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

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

Light particles blast the photosensitive material on a silicon wafer to create the features on a chip, just as in photography, photons transform silver molecules on photographic film into visible images.

FORBES: Trick Of The Light

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

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