Episode 530: X-ray diffraction
Lesson for 16-19
- Activity time 170 minutes
- Level Advanced
Most schools will not have an X-ray set, but there are plenty of effective analogue demonstrations using other types of waves (laser light, microwaves, and water waves).
- Discussion and student questions: Students’ knowledge of X-rays (30 minutes)
- Discussion: Diffraction and the limits of resolution (20 minutes)
- Demonstrations: Diffraction of laser light; crystal models (20 minutes)
- Discussion – optional: Deriving Bragg’s law (20 minutes)
- Demonstration: Crystal spacing by X-rays (30 minutes)
- Demonstrations: Various analogues of X-ray diffraction (30 minutes)
- Student activity: Chemical composition by X-ray analysis (20 minutes)
Note that students who are also studying chemistry may already have come across Bragg’s law.
Discussion and student questions: Students’ knowledge of X-rays
Rehearse students’ assumed knowledge of X-rays. If a typical wavelength is 1 nm , what is the frequency of such an X-ray?
(f = c λ
f = 3 × 108 m s-11 × 10-9 m
f = 3 × 1017 Hz)
Students can learn about how X-rays were discovered, and how they are used.
Discussion: Diffraction and the limits of resolution
The shortest wavelength of visible light ~ 450 nm (450 × 10-9 m) sets a limit for the smallest thing that can be
seen using visible light. This limit comes about because of diffraction effects, when the wavelength is comparable to physical dimensions. To investigate matter on a smaller scale requires that we
look at it using shorter wavelengths. X-ray wavelengths are < 1 nm.
The wavelengths of X-rays are comparable to the atomic spacing in solid matter. Hence X-rays will be diffracted by planes of atoms in crystalline solids.
Show some X-ray diffraction patterns. Emphasise the idea that, the narrower the spacing, the greater the diffraction. This means that diffraction patterns can be used to determine the arrangement of atoms within a solid, and their separations.
You could also point out that a single crystal gives a pattern of discrete dots; a polycrystalline material or powder gives rings (because all orientations are present), and an amorphous material gives blurred rings or dots.
Demonstration: Diffraction of laser light; crystal models
Shine a laser beam at normal incidence onto a grating and note the separation of the diffracted beams. Now rotate the grating about a vertical axis. Observe that the separations of the diffracted beams increase as the effective slit width decreases. (Alternatively, use gratings with different spacing.)
Provided the laser is class 2 (less than 1 mW of visible light), the warning ‘Do not stare down the beam' is sufficient. Avoid specular reflections.
If you have crystal models handy (ask your chemistry department), look through in different directions. Many
planes of atoms reveal themselves, each with its own separation d.
Discussion – optional: Deriving Bragg’s law
You may have to derive Bragg’s law. Beware of potential confusion: students will have met the formula for diffraction by a (transmission) grating. In diffraction from crystals the angle is defined differently, and the crystal is acting as a reflection grating. Furthermore, the theory uses reflection rather than diffraction!
Demonstration: Crystal spacing by X-rays
There are problems with using X-rays in school. However, some schools do have X-ray sets. A good alternative is to arrange a visit to a university department (Physics, Chemistry or Materials Science) to see one in use, and to learn about contemporary applications.
You could arrange to show a determination of the crystal plane spacing in alkali halides.
Demonstration: Various analogues of X-ray diffraction
Here are some further analogues, for you to choose from (perhaps dependent on the equipment available).
Using laser light and diffraction gratings: Two
crossed diffraction gratings represent the atomic planes, and give an array of diffracted spots. The fact that many solids are polycrystalline and are made up of many small crystallites orientated randomly can be simulated by slowly rotating the crossed gratings. The array of diffracted spots rotates too. What would the diffraction pattern look like if the crossed gratings were rotated quite quickly to simulate all the possible orientations? (A series of rings.) Rig up two crossed gratings with an electric motor to spin them round while diffracting the laser beam. This always makes a visual impact.
Finding the structure of DNA is perhaps the best-known use of X-ray diffraction. The iconic
X shaped diffraction pattern from the helix can be simulated by diffracting a laser beam off a fine bolt thread. Using bolts of different pitches alters the angle of the
(See School Science Review vol 85 (312) pp 18-19.)
Use a ripple tank
Try using microwaves and /or a ripple tank with an array of pins
Student activity: Chemical composition by X-ray analysis
An interesting use of the Bragg equation is to find the wavelength of X-rays emitted by a substance or object, so that information about its chemical composition can be found.