Properties of Cathode Rays

What are Cathode Rays?

• Cathode rays (also known as electron beams or e-beams) are streams of electrons that can be seen in vacuum tubes.

• When a voltage is applied to an evacuated glass tube with two electrodes, electrons emitted from the cathode cause the glass opposite the negative electrode to glow.

• Cathode rays were the first to contain electrons, which led to their discovery.

• Johann Hittorf discovered cathode rays in the year 1869. He used Crookes tubes to observe shadows cast on the glowing wall opposite the tube’s negative electrode.

• A focused beam of electrons deflected by electric or magnetic fields in cathode ray tubes generates the image in a television set (CRTs).

• Cathode rays are so-called because they are emitted by a vacuum tube’s negative electrode or cathode.

• Electrons must first be separated from the cathode atoms before being released into the tube.

• Though cathode rays are invisible, their existence was first detected in vacuum tubes when they fell on the glass wall, vibrating the glass atoms making them emit a glowing light known as fluorescence.

• Researchers discovered that if objects were placed in the tube in front of the cathode, they could cast shadows on the glowing wall, implying that there was something travelling in straight lines from the cathode.

• After reaching the anode, electrons travel through the anode wire to the power supply and back to the cathode, resulting in cathode rays carrying electric current through the tube.

Experiment by J.J Thomson

• Cathode ray tubes were observed by J.J. Thomson. He reached the conclusion that the particles in the cathode beams must be negatively charged because they were deflected by negatively charged objects (either the cathode or a negatively charged plate in the cathode ray tube) and attracted by objects that are positively charged (either the anode or the positively charged plate in the cathode ray tube).

• He dubbed these minuscule atom fragments “electrons.”

• Dalton’s atomic theory was disapproved by Thomson through his experiments. According to Dalton’s atomic theory, atoms were the smallest piece of matter in the universe and were indivisible.

• The presence of electrons clearly invalidated these aspects of Dalton’s atomic theory.

Important Properties of Cathode Rays

The important properties of Cathode rays are stated as below:-

Property 1. – Cathode rays travel in a straight path and cast shadows that are sharp.

Property 2. – Cathode rays have a negative charge.

Property 3. – Cathode rays are deflected by electric and magnetic fields.

Property 4. – They are generated at the cathode and move towards the anode in a vacuum tube.

Property 5. – The cathode rays’ properties are unaffected by the electrodes or the gas used in the vacuum tube.

Property 6. – Cathode rays travel at a slower rate than light.

Property 7. – Cathode rays cause heat to be emitted from the objects they strike.

Property 8. – Cathode rays possess the ability to pass through thin metal plates.

Property 9. – Cathode rays cause phosphorus to glow while falling on it.

Property 10. – Cathode rays ionise the gas.

Property 11. – Cathode rays are 180 times lighter than the lightest element, hydrogen.

Conclusion

A cathode ray is a stream of electrons that leave the negative electrode (or cathode) in a discharge tube containing gas which is at low pressure, or electrons emitted by the heated filament in certain electron tubes. Cathode rays focused on a hard target on a small object in a vacuum produce X-rays or extremely high temperatures.

They have a number of properties which are stated above.