The Science Behind Soap Bubbles: A Dance of Light and Surface Tension

From their mesmerizing colours to their delicate shape...Soap bubbles hold more science than meets the eye.

Why do soap bubble show different colours?

What is the role of surface tension in keeping the bubbles spherical?

What is the role of gravity in colours shown by the bubbles?

Explore the answers in this blog post.

---

Soap bubbles are far more than mere playthings: they are hollow spheres of soapy water enclosing air. Each bubble is composed of three delicate layers: an inner soap layer, a middle water layer and an outer soap layer.

The formation of these layers is a result of the unique properties of soap molecules, which possess both hydrophobic (water-repelling) and hydrophilic (water-attracting) ends.

The thickness of a soap bubble is not uniform; gravity plays a pivotal role in creating a gradient of thickness. The bubble is thickest at the bottom and thinnest at the top. This phenomenon occurs as gravity pulls the water molecules downward, yet surface tension acts to stabilize the membrane formed by the soap and water. 

Over time, as gravity continues to draw water molecules closer together, the surface tension is eventually overcome, leading to the rupture of the bubble when most of the water has settled at the bottom.

The iridescent colors (i.e dazzling rainbow colours) we observe in soap bubbles are a direct consequence of the varying thickness of the soap film.

When light encounters a soap bubble, it undergoes two significant changes in medium: First from air to water, and then from water back to air. Each change in medium results in partial reflection of the light wave. The initial reflection occurs at the air-water interface and the second at the water-air interface.

These two reflected light waves are inherently identical but travel different distances due to the thickness of the soap film. This variation in travel distance, or path difference, between the two light waves leads to interference.

When these light waves converge, they interfere with each other to create the colorful patterns we see.

• Constructive interference: where the waves align and amplify each other to result in the reflection of the same color as the wavelength of light.

• Destructive interference: where the waves are out of phase and cancel each other out to give the perception of a different color.

Two main primary factors that influence this interference and the resultant colors (display) are:

1. The Angle of Incidence (how the light enters the surface), and

2. The Thickness of the Soap film.

A thicker soap film increases the path difference, altering the interference pattern of the reflected light waves. Consequently, the thickness of the soap film is the determining for the vibrant colors observed on the surface of soap bubbles.

In essence, the mesmerizing colors of soap bubbles are a beautiful interplay of Physics and Chemistry. The delicate balance of gravity, surface tension and light interference creates a captivating display that continues to intrigue scientists and layperson alike.

Understanding the underlying principles of soap bubble formation and coloration not only deepens our appreciation for these ephemeral wonders but also highlights the intricate connections between everyday phenomena and the fundamental laws of nature.

---

Written by: Kushal B R