"There's an IR LED and a phototransistor pair mounted under the outer rim of the platter. A thin piece of black foam absorbs IR light and it generates a fairly sharp negative pulse on the output of the phototransistor."
Let's talk time scales real quick. Your computer's CPU lives by the nanosecond: most CPUs can get a few things done in each nanosecond – mostly simple math and comparisons. To make this easier to grasp, suppose you're the CPU and instead of nanoseconds, you live and work second by second. For clarity I'll keep this metaphor to a single-core of a single processor. You can hold a few things in your head (register). Not more than a dozen or two in your active memory, but you can recall any of them pretty much instantly. Information that's important to you you'll often keep close by, either on sheets of loose-leaf paper on your working desk (L1 cache) a couple seconds away, or in a one of a handfull of books in your place (L2 and up cache) which is so well organized that no individual piece of information is more than a dozen or so seconds away. If you can't find what you're looking for there, you'll have to make a quick stop at the library down the street (RAM, i.e. main memory). Fortunately, it's close enough that you can go down and grab a book and get back to work in only ~8 and a half minutes, and it's enormous, some are thousands of times the size of a typical strip-mall book store. A little inconvenient, until you remember that this library has a free delivery service, so it's really no bother at all so long as you can still find things to work on while you wait. But the local library mostly just stocks things on demand (which is fair, your bookcases, worksheets, and even the dozen or two facts you hold in your head are mostly the same way). The problem is that when you need something that's not there, it can take a while to get it. How long? Think Amazon.com in the age of exploration. They send out an old wooden boat and it could be a week, could a month, and it's not unusual to wait 3 years before you hear a response. Welcome to the world of hard disk storage, where your information is retrieved by making plates of metal spin really fast. Many metric tons of sweat have been spent making this as fast as possible, but it's hard to keep up with electrons flowing through wires. So when someone says that Solid State Disks are awesome, it's because they're able to turn that slow, unpredictable old sailing ship into a streamlined steam-powered vessel. A good SSD can often make the voyage in less than a week, sometimes in little more than a day. It can also make many thousands more quests for information per year.