Why Soap Bubbles are Colorful

A previous post, Beyond Visible Light Photography, described how color is a function of the wavelength of light. This post considers why we see colors on soap bubbles.  (As a caveat, this post uses many simplifications. Optics is a complex subject.  A complete description of this topic would require a large and complex volume. I am only trying for “good enough.”)

This is the wavelength diagram from the earlier post. It describes light (and other electromagnetic energy) in terms of frequency and wavelength.

Imagine light waves to be like waves on the ocean. They can be represented with the high points as the crests, low points as troughs, and a red arrow indicating the average height (i.e., a zero energy line):

The distance between successive waves is the wavelength, L. The time between peaks is represented by t and the frequency (how many peaks per second) equals 1/t and is represented by the letter v.

A nanometer (nm) is one billionth of a meter (pretty small). For blue light, L would be about 400 nm.  For a red light, L would be about 700 nm. The time between peaks, t, for blue light is 1.5*(10^-15) seconds (divide 1.5 by 10 fifteen times). For red light, t is 2.33*(10^-15) seconds (a little slower but still pretty fast).

Wave Interference

Think about what happens when ocean waves crash into each other near a shoreline. An even larger wave is created when the crest of one wave merges with the crest of another. On the other hand, the crest of one can fill trough of another causing both to disappear. This works with light too. If a light wave whose time between peaks is t meets an otherwise identical light wave arriving t/2 seconds later, they will cancel each other out.

Wave Cancellation

To demonstrate this phenomenon, I made a diffraction grating by cutting two closely spaced slits in a piece of aluminum foil taped onto a cardboard frame. I then shone a laser pointer through them. This caused the light waves from each slot to arrive at the same points on a screen (I used the cover of my lightbox as a screen) at slightly different times.

Basic Setup

A photo of the results shows dark banding where the two light waves hit the same points on the screen t/2 seconds apart. An actual beam splitter would show the effect more strongly but I don’t have one.

Reflection and Refraction

When you look at someone’s glasses in bright light, there is some glare but you can also see the persons features distorted by the lens. The former is the reflection of the glasses and the latter is refraction. Refraction is the bending of light when it passes from one medium (e.g., air) to another (e.g., the glasses). The refractive index measures how much a medium (e.g., air, glass, soap film) bends the light as it enters. A key concept is that light reflected from a surface (e.g., glasses) with a larger refractive index (i.e., bends the light more) than the one through which it has been travelling (e.g., air) has its peaks shifted by t/2 seconds. The inner surfaces of glasses also reflect light but, since the air beyond it bends light less than glass, the peaks are not shifted.

Reflection and Refraction

A soap bubble is like a lens in a pair of glasses. It has an outer surface of soap molecules, a middle layer of water molecules, an inner layer of molecules, and air inside. Both layers of soap molecules reflect some light. Since soap molecules bend light more than air, the light reflected from the outside of the bubble is shifted by t/2 seconds. On the other hand, since the refractive index of the air inside the bubble is less than soap, the peaks of the inner reflection are not shifted in time.

If a bubble is extremely thin, distance between the inner and outer soap layers is approximately zero. The inner and outer reflections will (approximately) overlap.  The crests of the inner and outer reflections will be t/2 seconds apart and the light waves will cancel each other.  The surface of the bubble will appear black. This indicates that the water layer is almost gone and the bubble is about to pop.

Now consider a thicker soap bubble.  The light causing the inner reflection will take time to pass through the bubble. If this time equals a multiple of t for some color in the visible spectrum, the inner and outer reflections will cancel and that color will be removed from the reflection. The subtraction and reinforcement of light waves cause the iridescent beauty we see in soap bubbles.

Real Bubbles

Light hitting a soap bubble generally contains many colors (e.g., white light is a mixture of the visible colors). From a normal source, a light beam hits a bubble at many different angles causing many different inner and outer reflections. The thickness of the bubble’s water layer varies. Thus, the color cancellation patterns from the bubbles surface can vary dramatically. This causes the bubble reflections to display the patterns seen in the following photographs. Whew!

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