Before you read this, I suggest you read post 16.18. I have written this post not because I know a lot about the subject but because I find some aspects of it interesting. “The naming of cats is a difficult matter.” T.S. Elliot (American/British poet, 1888-1965) Old Possum’s Book of Practical Cats. So let’s see… Continue reading 23.5 Biological classification – taxonomy

If you want to understand the background to this post, I suggest you read posts 22.12, 22.14, 22.15, 22.20, 22.21, 22.23, 22.24, 23.1 and 23.3 (route 1). But, for a less mathematical introduction, you can read posts 18.10, 18.16, 22.21 (but not the last paragraph) and 23.2 (route 2). Most of the statements that appear… Continue reading 23.4 X-ray crystallography and molecular structure

Before you read this, I suggest you read posts 22.14 and 22.21. In post 22.21, we saw that an ideal crystal is an arrangement of atoms (that may be bonded together in molecules) or ions in a regularly repeating three-dimensional pattern. And, in post 22.14, we saw that the x-ray diffracted by an object could… Continue reading 23.3 X-ray diffraction by a crystal

Before you read this, I suggest you read posts 22.4 and 22.21. Post 23.1 was about x-ray diffraction by a crystal. If you learnt about this topic at school or university, you were probably taught Bragg’s law. We don’t need to know about this law to understand x-ray diffraction by a crystal; it uses a… Continue reading 23.2 Bragg’s law

This article was first posted on January 9, 2013; it was revised on January 27, 2013. In this post we will think about diffraction of a beam of x-rays, with a single wavelength, λ, as in post 22.14. But here we will consider the special case where the scattering object is an ideal crystal. My… Continue reading 23.1 Observing x-ray diffraction by a crystal

Probability is an important idea in science. An experiment to answer a question like, for example, whether a drug is safe can only be answered with a probability (post 16.1). When we measure anything we only really know that its value lies between certain limits with a given probability (posts 16.7, 16.24, 16.26, 16.28). Probability… Continue reading 22.25 The Monty Hall problem

Before you read this, I suggest you read posts 22.22 and 22.23. In post 22.22, we saw that a one-dimensional lattice was a line of points, each separated by the vector a. The picture above shows how a two-dimensional lattice can be generated by convolution of two one-dimensional lattices. A three-dimensional lattice can be generated… Continue reading 22.24 Reciprocal lattice

I mentioned the idea of K-space in post 22.22 and introduced the vector K in post 19.20; in post 22.12 I described the Fourier transform as a function of K. The purpose of this post is to bring all these ideas together. In the left-hand picture above, a ray is scattered by a particle at… Continue reading 22.23 K-space

Before you read this, I suggest you read post 22.12 and appendix 3 of post 22.20. When you read the title, this may seem like a very obscure topic. But it isn’t because we need to understand the Fourier transform of a lattice to understand x-ray diffraction by a crystal. And we need to understand… Continue reading 22.22 Fourier transform of a one-dimensional lattice

In post 16.37 we saw that the atoms or molecules or ions in a solid are packed together so that the solid is completely contained within its free surfaces – it forms an object that we can hold without it flowing through our fingers. In an ideal gas, the atoms or molecules are free to… Continue reading 22.21 An ideal crystal