23 Discoveries Leading to the Nuclear Atom Model
LumenLearning
Cathode Rays
Cathode rays are electron beams or streams of electrons that were observed for the first time in Crookes tubes (vacuum tubes).
LEARNING OBJECTIVES
Define a cathode ray
KEY TAKEAWAYS
Key Points
- Electrons accelerated to high velocities travel in straight lines through an empty cathode ray tube and strike the glass wall of the tube, causing excited atoms to fluoresce or glow.
- Researchers realized that something was traveling from the anode when objects placed in the tube in front of it could cast a shadow on the glowing wall. Cathode rays carry electronic currents through the tube. Electrons were first discovered as the constituents of cathode rays.
- J.J. Thomson used the cathode ray tube to determine that atoms had small negatively charged particles inside of them, which he called “electrons.”
Key Terms
- cathode rays: Streams of electrons observed in vacuum tubes
- crookes tube: An early experimental electrical discharge tube, invented by English physicist William Crookes and others around 1869-1875, in which cathode rays, streams of electrons, were discovered
Cathode Rays
Cathode rays (also called electron beams or an e-beams) are streams of electrons observed in vacuum tubes. If an evacuated glass tube is equipped with two electrodes and a voltage is applied, the glass opposite the negative electrode is observed to glow from electrons emitted from the cathode. 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 so named because they are emitted by the negative electrode, or cathode, in a vacuum tube. To release electrons into the tube, they must first be detached from the atoms of the cathode. The early cold cathode vacuum tubes, called Crookes tubes, used a high electrical potential between the anode and the cathode to ionize the residual gas in the tube. The electric field accelerated the ions and the ions released electrons from the cathode when they collided with the cathode.
Modern vacuum tubes use thermionic emission, in which the cathode is made of a thin wire filament that is heated by a separate electric current passing through it. The increased random heat motion of the filament atoms knocks electrons out of the atoms at the surface of the filament and into the evacuated space of the tube. Since the electrons have a negative charge, they are repelled by the cathode and attracted to the anode. They travel in straight lines through 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 no