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Light interference in thin films

Light interference in soap films occurs because the thin film has two reflecting surfaces, the lower and upper surface. When looking at the film from either side, light will reflect from both surfaces. These surfaces are very close together, typically not more than a few microns (10-3 mm). The reflected parts of a white-light wave, which after reflection have traveled different distances, will interfere with eachother constructively or destructively. Since the wavelength is different for every color (frequency) in the light spectrum, the angles over which the interference occurs differ for every color: you observe patterns of colors.

Introduction

Soap films, films of oil on water, and some kinds of thin plastics may exhibit light-interference colors. These interference colors also occur in nature. Examples are cloud irisation and solar or lunar coronas in clouds.

About soap bubbles and films

To study (and photograph) the interference colors, it is handy to know the basics of soap films. You will have seen them when you were a youth, blowing soap bubbles, or as an adolescent, doing the dishes.

Soap bubbles are soap films with inclusions of air. They may drift in the air or on froth, appearing spherical, or they may sit on a liquid (water) surface, where they will usually be half-round. The inclusion of air is therefore trapped, and the soap bubble will remain as long as the soap film is able to support the surface pressure (tension) it exerts on its own surface. The soap bubble wants to contract; the trapped air presses outwards on the film surface, and equilibrium is reached.

Larger soap bubbles are more unstable in time than smaller ones. In particular, a soap film which you may find inside a glass after doing dishes, will usually disappear within a few minutes, while the tiniest soap bubbles which make up the froth in the sink may remain for hours.

Soap films become thinner with time, due to evaporation (for the smaller bubbles) and additionally by gravity (for very large bubbles). You can study this by means of the interference colors. If you observe a wide range of colors at multiple repetitions (red-green-blue-red-green-blue-... etc.), the film is still fairly thick compared to the light wavelength, as the colors indicate changes in thickness.

Color bands in a soap film, due to variable film thickness. The film is thinnest at the top left, where no constructive interference of colors in the visible range is possible and the film appears black.

As the color bands move further apart, the film surfaces become more parallel while the film becomes smaller. In the beginning, you may see predominantly red colors, which have a longer wavelength. As the film becomes thinner, constructive interference for the longest, red wavelengths, is no longer possible and you see predominantly green, and later blue colors. As the film gets even thinner, you observe that even the blue color disappears; the film appears now completely dark to the eye.

This thin state does not last long; in a few seconds up to one minute after the film colors disappear, the film will burst.

With this in mind, you can estimate when a film will exhibit beautiful color bands, and how long you will have to photograph. When photographing, which I assume you will do on macro-mode, you spend a great deal of time focusing the camera and getting the light right. It helps to make a few soap films beforehand and watch their characteristics to learn how your specific mixture of soap and water behaves.

Photography

Thin films show predominantly blue colors. "Droplets" of higher soap concentration in the film make the film slightly thicker and create the colorful globules.

You will need:

  • Wire loop, such as from a metal coathanger
  • Black sheet of paper, or (better) black cloth
  • Bowl of water
  • Dishwashing soap
  • Sugar or glycerin
  • Camera with 135mm telephoto lens
  • Cable release
  • Height-adjustable tripod
  • Macro-bellows or adaptor rings, enough for 10cm travel (4")
  • Some books or other heavy objects

Bend the wire into a rectangular or square loop, which closes onto itself, and measures about 5cm x 7cm (2" x 2.5"). Put the black board or cloth flat on a table, which is in either direct or indirect sunlight (an overcast sky works best; do not use artificial lighting. The light should be omnidirectional, or you will have trouble with the exposure). Use the books to position the wire frame a short distance above the black surface, keeping the wire loop hanging free, hovering above the cloth.

All this beauty is really just in a soap film!

Mix some dishwashing soap with water, and if you like the sugar/glycerin. The sugar or glycerin helps to lengthen the lifetime of a soap film. Try making some soap films by dipping the wire loop into the mixture and observe the film.

For photography, I found it was easiest to mount a 135mm telephoto lens on an SLR camera, with bellows of 10cm (4") in between. This should give you a macro mode for the telephoto lens which allows you to photograph subjects at close range (up to 20cm or 8in, as I found).

The technique is to put the tripod next to the table, let the camera point down from nearly directly above the wire frame, with the sun shining on the film at an angle. Then wait for the color pattern to "freeze" (this happens when you have a small wire loop like this), and take photos. Make sure that the color patterns will be sufficiently still at the chosen exposure time to yield sharp photos, if you photograph before the pattern freezes. If you use a lot of sugar/glycerin in the mix, the soap film colors will freeze very well and show striking colors.

Note: you can also rotate the whole setup if you can't position your camera on the tripod to point downward; just make sure that sunlight will be incident on the film and that you have the black background.

A few tips

A spherical soap film will show circular color patterns with lots of details.

  • You can also use a 50mm standard lens on macro mode (bellows), but the camera may get too close to the wire frame and actually block the sunlight. This results in shadowed parts of your photo. You cannot circumvent this by photographing the film at an angle, as your depth-of-focus on macro mode is very shallow and parts of your photo would be unsharp.

  • Try to leave the wire frame out of your photo. Keep in mind that the camera viewfinder may show less of the actual photo (many cameras show only 80% to 90%).

  • You need to wait a few minutes for the soap film to become stable. Initially, you will see that the color bands appear, move and deform quickly. Since you may need exposure times down to 1/4 second, make sure that the colors are stationary.

  • A high aperture number to increase depth-of-focus is only possible in case the film colors are stationary, since the high aperture will require a long exposure time (up to a few seconds).

  • Use a sturdy tripod and cable release, or your photos will be unsharp.

  • For variety, you can hold the frame with your hands initially, and shake it around just after dipping it in the soap solution, in order to get some more interesting color patterns. You can make this into an art after some practice.

  • Larger wire frames will cause more unstable films which will not freeze the color pattern, but which will exhibit other beautiful color patterns. Experiment with different sizes of wire frames.

  • You will see an interesting effect by making a soap film on a big mug (choose a black one for better color contrast!), wet a straw, stick it through the film and blow air in the glass in order to make the film spherical. This way, you will observe circular color bands and the film will get thinnest at the center (top).