A question occurred to me the other day — how big (physically) are the bits on a CD?
Measuring a CD’s inner and outer diameters, I got about 119mm for the outer diameter and 44mm for the inner diameter. Multiplying these by π*r2, I got about 38,400mm2 for the area occupied by the data. According to Wikipedia, a CD-R has 867,041,280 bytes of raw data (including the error-correction code and everything). Multiplied by 8, this is 6,936,330,240 bits.
Dividing, I got about 180,600 bits per mm2. Taking the square root, it turns out that about 425 bits placed end-to-end would be 1 millimeter long. Wow.
Imagine DVD-Rs (with 6-7 times the capacity, for single-layer) or BD-R discs (with about 36 times the capacity, for single-layer). 2,400 bits to the linear millimeter? Dayumn.
I took the CD-R and looked at it using one of the stereo microscopes we have in the lab — at full magnification, I was just able to see some wavy structure. You’d need something a bit more expensive (this was just a standard $250ish lab ‘scope) to see the actual bit patterns, I think.
On a somewhat- related note, happy Pi Day, everyone!
