Compact disc is a new kind of disc which is recorded and played by laser beam. Compact disc has silvery, mirror-like surfaces which reflect light in a rainbow spectrum. The music disc is about 12 cm dia. while video disc is about the size of an LP and hold both pictures and sound.
In the recording process, sound signals are converted into number so that each part of the signal has a precise code. These numbers are recorded as the binary digits 0 and 1. Physically, sound is recorded on a CD as a series of minute pits and flats which relate to the two digits.
The laser disc has a very reflective metallic surface, covered by a protective coating of clear plastic. A semiconductor or small He-Ne laser is used for scanning. The player spins the disc and scans with laser beam which moves straight across the disc from the centre to the edge. The shiny surface reflects the beam back into the player, where it is picked up by electronic device. This produces an electrical signal which the player decodes back into video pictures and sounds. The laser beam reads about 20,000 digits every second which are converted into sound signals.
The biggest advantage of compact discs is that they never wear out because there is no physical contact between the disc and the player – only a beam of light.
Most large planes have an autopilot. This is a device operated by a computer. It will fly the plate without the pilot’s touching the controls. These autopilots can even control take offs and landings.
Tubes are present inside a conventional television set. In these tubes the images are broken down into small pixels with the help of a beam of electrically charged particles. The beam sends information about the colour and brightness of each pixel to a chemically luminescent layer on the rear of the screen. The electron beams light up the pixels on this layer depending upon the speed. They become blurred to form a picture for our eyes. Along with the fat-bellied tube televisions, there are also televisions with flat screens that have been improved very rapidly. 
Pictures can ‘run’ only because our eyes cannot register more than 20 pictures per second. If we see more than 20 pictures, they look very similar but differ only in minor things; our brain perceives them as moving. Flip books or cartoon films work on this principle. The movements could not be recorded with cameras for a longtime because the exposure times were too long. The first serial photography succeeded in 1872. The British photographer Eadweard Muybridge captured a running horse with 30 cameras. He mounted the finished pictures on a disc, which was rotated in a device. Through a viewing hole you could see a running horse. Later individual photos were recorded with just one camera on a single film. This was the birth of the cinema. 
The film as the storage option for images was invented at the beginning of the 19th century. The researchers at that time were experimenting with toxic chemicals to store light reflected from the objects on a long-term basis. The Frenchman, Louis Daguerre, finally succeeded in 1837 to fix the images on a light-sensitive material in a salt bath. This was the first film. However, the exposure times were very long. People had to sit for a long time while the photographer captured their image on a photographic plate. The path to the film roll was paved in 1889 by the American George Eastman – a thin layer of silver salts was applied on flexible plastic celluloid. Digital cameras do not need this because they store everything on a computer chip.