TLDR: A cathode-ray tube (CRT) is a vacuum tube that uses electron beams to display images on a screen. It was commonly used in old TVs and computer monitors.

A cathode-ray tube, or CRT, is a type of vacuum tube that contains one or more electron guns. These electron guns emit electron beams that are manipulated to display images on a phosphorescent screen. CRTs were used in various devices, such as televisions, computer monitors, and oscilloscopes. In a CRT television, the entire front area of the tube is scanned repeatedly in a fixed pattern called a raster. In color devices, the intensity of each electron beam is controlled to produce different colors. CRTs have been largely replaced by flat-panel display technologies like LCD and OLED.

The history of CRTs dates back to the discovery of cathode rays in the 19th century. Scientists observed that these rays could be deflected by electric and magnetic fields. In 1897, J.J. Thomson measured the charge-mass ratio of cathode rays and discovered that they consisted of negatively charged particles called electrons. The first CRT, known as the Braun tube, was invented in 1897 and became the foundation of 20th-century television.

CRTs are made up of three main parts: the screen, the funnel, and the neck. The screen is the part of the tube that displays the image and is made of a special glass that can shield against X-rays. The funnel and neck are made of leaded glass to further protect against X-rays. The size and weight of a CRT depend on its screen size, with larger screens being heavier. The outer coating of the CRT is connected to ground, while the inner coating serves as the anode and is connected to a high voltage source.

Over time, CRTs became larger and more advanced, with color CRTs being introduced in the 1950s. However, with the development of flat-panel display technologies like LCD and OLED, CRTs started to decline in popularity. They were eventually phased out due to their bulkiness, weight, and the availability of cheaper and lighter flat-panel displays.

Despite their decline, CRTs still have some niche uses, such as in the airline industry and for retro gaming. They are also still manufactured for specific markets. However, their widespread use has been replaced by flat-panel displays.

In summary, a cathode-ray tube is a vacuum tube that uses electron beams to display images on a screen. It was commonly used in old TVs and computer monitors but has been largely replaced by flat-panel displays. CRTs have a long history and were once the foundation of television technology. They are made up of different parts and have specific properties to shield against X-rays. While they are no longer widely used, they still have some niche applications.