How do we get so much more information on a DVD than on a CD?

It’s a challenge, all right. DVDs use similar material and manufacturing processes, but there are several differences.

Part of the solution was making the bumps that carry data smaller and jamming them closer together, making the “groove” of a DVD 7.5 miles long, more than twice what you get on a CD.

But wait, there’s more. The designers managed to double that capacity to 15 miles per side, because the DVD uses a sandwich of two different levels of bumps.

Like a CD, the DVD has a reflective aluminum base behind the inner layer. However, on top of that is a semi-reflective gold layer, which also contains data bumps.

The result is that the laser can read the top layer, then focus through it and read the inner layer as well.

The cumulative effect is that a typical DVD has 7.5 gigabytes of storage.

DVD stands for Digital video disc.