**conversational tone**

Good afternoon. If I asked how many of you used a computer to prepare for your speech tonight, I would expect the majority of the hands in the room to rise. We all use computers, and we all generate information. We live in an age of information – information that needs to be stored somewhere. We use hard drives, Zip drives, CD-ROMs, and a host of other media to store our information. Though it plays a crucial role in our lives, most of us never think twice about information storage. It’s just supposed to work, like magic. As a system administrator, it’s my job to know the ins and outs of the computers I work with. I can’t afford to simply take things for granted, though I find myself doing it all too often. **hold up a CD-ROM** See this CD-ROM? It seems like such a simple thing, doesn’t it? Every one of you has likely used one of these. Do you know how it works? Probably not. Tonight, I’m going to give each of you the opportunity to stop taking it for granted. First, I’ll introduce you to this CD-ROM disc, and how it actually works. I’ll also introduce you to one of its younger cousins, the CD-Recordable. You will see that there is more going on than meets the eye when you stick one of these discs in your computer.

In order to understand how your computer reads a CD-ROM disc, you will need to understand both the nature of lasers, and the nature of the digital data we read with them. **Visual Aid: laser light vs. white light** What’s special about lasers is that their light is coherent. In other words, laser light is very organized. It has a specific wavelength and color, as you can see here. **point to the laser light diagram** It travels a very predictable and orderly path. White light, on the other hand, has no pattern to it. It contains many colors, and has no particular wavelength. **point to the white light diagram** It’s because of this that we use lasers. We need that specific wavelength, to read the data on the CD. "So," you may ask, "how does the laser read the CD?" Well, you may already know that computers think in binary. That is, everything a computer thinks is in 1’s and 0’s. It stores its information in binary as well. A CD-ROM disc is no exception – everything on the CD is either a "1" or a "0". **Visual Aid: pits and lands** If you could look at a CD very closely, you’d see tiny little indentations stamped all over the surface. These are called "pits". The places that aren’t indented are called "lands". What’s significant about these pits and lands is their height. Remember that laser light has a specific wavelength. The difference in height between a pit and a land just happens to be one half the wavelength of the laser light. When the laser light bounces back from a pit, it’s wave is in a certain position when it gets "read" by the CD-ROM drive. **point to diagram** When the laser light bounces back from a land, it’s wave is in the exact opposite position. This is because of the precise height difference between the pits and lands. When the laser light is backwards like that, it’s called "being 180 degrees out of phase." It’s this 180 degree difference that lets the CD-ROM drive know what’s a "1", and what’s a "0". String millions of these little pits and lands together, and you have a complete CD-ROM. Nifty, eh? **smile, and look friendly**

Now that you’ve been introduced to the idea, you may not even care. That’s okay -- at least you’re not completely in the dark any more. But that’s only the beginning. **smile again** Let me introduce you to a personal friend of mine, the CD-Recordable Disc, or CD-R. **hold up CD-R discs, then hand them to someone near the front of class** Feel free to take a look at them. Don’t worry about touching it – they’ve already been ruined. **return to front, by podium** CD-R discs accomplish the same thing as a regular CD, but they do it in a different way. A CD-R disc is not stamped with little indentations, like a CD. Rather, the indentations are actually "burned" into the disc by a CD-ROM burner, one of which I have (by the way). The disc comes completely blank, with no data on it. There’s a recording layer inside the disc, which consists of a kind of dye. When the laser of a CD-ROM burner switches to recording mode, it actually melts some of the dye inside the disc, reducing its volume, lowering it a bit. **do hand gesture for melting dye** This produces what we now know as a "pit". Where the dye has not been melted are the "lands". Of course, the actual process is a bit more complicated, dealing with laser wattages and other things, but that’s the essence of it.

There is another new CD technology, called CD-Rewritable, or CD-RW. It’s far too complicated to include in this speech, but I will mention it briefly. CD-Rewritables allow you to write and erase a single disc up to 1,000 times, making it a highly flexible medium for storage. Perhaps I’ll cover this new technology another time. For now, I hope that I’ve left you with a better understanding of how CD-ROMs work, and perhaps an increased interest in not taking so many things for granted. If not, then at least you got to see a pretty CD-R disc. I expect to get those back, by the way. **smile** In any case, thank you all very much for listening, and I’ll see you next time.