The Deeper the Vacuum, the More Colors It Holds! The Mysterious World of Vacuum Tubes Glowing Under High Voltage

I am Ken Kuwako, a science trainer. Every day is an experiment.

In a pitch-black room, a glass tube glows softly. Blue, pink, orange… like shifting aurora light. This is neither special lighting nor CG. It is a physics experiment in which the color changes depending on the degree of vacuum. In this article, we introduce the mysterious light-emission phenomenon that occurs when high voltage is applied to a vacuum tube, along with its underlying mechanism.

What does a vacuum tube do?

Have you ever heard the term “vacuum tube”? It is an electronic component that was once essential in radios, televisions, and early computers, and is considered a “predecessor” of modern transistors and IC chips. The basic principle of a vacuum tube is very simple. Air is removed from a glass tube to create a near-vacuum, and electrodes are placed inside to allow electric current to flow. In a vacuum, electrons can travel freely without colliding with air molecules, enabling precise control of electric current.

Why the color changes depending on the vacuum level

In this experiment, four tubes with different vacuum levels glow in different colors when high voltage is applied. Please watch the video below.

Let’s look at the color of the discharge tube on the far left.

Second tube

Third tube

Fourth tube — it appears white.

Why does the color change? The key lies in the type and amount of gas remaining inside the tube. When voltage is applied, high-speed electrons are emitted from the electrodes. When these electrons collide with gas molecules remaining inside the tube, the molecules become excited to a higher energy state. When they return to their original state, they emit light, and the wavelength (i.e., color) of the emitted light depends on the type of gas.

In tubes with low vacuum (more air remaining), nitrogen and oxygen molecules contribute to the emission, producing pinkish and orange colors. As the vacuum level increases and residual gas decreases, the light shifts toward bluish-white light and even ultraviolet-like emission. When the vacuum becomes extremely high, electrons hit the inner wall of the glass tube, producing fluorescence (green light). This is the phenomenon known as “cathode rays,” which is the very principle behind cathode-ray tube televisions. In other words, the color changes are a sign that tells us what the electrons are colliding with.

This is the overall setup of the apparatus.

The day I learned the danger of an induction coil with my own body

While conducting this experiment, something unforgettable happened. While explaining the vacuum tube and demonstrating the setup, I accidentally touched the electrode of the induction coil. In that instant, I nearly lost consciousness. I was thrown backward by a violent shock and ended up crouching down. I had often told students that “high voltage is dangerous,” but that day I truly experienced the fear of electric shock with my own body.

An induction coil may look like a small device, but it can generate voltages of tens of thousands of volts or more. Behind the beautiful glow lies a dangerous form of energy that can be extremely hazardous if handled incorrectly. When observing experiments, it is important to keep a safe distance.

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