What is a Cathode Ray?
A cathode ray is defined as a stream of electrons leaving the negative electrode (or cathode) in a discharge tube that contains gas at low pressure or the electrons emitted by the heated filament in certain electron tubes. X-rays or a very high temperature are created by cathode rays centred on a hard target (anticathode) on a small object in a vacuum (cathode-ray furnace).
Light Effect
When cathode rays collide with particular molecules used to cover a cathode plate, the molecules (and thus the screen) emit light. This particular effect, when coupled with the controlled deflection of a cathode ray either by a magnetic or electric field, the cathode-ray oscilloscope (rather than the cathode-ray tube CRT) is used to track the values and changes of an alternating current or voltage, as well as the image tube used in radar and television.
Properties
Like a wave, the cathode rays travel in straight lines and form a shadow when obstructed by the objects. Ernest Rutherford has demonstrated that the rays could pass through the thin metal foils, the behaviour expected of a particle. These conflicting properties have caused disruptions when trying to classify it either as a particle or wave. Also, Crookes insisted it was a particle, while Hertz maintained it as a wave.
This debate between them was resolved when an electric field was used to deflect the rays by J. J. Thomson. Since scientists knew it was difficult to deflect electromagnetic waves with an electric field, this was evidence that the beams were made up of particles. Also, these can create fluorescence, mechanical effects, and more.
Later in 1924, Louis de Broglie showed in his doctoral dissertation that, in fact, electrons are much like photons in the respect that they act both as particles and as waves in a dual manner as Albert Einstein had shown for light earlier. In 1927, Germer and Davisson used a crystal lattice to specifically illustrate the cathode rays' wave-like behaviour.
Description
Cathode rays are so-named due to the reason they are emitted by the negative cathode or electrode in a vacuum tube. To bring the electrons into the tube, they must first be isolated from the atoms in the cathode. Crookes tubes were the first cold cathode vacuum tubes, and they worked by ionising the remaining gas atoms in the tube between the cathode and the anode using a high electrical potential of thousands of volts.
The ions that are positively charged accelerated with an electric field towards the cathode, and when they collided with it, they knocked the electrons out of its surface. These electrons were the cathode rays. Modern vacuum tubes often use thermionic emission, in which the cathode is a thin wire filament heated by a separate electric current flowing through it. The increased random heat motion of the filament knocks electrons out of the filament's surface and into the tube's evacuated space.
Since electrons hold a negative charge, they are attracted to the positive anode and repelled by the negative cathode. They also travel in straight lines via an empty tube. The high velocities of these low mass particles are accelerated by the voltage applied between the electrodes. Cathode rays are invisible, but when they reach the glass wall of an early vacuum tube, they excite the atoms and cause them to emit light, which is known as fluorescence.
Researchers observed that the objects placed inside the tube in front of the cathode could cast a shadow on the glowing wall and realized that something (a kind of ray) must be travelling from the cathode in straight lines. After reaching the anode, electrons pass through the anode wire, present to the power supply, and then return to the cathode. Hence, cathode rays carry electric current through the tube.
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(In the above figure, the cathode rays pass from the cathode at the rear part of the tube, striking the front glass and making it glow green by the fluorescence. A metal cross present in the tube casts a shadow, describing the rays travelling in straight lines.)
A small negative voltage can be applied to a metal screen of wires (otherwise known as a grid) between the anode and cathode to regulate the current in a beam of cathode rays passing through a vacuum tube. The wire's electric field deflects a few electrons, preventing them from reaching the anode. The range of current that gets through to the anode depends upon the voltage present inside the grid. Therefore, a small voltage present on the grid can be made to control a much larger voltage present on the anode.
This is the main principle, which is used in vacuum tubes for electrical signal amplification. The triode vacuum tube, which was developed between 1907 and 1914 and is still used in some applications such as radio transmitters, was the first electronic system that could amplify.
Electric fields created by additional metal plates in the tube where the voltage is applied, or magnetic fields created by wire coils, can also direct and control the high-speed beams of cathode rays (otherwise called electromagnets). These can be used in the cathode ray tubes that are found in computer monitors and televisions and also in electron microscopes.
