ID:
1060
USA
Tour Audio:

This laser disc comes from a set of  twenty LaserDiscs produced in the early 1990s by the Education Group as the Video Encyclopaedia of Physics Demonstrations. It contains lecture demonstrations suitable for first year physics students. The LaserDisc can be played on a LaserDisc player.

The wave model of light can be used to explain the phenomena of interference, which has many modern applications. One of these applications is in storing and reading of information on disks like this laser disc, CD and DVDs.

The color of light is caused by its wavelength. The visible spectrum of light ranges from 400 nm (blue) to 700nm (red). If two waves of light with the same wavelength meet they interfere with each other and this interference can be constructive or destructive. If the two waves are in phase then their amplitudes will add together, constructive interference. If the two waves are not in phase then their amplitudes will cancel each other out, destructive interference. This causes the rings of light that Newton observed.

Information is stored on disks in the form of small pits that are pressed into the reflective aluminium film and then covered with a transparent plastic to protect them. A laser beam is focussed on the disk and reflects into a sensor. The pits have a depth of a quarter of the wavelength of the laser that is used to read them. The part of the light that enters these pits hits the bottom and bounces back up, travelling half a wavelength further than the part that bounces off the upper surface of the disk. This means that when the two parts intersect in the sensor above the disk they are out of phase and destructively interfere. The reader sees this as dark and light patches which it interprets as a pulse width modulated analogue signal. Later formats like CD and DVD use binary codes, just zeros and ones. The shorter the wavelength of the light, the smaller the pits can be and the more pits a disk can hold. This is why a blu-ray disk, which is read by a blue laser, can store more information than normal CDs, which use a red laser.

This technology was pioneered by Dr David Gregg who first envisioned the optical disk in 1958. He patented the technology in 1961 and 1969. In the early 1960s Gregg’s company Gauss Electrophysics was bought by MCA, who also bought the patent rights. In 1978 MCA Discovision released the first consumer optical disk player. The optical disk was replaced by the DVD in 1997

This item is part of the UQ Physics Museum ‘2015, International Year of Light’ Tour