When answering examination questions, it is important that your excellent knowledge of physics isn’t undermined by your calculator behaving in an unexpected way. You MUST be able to trust the answer given. But the situation is potentially complicated, not only because calculators don’t all behave the same way but also because they may not even … Continue reading Know your calculator!
At what rate is the Universe expanding?
When Edwin Hubble first used data to determine whether “nebulae” were part of our own galaxy, he found they were in fact moving away from the Milky Way at a rate proportional to their observed distance. The most distant of these objects (now known to be galaxies) were receding at the greatest speed. This in … Continue reading At what rate is the Universe expanding?
Ray Diagram for a Telescope in Normal Adjustment
There is a summary of the key characteristics of astronomical telescopes in a previous post, at https://physbang.com/2026/03/15/a-level-guide-to-astronomical-telescopes/. This article builds on that general overview by explaining a specific skill; how to draw a ray diagram for a refracting telescope in normal adjustment. First, what is "normal adjustment"? It is when the objective and eyepiece lenses … Continue reading Ray Diagram for a Telescope in Normal Adjustment
Red-Shift and the Age of the Universe
Stars can be characterised by their absorption lines, which reveal their temperature and composition. There is more detail about this in two recent posts about the HR diagram and stellar classification. A similar approach can also be used to characterise entire galaxies but in this case the aim is to determine how quickly they are … Continue reading Red-Shift and the Age of the Universe
The formation, life and death of stars
All stars are born through the same basic mechanism but their evolution depends on their size. In particular, all stars go through a hydrogen-fuel stage but the time spent fusing hydrogen is determined by the star's initial mass. The final fate of a star is also predicted by its mass; some stars will simply fade … Continue reading The formation, life and death of stars
Classification of Stars
The obvious way to classify stars is by their appearance; how bright they are and their colour. In both cases, we are referring to properties judged using the human eye from the location of planet Earth. Sadly, our eyes are not a good judge of colour (equal amounts of red and green light are perceived … Continue reading Classification of Stars
Electricity: models and theories
Physics can get complicated at times and when it does, we use models to simplify things. The models aren’t wrong: in fact they work remarkably well, but they aren’t complete. Electricity provides probably the best examples of this state of affairs. Models normally start at the level of “electrons as buckets carrying energy” and include … Continue reading Electricity: models and theories
A-level guide to astronomical telescopes
The first section of the Astrophysics module for AQA A-level Physics is all about telescopes. This post is a brief summary of that section’s key learning points. Telescopes aren’t used simply to make astronomical objects “bigger” or to get a “closer” view. It is true that nearby objects, such as our moon and other bodies … Continue reading A-level guide to astronomical telescopes
Radiation detectors
The first device that comes to mind as a radiation detector (for use in schools) is probably the Geiger-Mueller (GM) tube. Although GM tubes can be used to detect all three types of ionising nuclear radiation (alpha, beta and gamma) they are subject to some important limitations. These shortcomings will be discussed below, together with … Continue reading Radiation detectors
Does radiation make things radioactive?
The short answer is “no” but the devil is always in the detail. Firstly, “radiation” here applies specifically to ionising radiation; alpha particles, beta particles and gamma photons. In particular, we are excluding non-ionising nuclear radiation in the form of neutrons, which definitely can turn previously stable nuclei into radioactive materials. This effect is a … Continue reading Does radiation make things radioactive?
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