FAQs on Cathode Ray
1. What exactly are cathode rays?
Cathode rays are streams of fast-moving, negatively charged particles called electrons. They are produced in a special glass tube called a discharge tube when a very high voltage is applied across two metal electrodes in a near-vacuum. They get their name because they originate from the negative electrode, known as the cathode.
2. How are cathode rays produced inside a discharge tube?
Cathode rays are generated under specific conditions in a setup called a cathode ray tube:
- A glass tube is filled with a gas at very low pressure.
- A high voltage (several thousand volts) is applied between two metal plates, the cathode (negative) and the anode (positive).
- This high voltage ionizes the gas atoms, knocking electrons free.
- The powerful electric field then accelerates these free electrons from the cathode towards the anode at very high speeds, forming the cathode ray.
3. What are the main properties of cathode rays?
Cathode rays have several key properties that were discovered through experiments:
- They travel in straight lines from the cathode.
- They consist of negatively charged particles, now known as electrons.
- They are deflected by both electric and magnetic fields, which confirms their charged nature.
- They can heat the material they strike and can cause certain materials to fluoresce (glow).
- Their properties are the same regardless of the electrode material or the gas inside the tube.
4. How did J.J. Thomson's experiment show that cathode rays were particles?
J.J. Thomson's experiments provided strong evidence that cathode rays were particles, not waves. He placed a small paddle wheel in their path, and the rays caused the wheel to spin. This demonstrated that the rays had mass and momentum, which are defining properties of particles. This experiment was crucial in identifying the electron.
5. Why are cathode rays considered negatively charged?
The negative charge of cathode rays was confirmed because they are consistently deflected towards the positive plate when an electric field is applied across their path. Since opposite charges attract, this behaviour proves that the particles making up the rays must carry a negative charge. They also bend in a magnetic field exactly as negatively charged particles should.
6. Why do cathode rays travel in straight lines?
Cathode rays travel in straight lines because, in the absence of any external electric or magnetic fields, there is no force to bend their path. Just like an object moving through space, these high-speed electrons continue along their initial trajectory unless a force acts on them. This property is easily observed by placing an object in their path, which casts a sharp shadow on the opposite wall of the tube.
7. Does the type of gas inside the discharge tube change the nature of the cathode rays?
No, the fundamental nature of cathode rays does not change. Cathode rays are always streams of electrons, regardless of the gas used. J.J. Thomson discovered that the charge-to-mass ratio (e/m) of these particles was always the same. This was a critical discovery, suggesting that electrons are a universal constituent of all matter. The type of gas only affects how easily the rays are produced and the colour of the glow seen in the tube.
8. What is the main difference between cathode rays and anode rays?
The main difference lies in their composition, origin, and properties:
- Cathode Rays are streams of electrons that originate from the cathode. Their properties are universal and do not depend on the gas in the tube.
- Anode Rays (or Canal Rays) are streams of positive ions. They are formed when electrons collide with gas atoms, knocking off other electrons. Their properties, like mass, depend on which gas is used inside the tube.
9. How did cathode ray tubes (CRTs) create images in old televisions and monitors?
In old CRT devices, an 'electron gun' fired a focused beam of cathode rays towards a screen. The screen was coated with materials called phosphors that glow when struck by electrons. Powerful electromagnets would steer this beam rapidly across the screen, line by line, while varying its intensity. This process would 'paint' a full image on the screen so quickly that our eyes perceived it as a single, steady picture.
10. If cathode rays are invisible, why do we see a glow in the tube?
The cathode rays themselves, being just a stream of electrons, are indeed invisible. The glow we see is the result of the rays interacting with their surroundings:
- The high-speed electrons collide with the few remaining gas atoms in the tube, exciting them and causing them to emit light.
- When the rays strike the glass wall at the end of the tube, they cause the glass (or a special phosphor coating) to fluoresce, creating a bright spot of light where they hit.
