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 inside the Solar System, do…
Read moreRadiation 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 other techniques for detecting particular…
Read moreDoes 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 significant contributor to the clean-up…
Read moreWhat are lumens? (part 2)
In the previous discussion about lumens (https://physbang.com/2025/12/30/what-are-lumens/) some numbers were quoted without any explanation. This post adds a bit more detail to those numbers. Firstly, the maximum theoretical output of a light source was stated to be 683 lumens-per-watt. Why that number? The answer is to be found in the definition of the candela, which is defined as a source…
Read moreWhat are lumens?
Modern light sources are rated in lumens or, more importantly, lumens per watt. The first figure expresses the brightness of the source whereas the second indicates its efficiency. But what, exactly, is a lumen? To answer that question it is useful to recall that the first man-made light sources relied on heat and were very inefficient in terms of transferring…
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