Reposting this from last year, for those looking to hit the sweet spot between covering some physics, and yet acknowledging that Christmas exists.

I speak, of course, of 2014 q.12b, which involved a reflective Christmas bauble and asked students to find an image formed by it, treating it as a convex mirror.

Its quite difficult, with two likely pitfalls for even a well prepared student: the negative values of v and f, and the fact that the focal length is 1/4 the diameter. Many instinctively halved it instead, which in itself is a good example for students as to why they should always draw diagrams.

I'm using it to introduce a (not very festive) Christmas theme to a few classes today and tomorrow. __Its available here, if you wish to do the same, under the heading 'Christmas baubles'__

Much more festive, and presumably more fun, I really like the look of this Christmas quiz put together by @iteachboys92 on twitter, and mentioned recently on the sharing-physics groups page. I'm thinking I'll roll it out next week: :__ https://t.co/0OrmRK0Zzr #science #physics #edutwitter #maths https://t.co/Zqn8PYpTDD" / Twitter' __