Refraction, Lenses, Telescopes and Total Internal Reflection
This topic is covered in most specifications but they don't all have the same emphasis. Depending on which you teach, pick out the sections on telescopes or on refractive index.
The use of lenses fell out of fashion for quite some time in GCSE and so many schools have trays of lenses that have not been used much for a long time. Even if you were not directly teaching optics, lenses and telescopes it seems a real shame not to get the lenses out and at the very least find the focal length of a converging lens. It seems to be a ‘wow’ moment from a few people pretty much every time I do this activity with teachers or students. If you’ve never done this then there are nice clear instructions here from Practical Physics and if you don’t have any lenses then they can be got hold of pretty easily. In the absence of another opinions I’d suggest 50mm diameter converging lenses with focal lengths of 100mm and 500m. These work well for the focal length experiment as well as making a telescope where they need to be 600mm apart and so it all fits on a metre ruler.
The ray diagram provides a really nice example of a model in physics that is predictive and there are plenty online simulations around including PhETs Geometric Optics
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