An optical media is a storage device which can hold a considerable amount of electronic data written by a laser. The conventional optical media that you are probably familiar with are CDs (Compact Discs) and DVDs (Digital Video Discs or Digital Versatile Discs). You can store images, videos, and data files on these media, which have a very long storage times - on the order of decades.
How do these pieces of innocent round plastic store information and in such a huge amount?
It may be very difficult to understand the scientific and technical principles involved in optical media storage but I can make it a little simpler for you. Before we plunge to the exciting world of optical media, let us first discuss the different types that are available. Basically, there are three types of CD media: Permanent, Writable and the Re-Writable. The Permanent media represents those that you can buy from software shops and record bars. These discs were manufactured by writing data on them using large scale mass production machines. The second type, the Recordable, is also called a blank CD. You can put any type of data on them as long as you have a CD writer or burner. Lastly, the re-writable CDs are available with the same features as the writable ones only that you can actually erase previously “burned” (the term for writing or saving data on a CD) data and write new data on the disc.
All of the three types of discs are written using a laser, which is built into the optical writing device, such as your CD writer. The main concept is pretty simple, when you initiate the burning process, using your writer software, the data on your computer are converted into signals which are then interpreted by your CD writer. These signals are then converted into pulses of infrared laser light which are focused into a tiny, microscopic point inside the blank CD. Each pulse of laser light, or absence of a pulse, corresponds to the digital information being recorded. As the disc spins and the laser pulses, a series of dots are formed in the disc and follow a spiral path that has been molded into the plastic. The data is recorded starting from the center and moving to the outside edge of the disc. When the burning is complete, you can then playback the data saved on the CD.
In permanent discs this information is imbedded in the disc as a series of pits. In a re-writable disc there is a layer of photosensitive dye that changes it's reflective properties when exposed to the laser light and thereby encodes the digital information.
Why etch a spiral track on the CD?
The track is used for the burner to properly place the data so that the disc can be read by another drive. The reading process is done by shining the laser light on the track and registering the reflected light, which is then translated into the proper date format by the software reading the disc.
What about DVDs?
DVDs have the same characteristics of ordinary Recordable and Re-Writable CDs, only they have larger capacities. They can store up to 4.7 Gigabytes of data for single sided disks, in contrast to about 700 Megabytes of data supported by most Compact Discs.
CD Burner: http://computer.howstuffworks.com/cd-burner6.htm