|Disks store digitally encoded video and audio information in spiral grooves called pits. These grooves run from the center of the disk to the edges. The data is accessed when a laser reads the other side of these pits i.e. the 'bumps' enabling us to watch a movie or listen to a CD.|
If a disk contains a large amount of information the pits are packed closely together and are much smaller. As the pits become smaller the laser needs to become more and more accurate in its ability to read the data.
DVDs use a red laser to read and write data but the Blu Ray disk uses a blue laser to access the information. Blue lasers have a shorter wavelength (405 nanometers) than red lasers (650 nanometers) and the smaller beam has the ability to focus on the pits more accurately.
Blue Ray disks can read information recorded in pits as small as 0.15 microns (µm) (1 micron = 10-6 meters) long i.e. twice as small as the pits on a DVD. In addition Blu Ray has reduced the track pitch from 0.74 microns to 0.32 microns.
The combination of the smaller pits, smaller beam and shorter track pitch mean that a single-layer Blu Ray disk can hold in excess of 25 GB of information, that's about five times the amount of information that can be stored on a DVD disk.
A Blu-ray disk is about 1.2 millimeters thick (the same as a DVD), but the disks store data in different ways. DVD disks store data between two polycarbonate layers, each 0.6-mm thick. This polycarbonate layer can cause a problem called birefringence i.e. the substrate layer refracts the laser light into two separate beams.
If the beam is split too wide apart, the disk cannot be read and if the DVD surface is not perfectly and perpendicular to the beam, it may lead to 'disk tilt', in which the laser beam is distorted. For these reasons the manufacturing of DVDs is a very involved process.
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Hope my answer is clear enough.