Cathode rays (A-level only)

What you'll learn

• What a "cathode ray" actually is.
• The setup of a classic gas discharge tube.
• The step-by-step physical process of how cathode rays are produced.
• Why the gas inside the tube glows.

Welcome to the start of "The discovery of the electron" topic! Before physicists knew what an electron was, they observed strange glowing beams in glass tubes. They called these mysterious beams "cathode rays". To understand the experiments that led to the discovery of the electron, we first need to understand how these beams were made.

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What is a cathode ray?

In the late 19th century, physicists discovered that if they trapped a gas inside a glass tube at a very low pressure and applied a massive voltage across it, the tube would light up, and an invisible "ray" would shoot from the negative electrical terminal to the positive one. Because the negative terminal is called the cathode, they named them cathode rays.

At the time, scientists didn't know they were electrons—they just knew something was travelling through the tube.

The Discharge Tube Setup

To produce cathode rays, you need a specific piece of equipment called a discharge tube. The basic setup includes:

1. A sealed glass tube containing a gas (like neon or argon).
2. A vacuum pump to lower the pressure of the gas to a fraction of normal atmospheric pressure.
3. Two metal plates inside the tube: a cathode (connected to the negative terminal of a power supply) and an anode (connected to the positive terminal).
4. A high-voltage DC power supply (often generating several thousand volts).

How are cathode rays actually produced?

The production of cathode rays in a cold discharge tube is a chain reaction. It relies on the immense electric field created by the high voltage, calculated by $E=\frac{V}{d}$, where $V$ is the potential difference and $d$ is the distance between the anode and cathode.

Here is the exact sequence of events you need to know:

1. Stray electrons: Even in a normal volume of gas, background radiation (like cosmic rays) occasionally knocks an electron out of a gas atom, creating a few stray "free" electrons and positive gas ions.
2. Acceleration: The extremely high potential difference creates a strong electric field. This field exerts a force ($F=Eq$) on those stray electrons, rapidly accelerating them towards the positive anode.
3. Ionisation: Because the pressure is low, the electrons can travel far enough to gain significant kinetic energy before hitting anything. When they finally do collide with a neutral gas atom, they hit it with enough energy to knock out more electrons. This process is called ionisation.
4. Positive ion acceleration: The ionisation creates heavy, positive gas ions. The electric field pulls these positive ions in the opposite direction, accelerating them forcefully towards the negative cathode.
5. Emission of cathode rays: When these heavy positive ions smash into the metal surface of the cathode, the impact transfers enough energy to knock free electrons right out of the metal lattice.
6. The final beam: These newly freed electrons are instantly repelled by the cathode and accelerated towards the anode. This continuous stream of fast-moving electrons is the cathode ray!

Why does the tube glow?

As well as ionising the gas, the flying electrons also cause excitation.

When an electron collides with a gas atom but doesn't have quite enough energy to completely remove an electron, it can still bump one of the atom's orbital electrons up to a higher energy level.

The atom is now unstable. Shortly afterwards, the orbital electron falls back down (de-excites) to its original energy level. As it drops, it releases the excess energy as a photon of light ($\Delta E=hf$). Because millions of these collisions happen every second, the entire tube emits a continuous, visible glow.

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Worked Example

Let's look at how AQA typically assesses this descriptive process. They love asking for clear, sequential explanations.

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