Deep inelastic electron-photon scattering is studied in the Q 2 ranges from 6 to 30 GeV 2 and from 60 to 400 GeV 2 using the full sample of LEP data taken with the OPAL detector at centre-of-mass energies close to the Z 0 mass, with an integrated luminosity of 156.4 pb −1 . Energy flow distributions and other properties of the measured hadronic final state are compared with the predictions of Monte Carlo models, including HERWIG and PYTHIA. Sizeable differences are found between the data and the models, especially at low values of the scaling variable x. New measurements are presented of the photon structure function F γ 2 (x, Q 2 ), allowing for the first time for uncertainties in the description of the final state by different Monte Carlo models. The differences between the data and the models contribute significantly to the systematic errors on F γ 2 . The slope d(F γ 2 /α)/d ln Q 2 is measured to be 0.13 + 0.06 − 0.04 .

In this resource article, an exceptional bubble chamber picture-showing the annihilation of a positron (antielectron e + ) in flight-is discussed in detail. Several other esoteric phenomena (some not easy to show on their own!) also manifest themselves in this picture-pair creation or the materialization of a high energy photon into an electron-positron pair; the 'head-on' collision of a positron with an electron, from which the mass of the positron can be estimated; the Compton Effect; an example of the emission of electromagnetic radiation (photons) by accelerating charges (bremsstrahlung).It is hoped that this article can be useful on several levels.

This article provides a simple practical introduction to wave-particle duality, including the energy-time version of the Heisenberg Uncertainty Principle. It has been successful in leading students to an intuitive appreciation of virtual particles and the role they play in describing the way ordinary particles, like electrons and protons, exert forces on each other.

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A bubble chamber plcture of part of an ala&* magnetk shower demonatrates many lnterestlng phenomena, lndudlng the existence of antlmaner, the maleriaiizaffon of a hlgh anergy photon Into en electron-posltron pair, the em1!3slon of radlanon by accelaratlng charges, and the Compton affed. Thls alticle Introduces the subject of particle physlca, explaining In particular the role of the bubble chamber, and It carefully deacrlbes the parts of the plchlre that demonatrate the phenom ena ot interest. Some relevant kinematies Is given In an appendlx.' The bubble chamber picture to be discussed in this article has been shown to, and discussed with, hundreds of A-level students and teachers during University of Birmingham Open Days. Many expressed amazement that so many phenomena (some not easy to demonstrate on their own!) could manifest themselves in a reasonably straightforward way on one photograph.The phenomena come under the rather formidable title of Quantum ElectroDynamics or QED. Actually, the QED label is a sign of comparative simplicity, because, of the four traditional forces of nature-gravity, electromagnetism, and the weak and strong nuclear forces-QED limits itself to electromagnetic forces. (Quantum mechanics is always needed when talking about the behaviour of particles; here, all we need to know is that it is useful to describe electromagnetic radiation in terms of the flow ofphotons.)What Is parilcle physlca?