The media that uses laser light technology for data access (reading and writing) is called Optical Media. Examples of optical media are CD and DVD. The laser beam reads and writes data starting from the centre of the disc and proceeding in spiral direction towards the outer edge.
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a.      Long Life – With proper care optical media can last long time

b.   Random access – OMD has the capability of random accessing of the data independent of the location of the data on the disc.

c.   Excellent storage and archiving

CD-R is short for Compact Disc Recordable. Its a write once and read multiple media. We cant reuse a CD-R. It is made up of polycarbonate substrate, a layer of organic dye, a metalized reflective layer and a protective lacquer coating.
CD-RW is a write multiple read multiple technology that can be written up to 1000 times which is designed for daily back-up & short term storage. The structure of the disc has a polycarbonate substrate layer, a protective layer and a reflective metal layer, plus two dielectric layers and a layer of phase-changing metal alloy.
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1. Polycarbonate layer
   This layer covers the most disc area, including the area that is read by laser. It is present on both sides of a DVD. This layer mainly maintains disc structure, gives enough strength to remain flat. Contact of any foreign material, dust, dirt etc may lead to misreading of data.
   

2. Data layer
   The layer, where the data has stored is called a data layer. In CD, the layer is very close to the label side of the disc. In DVD, they are in the middle.
   
3. Metal reflective layer
   The metal layer reflects the laser beam back to the photo sensor in the laser head. The metal  typically used for this layers are aluminum, gold, silver and silicon.
   
4. Lacquer layer
   This layer act as a protective cover to the label side. It protects the metal from exposure to the environment.

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Multisession disc is a compact disc to which data is added incrementally in more then one recording. A session is a recorded segment that may contain one or more tracks of any type. Multisession disc consist of a chain of single sessions, each having lead-in and lead-out areas. After each session is closed, the lead in of that session contains the recordable address of the next future session.
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The life of an optical disc depends on many factors such as type, quality, recording, handling, maintenance and environmental conditions. The disc will be able to read in 10-20 years time, given that they are not damaged or degraded in any way.
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Information is storing in CD as digital format. Pits represent logical 0 and pumps represent logical 1. The CD consist of helical shaped track on the surface. The laser from the drive moves through the track from the centre of the CD  to outer edge until the end of the track has been reached. The sensor accept reflected light from the disc and interpret the result.

                                            
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1. Handle discs by the outer edge or the centre hole. Don't touch the surface of the disc.
2. Use soft marker to mark the label side of the side.
3. Keep discs in storage cases after use. Thus reduce the effects environmental changes.
4. Don't expose disc to extreme temperature, prolonged sunlight or humidity changes.
5. Store discs in cool, dark and dry environment. Keep disc away from dirt or other foreign matters.
6. Clean the disc using smooth cotton fabric. You can use cleaning detergent, isopropyl alcohol or methanol.
7. Don't scratch, peel off or reposition the label of a disc .

In a CD-RW phase changing metal alloy film is present in between the polycarbonate substrate layer and metal reflective layer. Data is written to the disc by heating the film above the melting temperature. By heating the phase-changing film to a specific temperature above the crystalline temperature but below the melting temperature, the film can revert back to the crystalline state, thereby erasing previous bits. The writing and erasing processes can be done together in a single pass when rewriting a disc.
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It is a type of DVD with double the storage capacity of standard discs. It contains  two recording/ data layers. A transparent spacer is exist between the two recording layers. Semitransparent reflector layer of first recording layer accepts the laser to access data which is then passes to the second layer. Recording of first layer starts from  inner edge to outer edge, where as in the second layer it is from outer to inner.
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HD DVD stands for High Definition Digital Versatile Disc with high capacity(15 GB/layer) than normal DVD. It is considered as the successor to the standard DVD format. It uses blue laser of shorter wavelength to access data. Thick protective coating is available on the surface of the disc.
As the name indicates blue laser discs uses blue violet laser of shorter wavelength(405nm) to read and write data. The capacity is 25 GB/layer which is more than that of HD DVD. It uses a protective coating on the surface to ensure scratch resistance.
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1. Type and manufacturing quality
2. Recording
3. Handling and maintenance
4. Environmental conditions.

1. Temperature: The disc should be stored at a temperature similar to that environment it is using. Temperature variation may cause greater stress on the disc.
2. Exposure to sunlight: Prolonged sunlight will cause degradation of polycarbonate layer, which is then form coloring or clouding mark on the disc surface.
3. Moisture: Exposure to moisture will cause absorption of water in to the disc, which in turn react with any of the layers. Drying will dissipate out solvents from disc with water.
4. Organic solvents: Contact the disc with hard organic solvents must be avoided                    

1. Jewel Case: Hinged transparent plastic case with two opposing halves. Second half  holding a media tray that grip the disc by its media hole. It is the old and standard disc case. It is available in different configurations.
2. Slim Case: The structure of the slim case is same as Jewel case. But the thickness only the half allowing twice as many CDs to be stored in the same space.
3. Rollo Disc:
4. Cake Box: This is used for bulk storage of discs. This packaging consist of round plastic cup with central post that holds the stack of discs.
5. Eco:
6. Movie Box: Movie Box is the traditional name of the big casing for the discs. Basically this is comfortable packing for disc with good space length wise. This name is derived from the tradition of keeping the good and important data in such case for example movie.

That depends on what was being recorded, how it was being recorded, and how far along the process thing were. If it is failed while writing the lead in, before any data was written, the disc probably isn't usable. But some drives have a "repair disc" option that forcefully closes the current session. This would allow you to add extra data in a second session on the disc, but anything written in the first session will be unavailable.
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