms in Motion, which instantly gives the idea some street cred, the most bit of important scientific knowledge that can be expressed in one statement is the “atomic hypothesis (or the atomic fact, or whatever you wish to call it) that all things are made of atoms–little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another.”

The interferometer is a quick and easy way to measure the thickness of a dielectric thin film.

When an electromagnetic wave is shined on a thin film, the reflection and transmission of the wave between media creates interferences patterns.  Component wavelengths from 200 nm to 900 nm are typical.  These wavelengths are ultraviolet (10 nm to 400 nm), visible light (380 to 750 nm), and short infrared (700 nm to 300 um).

Spectrophotometry is used to calculate the intensity of the reflected light as a function of wavelength.  The Fresnel equations are used for curve fitting and the thickness of the single-layer transparent film are obtained.