Cathode Rays | Definition, Discovery, Experiments

Definition of Cathode Rays

Cathode rays are the beam of electrons traveling from the negatively charged Cathode to the positively charged anode at the other end of the vacuum tube. These Cathodes travel in a straight-line path at high speed when a voltage difference is applied to the electrodes.

cathode rays

The detection of cathode ray was a by-product of investigating the discharge of electricity through rarefied gases. The latter phenomenon had been studied since the early eighteenth century.

By the middle of the nineteenth century, it was known that the passage of electricity through a partly evacuated tube produced a glow in the gas, whose color depended on its chemical composition and its pressure. Below a certain pressure, the glow assumed a stratified pattern of bright and dark bands.

Discovery

Johann Hittrof discovered cathode rays in the year 1869. He used Crookes tunes and observed shadows and observed shadows cast on the glowing wall opposite the negative electrode in the tube.

J.J. Thomson discovered a new subatomic constituent of cathode rays in the year 1897. This subatomic particle was later named electrons. He found that the mass of the particles on the cathode ray is 1800 times lighter than hydrogen, the lightest element J.J. Thomson was awarded the Nobel Prize in Physics for this work.

Working of Cathode Rays

working of cathode rays

The electrode at the negative end is called a cathode. The electrode at the positive end is called an anode. Since the negative charge repels electrons, the Cathode is seen as the source of the cathode ray in the vacuum chamber. Electrons are attracted to the anode and travel in straight lines across the space between the two electrodes.

          Cathode rays are invisible, but their effect is to excite atoms in the glass opposite of the Cathode by the anode. They travel at high speed when voltage is applied to the electrodes. And some bypass the anode to strike the glass to be raised to a higher energy level, producing a fluorescent glow. This fluorescence can be enhanced by applying fluorescent chemicals to the back wall of the tube. An object placed in the tube will cast a shadow, showing that the electrons stream in a straight line, a ray.

                       Cathode rays can be deflected by an electric field. Which is evidence of it being composed of electrons particles rather than photons. The rays of electrons can also pass through the thin metal foil. However, cathode ray also exhibit wave-like characteristics in crystal lattice experiments. A wire between the anode and the Cathode can return the electrons to the Cathode, completing an electrical circuit.

       Cathode ray tubes were the basis for radio and television broadcasting. Before the debut of plasma, LCD, OLED screens, television sets, and computer monitors were cathode ray tubes.

Important properties of Cathode Rays

Here are some important properties of cathode rays;

  • Cathode rays travel in a straight line and can cast sharp shadows.
  • Cathode rays are negatively charged.
  • Electric field and magnetic field deflect cathode rays.
  • They are produced at the Cathode (negatively charged electrode) and travel towards the anode (positively charged electrode) in a vacuum tube.
  • The properties of the cathode rays do not depend on the electrodes and the gas used in the vacuum tube.
  • Speed of cathode rays is slower than light.
  • The object hit by cathode rays gets heated.
  • They can penetrate through thin metal plates.
  • Phosphorus glow when cathode rays fall on them.
  • Gas gets ionized by cathode rays.
  • Cathode rays are 1800 times lighter than hydrogen, the lightest element.

Cathode Ray Tube Experiment

cathode ray tube experiment

There are three experiments of cathode ray tube:

Experiment-1

In 1897, great physician J.J. Thomson conducted his first cathode ray tube experiment to prove that rays emitted from an electron gun are inseparable from the latent charge.

He built his cathode ray tube with a metal cylinder on the other end. The metal had two small diversions(slits), leading to an electrometer to measure small electric charges.

From the first experiment, he discovered that the electrometers stopped measuring electric charge. He deduced that the electric charge and the cathode ray must be combined and are the same entity.

Experiment-2

Then, he conducted a Second experiment to prove the charge carried by the cathode ray was negative or positive. He put a negatively charged metal plate on one side of the cathode ray to go past the anode and a positively charged metal plate on the other side.

Instead of an electrometer at one end of the Cathode Ray Tube, he used a fluorescent-coated tube that would glow were the cathode ray hit it. When the charged metal plates were introduced. He found that the cathode ray bent away from the negative plate and towards the positive plate.

This proved that the cathode rays were negatively charged.

Experiment-3

Then, he performed the third experiment to know the particles’ nature and reduce the particle’s mass as they had too small of a mass to be calculated directly.

For the experiment, he used the cathode ray tube with a high applied potential difference between the two electrodes, with the negatively charged cathode produced the cathode ray. He had already deduced that the particles were negatively charged.

Firstly, he applied an electric field in the path between anode and cathode and measured the deflections from the straight path. Now he applied a magnetic field across the cathode ray tube by using an external magnetic field. The magnetic field deflects the cathode ray.

Now he changed the direction of the external magnetic field and found that the beam of electrons is deflected in the opposite direction.

This experiment concluded that the electrostatic deflection is the same as the electromagnetic deflection for the cathode rays. He was able to calculate the charge-to-mass ratio of the electron.

After these three experiments, he deduced that inside the atom, there consists a subatomic particle, named initially ‘corpuscle,’ then changed to ‘electron,’ which is 1800 times lighter than the mass of hydrogen atom (Lightest atom).

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