The Physics of CDs and DVDs

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The Physics of CDs and DVDs

Everywhere you look now days, you see or interact with CDs or DVDs; whether it is in the car, at work, or at home for pleasure, you deal with these modern marvels. Whatever the use, CD has become a reliable medium in which to distribute information, in a reliable way. The cost of a CD is very inexpensive, making it a popular choice by companies trying to advertise, or teach you about something. American On Line (AOL) is one example of companies who have chose this reliable source for communication. CD is the abbreviation for compact disk; DVD is the abbreviation for digital video disk or digital versatile disk. The difference between the two is the CD is audio, and the DVD is audio and visual. The objective of this paper is to help you to understand the physics surrounding the CD and DVD. The main focus of this paper is to inform the reader of all the things that take place when you watch a movie on a DVD player, starting with the CD itself. The laser reads the CD and converts it to the final product of what we see and hear.

Most CDs are made from polycarbonate plastic. This polycarbonate is a tough material that helps to prevent scratching. Scratching of a CD causes the laser, which reads the spiral data, to jump off track, allowing the CD to skip around and to distort whatever you are watching or listening to. According to How Things Work,

A CD is a fairly simple piece of plastic, about four one-hundredths (4/100) of an inch (1.2 mm) thick. Most of a CD consists of an injection-molded piece of clear polycarbonate plastic. During manufacturing, this plastic is impressed with microscopic bumps arranged as a single, continuous, extremely long spiral track of data. Once the clear piece of polycarbonate is formed, a thin, reflective aluminum layer is sputtered onto the disc, covering the bumps. Then a thin acrylic layer is sprayed over the aluminum to protect it. (Brian 1)

After that, you have a final product called a CD.


Often, you will hear about bumps on a CD. These bumps are what the laser reads. They are also sometimes referred to as pits. Bumps and pits are the same thing; it just depends on what side of the CD you are looking at.

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On the aluminum side, they appear as pits, but they appear as bumps on the side the laser reads. All of these bumps are little pieces of data from the music or video. Data storage is different on a CD vs. a DVD. CDs require a longer track pitch than a DVD. A DVD requires about half of that of a CD. This allows for a DVD to hold more data. Not only do DVDs have shorter track pitches, they have multi-layer levels that store even more data.

I Specification I CD I DVD 1600 740 Track Pitch nanometers nanometers Minimum Pit Length
(single-layer DVD) 830 nanometers 400 nanometers Minimum Pit Length (double-layer
DVD) 830 nanometers 440 nanometers

Single-sided, single-layer DVDs can store about seven times more data than CDs. A large part of this increase comes from the pits and tracks being smaller on DVDs. To increase the storage capacity even more, a DVD can have up to four layers, two on each side. The laser that reads the disc can actually focus on the second layer through the first layer. (Nice 1)

This technology allows DVDs to hold up to 8 hours of data.

Now that we know a little bit more about how a CD is made and how much information it can hold. It is time to learn about the player. The player is the ting that does all the work. All we have to do is to throw in a CD or a DVD, and sit back and enjoy. CD and DVD players are made up of basically three main things: a drive motor, a laser, and a lens. Each part of the player has an important role. The drive motor’s job is to spin the CD. It can spin the CD anywhere from 200 to 500 RPM’s, depending on which track is being read. The laser’s job in the player is to read the bumps on a CD. The laser reads these bumps and produces what is needed to hear what is on a CD or to see what is on the DVD. Our book says a laser is an acronym for, “light amplification by stimulated emission of radiation (Kirkpatrick 599).” The lens is part of a tracking system that reads the spiral of data. It keeps the laser on track, so that everything is played in the order in which it is suppose to be played. Once this all comes together, the player works like this:A CD player uses a laser beam to determine the lengths of a series of ridges inside a compact disc. Infrared light from a solid-state laser is sent through several lenses, a polarizing beam splitter, and a special polarizing device called a quarter-wave plate. It's then focused through the clear plastic surface of the compact disc and onto the shiny aluminum layer inside the disc. Some of this light is reflected back through the player's optical system so that it passes through the quarter-wave plate a second time before encountering the polarizing beam splitter. The two trips through the quarter-wave plate switches the light's polarization from horizontal to vertical (or vice versa) so that instead of returning all the way to the laser, the light turns 90° at the polarizing beam splitter and is directed onto an array of photodiodes. These photodiodes measure the amount and spatial distribution of the reflected light. From this reflected light, the CD player can determine whether the laser beam is hitting a ridge or a valley on the disc's aluminum layer. It can also determine how well focused or aligned the laser beam is with the aluminum layer and its ridges. (Bloomfield 3)

The information gathered from the bumps are measured and then sent to produce the music. This is where the computer portion of the player comes into play. The computer takes the information gathered from the CD and turns it into what we hear.

In closing, I would like to talk about what we have learned. The CD is a magnificent thing that is very much a vital part of our society. We use this great technology everyday. Weither it is watching a movie, playing a video game, or even typing a paper on the computer, we use this technology. A CD can hold a lot of data, and, as technology advances, the boundaries will keep growing and growing.

Works Cited

Bloomfield, Louis A. “Compact Disc Players.” How Things Work. 24 Apr. 2003. <http://howthingswork.virginia.edu/compact_disc_players.html>.

Brian, Marshall. “Understanding the CD.” How CDs Work. 24 Apr. 2003. <http://electronics.howstuffworks.com/cd1.htm>.

Kirkpatrick, Larry D., and Gerald F. Wheeler. “Physics, A World View.” Chapter 23, “Lasers.” 2001 by Harcourt College Publishers.

Nice, Karim. “Data Storage: DVD vs. CD.” How DVDs and DVD Players Work. 27 Apr. 2003. <http://electronics.howstuffworks.com/dvd3.htm>.


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