Soap film colors

Introduction

A color figure will help you understand the colors you see in a soap film.

Material
The color image below
A ruler

To Do and Notice

If you have not studied the colors in a soap film do the Soap film in a can activity first.
To Understand the reflected waves better do the Soap Film Interference activity.

Look at the color image above.
Notice that along the right edge appear the colors that you see in a soap film. The colors have been scanned into a computer from a color photograph of an actual soap film. However the soap film has been turned upside down so that the thinnest part of the soap film is at the bottom. Real soap films are thinner at the top because the water drains down out of them due to gravity.

Notice that the image of the soap film is black at the bottom where the soap film is thinnest, much thinner than a wavelength of light. As you examine higher portions of the soap film it becomes thicker. To the left of each color is printed the thickness of the soap film in micrometers (millionths of a meter.) which produces that color.

To understand how these colors are produced look to the left of the soap film colors. You will see a spectrum of light arranged by wavelength with violet at the left and red at the right. The spectrum is crossed by diagonal black bands.
To find the color of the soap film place a ruler horizontally across the banded spectrum so that the ruler crosses the color you wish to understand. The color of the soap film is the sum of all the colors crossed by the ruler. For example, the color of a soap film of thickness 0.80 micrometers (indicated by the lower horizontal dotted line) is magenta, at this thickness the black band removes green light from the spectrum, the remaining blue and red colors add to produce magenta. At a thickness of 0.9 micrometers the soap film is cyan. Two different black bands have removed the red and a portion of the blue light.

What’s Going On?

A thin soap film is black (transparent)

The light that reflects from a soap film is the sum of the light that reflects from the front of the film and from the back of the film. When the soap film is much thinner than a wavelength of any color of visible light the light reflecting from the front and the back cancel each other for all wavelengths. When light reflects while going from air to water its wave is inverted, while it reflects while going from water to air it is not inverted. A light wave and its inverse cancel each other. As a result thin films reflect no light and so are transparent. Against a black background they look black. A ruler placed across the black band at the bottom of the figure touches no colors, so the sum of all colors touched by the ruler is black.

A light wave hits a thin bubble film. The wave reflects from both Some of the light goes through the bubble. surfaces. The reflections cancel.

A 200 nm thick soap film is orange
When a soap film is 1/2 as thick as a given wavelength of light, that wavelength of light will not reflect from the soap film, see Soap Bubbles. This is a combination of the inversion of the light wave at the front of the bubble and the extra full wavelength the light travels through the bubble to the back plus its return to the front. So when the soap bubble thickens to .20 micrometers, 1/2 a wavelength of blue light in soap, the soap film will not reflect blue light. It will reflect other colors however, these colors add to produce a pastel orange color. A 0.2 micrometer thick soap film is 1/4 of a wavelength of red light thick, red light reflected from front and back surfaces of the soap film adds up in-phase and is brightly reflected.

blue light of wavelength 400 nm hits a soap film 200 nm thick. The reflections off the front and the back cancel.

A 100 nm thick soap film is bluish silver
A soap film that is 0.1 micrometers thick is 1/4 of a blue wavelength thick. At this thickness the reflected blue light from the front and back surfaces adds up in-phase and the blue is strongly reflected. All other colors are also reflected, although somewhat more weakly. The sum of all reflected colors is whitish.

Light hits a soap film The reflections from the front and rear
that is 1/4 wavelength thick. surfaces add up in-phase.

A soap film that is 1/2 of a wavelength of red light thick does not reflect red light.
In fact a soap film whose thickness is 1/2 of the wavelength corresponding to any color of light does not reflect that color. Since red light has a longer wavelength than blue light a black band stretches across the spectrum showing the bubble thickness at which each color is removed.

In fact, at any multiple of 1/2 wavelength of light that color of light does not reflect from the soap film. These non-reflections are shown as black bands crossing the spectrum. The colors seen in the bubbles are made by all of the colors which do reflect from the bubble film.

Etc

Note, the wavelength of light in the water/bubble solution is actually shorter than the wavelength of light in air. It is shorter by about 40%. This shortening has not been illustrated in the above diagrams. See the Soap Film Interference activity for a longer discussion.

Etc

If you wish to print out just the color image above, it is available by clicking here, go to soap film color image.