Thanks to Emily for letting me investigate the properties of a glass sphere. It turns the world upside down because refraction in the glass bends light from the top downwards and light from the bottom upwards. This is because light slows down when it goes into the glass and speeds up when it comes out. A sphere doesn't focus the light perfectly to one point but the image is clear so the multiple focal points must be close together in this arrangement. This is called spherical aberration - see
https://en.wikipedia.org/wiki/Spherical_aberration for photographs.
