Home | Submit an Article or Contact Us Did you know1. DVDs are made from a 0.6 mm thick disc of polycarbonate plastic coated with a much thinner (reflective) aluminium layer. Two such discs are glued together to form a 1.2 mm double-sided disc. The substrates are half as thick as a CD to make it possible to use a lens with a higher numerical aperture and therefore use smaller pits and narrower tracks. 2. There is a 15-mm hole in the centre of the disc, usually used by some form of clamp or clip device within the player to hold it in place and allow it to be rotated by a motor. The diameter of the hole is the same as that of a former Dutch 10-cent coin. Philips engineers at the time used the coin to determine the size of the hole, and the coin fits inside the hole perfectly.
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