That bit of trickery works because the light can be manipulated when the material is shaped with features about as big as the light's wavelength - less than a millionth of a metre for light that we can see.

BBC: Invisibility cloaking benefits from crystal-clear idea

The DVD includes this material, plus an additional 81-minute disc of special features, many of which have never been seen before.

CNN: Behind the scenes: 'The Beatles Anthology'

The special features material on the second DVD improves on the Philosopher's Stone release as fans journey into the world of witchcraft and wizardry.

BBC: Dobby the house-elf