Optical disc an additional tolerance to allow the use of less-than-perfect analog audio pre-filters to remove any higher frequencies. The first version of the standard allowed up to 75 minutes of music which required 650MB of storage. The DVD disc appeared after the CD-ROM had become widespread in society. The third generation optical disc was developed in 2000–2006 and was introduced as Blu-ray Disc. First movies on Blu-ray Discs were released in June 2006. Blu-ray eventually prevailed in a high definition optical disc format war over a competing format, the HD DVD. A standard Blu-ray disc can hold about 25 GB of
Optical medium Optical medium An optical medium is material through which electromagnetic waves propagate. It is a form of transmission medium. The permittivity and permeability of the medium define how electromagnetic waves propagate in it. The medium has an "intrinsic impedance", given by where formula_2 and formula_3 are the electric field and magnetic field, respectively. In a region with no electrical conductivity, the expression simplifies to: For example, in free space the intrinsic impedance is called the characteristic impedance of vacuum, denoted "Z", and Waves propagate through a medium with velocity formula_6, where formula_7 is the frequency and formula_8 is the wavelength