A rainbow colored soap bubble is very beautiful.
Why is it rainbow colored?
How does it have many colors?

The colors of a bubble have something to do with the nature of light. Light has wavelike properties. The height of the wave determines brightness of light, and the width of the wave determines colors of light. The width of the wave is shown as the distance from peak to peak of the wave, and it is called the “wavelength”

When different colors of paint are mixed together, the color turns black, but when some colors of light are mixed, the color turns white. Sunlight and light bulbs are mixtures of colored lights. This is why the sunlight and light bulbs look white.

Since light is like a wave, it becomes brighter or darker depending on the following conditions :

Overlapping peaks of waves
→ If two peaks are added together, light gets brighter.

Overlapping a peak and a valley of waves
→ If the two waves cancel each other, there is no light.

This is called “interference” ;.

In sunlight, areas, where waves of red light are strong, look red, and areas, where waves of blue light are strong, look blue.
These colors make soap bubbles look rainbow colored.

How do the colors appear?
We see the soap bubbles due to two sources of visible light. One source of light is reflected from outer surface of the bubble wall, and the other light source passes through the bubble wall and is reflected from the back surface of the bubble wall.

Let’s draw waves along the paths of light. The waves do not overlap each other if the wavelengths are different, but they overlap when the wavelengths are the same. It means that the light with the same wavelength looks bright.

Now, let’s change the thickness of the bubble wall, and then, the wavelength of the overlapping waves changes.

The thickness of the bubble wall is almost the same as the wavelength of visible light. Light that strikes the bubble is white because it is a mixture of many different colors of light, and light waves that are overlapped in the white light are reflected brightly.
Since the thickness of the bubble wall changes with the area of the bubble, the colors of the reflected light change, and thus, the bubble looks rainbow colored.

What color does the bubble appear to be when the wall of the bubble thickens?
It is “colorless and transparent.”

How can the bubble, which is made from colorless liquid, be visible? Well, the overlapping light waves cause a rainbow of colors. Why do the beautiful colors disappear, as the bubble wall becomes thicker? When a bubble wall has a certain thickness, some of the light is intensified by interference. Look at the picture below.

The first waves from the left is light with the longest wavelength among the lights that can be intensified by interference. When a light wave that is reflected from the back surface of the bubble wall passes through the bubble wall, it overlaps the light wave that is reflected from the outer surface of the bubble. The lights with other wavelengths are also strongly reflected from the surfaces in the same manner as the first one.

When the thickness of the bubble wall is the same as the wavelength of visible light, only one or two colored lights are clearly visible in the reflected lights. As shown in the picture above, only red light is visible, so the reflected light looks red.

What happens when the bubble wall becomes thicker?

There are more colored lights that are visible. In other words, we can see a mixture of colors. When many colored lights are mixed, the wall looks white.

What happens when the bubble wall becomes thinner?
When the wall becomes too thin, interference from visible light with the shortest wavelength, which is violet-blue (wavelength is about 400 nm), does not occur. So, the wall looks colorless.

When the wall becomes much thinner, compared with the wavelengths of visible lights, lights reflected from the outer and back surfaces cancel each other. So, lights are not reflected from the surface of the bubble. Therefore, the bubble looks almost invisible.

The inside of a thick glass ball is hard to see through but when the wall of the ball becomes thinner, the inside can be seen well. When the wall becomes as thin as that of a soap bubble, we can see rainbow colors on the surface of the glass ball. When the wall becomes thinner, the glass ball is invisible.

Optical and electronic properties of materials totally change when the materials become nano-sized. This is why nanotechnology is very interesting.

Let’s make various kinds of soap bubbles!
 → Experiment : Mysterious soap bubbles

(nanonet : Kazunori Komori)
(Character Design : Fusae Tanaka)