How is informationstored in CDs/DVDs?

CDs and DVDs are random(or direct) access type storage devices or media. The file location information is written into a content page area, called the File Allocation Table(FAT), in the zero sector of the4 medium. This area is also known as meta data area.
The read-head is programmed to read the file address and directly access the file of interest. The digital information in the files is in the form of combinations of two clearly distinguishable states of the medium, designated, normally as zeros and ones.
Actually, the sensitive material of the Cds can exits in two states, transparent and opaque. These materials are known as phase change materials (PCM) because they change from amorphous (opaque) to crystalline (transparent) phase by suitable thermal treatment. The PCMs are often an alloy (AgInSbTe) of silver, indium, antimony and tellurium.
writing and erasing in a CD is carried out by an infrared laser spot focused to about a micrometer in size. This spot locally raises the temperature up to 400 degree Celsius and converts the transparent crystalline light sensitivity material of a "land" spot to opaque amorphous state of the "pit" spot.
The amorphous state reflects less light than the crustal line state does. Therefore, by starting with a disc surface in the crystalline state, heating with the laser can change small spots to the amorphous state and the rapid colling of the spot causes the material to freeze in the amorphous state.
This appears dark during "reading". After the information of the file is written, the address is entered into the meta-data area for reference.
During the erasing an amorphous spot, it is converted back to crystalline state by a process known as annealing, accomplished by heating the material to a lower temperature.
The exiting data in CD that has been "quick-erased", can be overwritten by turning the laser on continuously to the erase power, which will erase any existing marks. Switching the laser to deliver a higher power, which is sufficient to melt the material, enables the creation of a new data.
Although, the commercial claims are that the read write CDs/DVDs can be erased and rewritten 1000 times, the actual number is smaller; further, the shelf life of the data in a RW medium is short owing to slow shelf annealing over period of about 2 years.

H.K.SAHU
scientific officer
Indira Gandhi Central for Atomic Research
Kalpakkam,
Tamil Nadu.

TRANSFORMER

How does a transformer increase of decrease the voltage?
A transformer is an electrical device that transfer electrical power from one circuit to another. It can raise or lower the voltage in a circuit with a corresponding decrease in the current. It operates on the principle of mutual induction between two coils which are electrical separate but magnetically linked. A simple transformer consist of soft-iron laminated core over which two separate coil are wound called the primary and secondary coils. The coils are insulated from each other and also from the core. The alternating voltage to be transformed is applied toe the primary coil. This causes a varying magnetic flux in the laminated core, most of which is linked with the secondary coil in which mutually induced alternating voltage is produced. If the secondary coil circuit is closed, a current flows in it and thus electric energy is transferred from the primary coil to the secondary coil. The ratio of the secondary voltage(output Voltage) to the primary voltage (input Voltage) can be shown to be equal to the ratio of the number of turns in the secondary to the number of turns in the primary. By selecting this ratio properly, the transformation can be done from any input voltage to any convenient output voltage.
If the secondary has lesser number of turns than the primary, the output voltage is greater than applied voltage and the device is called a step-up transformer. If the secondary has lesser number of turns than the applied voltage and the device is known as a step-down transformer.
For an ideal transformer, power in the primary is equal to that in the secondary. This implies that the current is transformed in the inverse ratio of the voltage. Thus in a step- up transformer, the voltage increases but the current decreases and in a step-down transformer, the voltage decrease but the current increases.

K.Kannan,
Retired professor of physics ,
Erode, Tamil Nadu