Should the intro page define reflectometry?

[alexhroom]

Hi there - I'm reading this guide as someone with a strong mathematics & optimisation background, but little (no higher than A-level) theoretical physics background. This guide is well-worded and kept simple, but after reading the first section I stopped and thought "wait, what actually is reflectometry?", and had to dig around Wikipedia for a bit. The guide jumps into analysis using the field terminology, without explaining what a reflectometry profile is, what scattering length density is, why the scattering length density profile of a bilayer would be a Heaviside function, etc.
I found I had to simultaneously have Chapter 3 of Sivia's book open in order to understand what we were actually analysing, and to define symbols which are not defined in the guide - what is $q$? What is $q_x$? What is $q_y$?
Even though analysis is the main focus of the guide, I think it would be worth having a short page defining the actual concepts of what we are doing and what we are measuring, e.g. something like

What is reflectometry?

Neutron or X-ray reflectometry is a technique for analysing the properties of materials. It involves directing a beam of neutrons or X-rays from a flat surface, and measuring the way in which these neutrons or X-rays reflect from the surface, known as reflectivity.
Reflectivity can be measured using scattering length density, which is...

I think it may also be worth having the slab model page moved to this introduction - as soon as I saw Fig. 4, the scattering length density profile being a Heaviside function made immediate intuitive sense, and it would be instructive to explain scattering length density using this example.

[arm61]

Hey Alex, thanks for this. There is actually a hackathon event to work on this material in a few weeks (as part of the SXNS conference) so I will make this a priority for this event. That said, if you would like to make this change and open a pull request, you are more than welcome to!