Ans: Cathode rays (also called an electron beam or an e-beam) are streams of electrons observed in vacuum tubes. The glass opposite the negative electrode starts glowing when an evacuated glass tube equipped with two electrodes is applied a certain voltage and the electrons hit the aforementioned glass wall. Electrons were first discovered as the constituents of cathode rays. The image in a classic television set is created by focused beam of electrons deflected by electric or magnetic fields in cathode ray tubes (CRTs).
Cathode rays are named after their origin place – the negative electrode, or cathode, in a vacuum tube. Electrons are released into the tube when they get detached from the atoms of the material of cathode. Crookes tubes(The early cold cathode vacuum tubes), used a high electrical potential between the anode and the cathode to ionize the residual gas in the tube. The ions got accelerated by the electric field and the ions released electrons when they collided with the cathode.
Thermionic emission is used in modern vacuum tubes – the cathode is made of a thin wire filament that is heated by a separate electric current passing through it. The filament’s increased random heat motion of atoms knocks electrons out of the atoms at the surface of the filament and into the evacuated space of the tube. The electrons are repelled by the cathode and attracted to the anode due to their negative charge. They travel in straight lines across the empty tube. The voltage applied between the electrodes accelerates these low mass particles to high velocities.
Cathode rays are invisible, but their presence was first detected in early vacuum tubes when they struck the glass wall of the tube, exciting the atoms of the glass and causing them to emit light—a glow called fluorescence. Researchers noticed that objects placed in the tube in front of the cathode could cast a shadow on the glowing wall, and realized that something must be traveling in straight lines from the cathode. The electrons travel through the anode wire to the power supply and back to the cathode after they reach the anode, so cathode rays carry electric current through the tube.
History of Cathode Rays
Michael Faraday passed current through a rarefied air-filled glass tube and observed a strange light arc with its beginning at the cathode (negative electrode) and its end almost at the anode (positive electrode).