Soap Film Interference Model

Note: Prior to doing this activity, we recommend you try Soap Film on a Can to observe the colors on a soap film. Together, these two Snacks will help you move from observing soap-film phenomena to understanding the science behind them.

For this activity, you'll need to first make a sine-wave template, and then use the template to create a set of cards with sine waves drawn across both sides. 

Use this trick to draw more accurate sine waves:

1. Take one of the small index cards and fold it in half lengthwise. Unfold it, then fold the card in half crosswise then in half again. Unfold the card. You should see a long fold mark across the middle, and three shorter fold marks dividing the card crosswise into quarters.
2. Mark a point on the crosswise fold mark nearest the left edge, about 1/2 in (1.25 cm) down from the top of the card—your maximum.
3. Along the bottom of the card, mark a point on the right crosswise fold mark, about 1/2 in (1.25 cm) up from the bottom—your minimum.
4. Draw the sine wave starting at the center-left side of the card moving through the maximum, then back through center, and through the minimum point ending at the center-right side of the card.
   
5. Using scissors, cleanly cut along the sine wave that you’ve drawn. The top half and the bottom half produce two sine-wave templates—give one to a friend. 
6. To use this template, line up the bottom of the template with the bottom of a new card and, using a blue marker, draw along the wave edge. Repeat until you have created twenty-four cards with matching blue sine-waves drawn on them.
7. You'll need sine waves on the back of the cards too, so flip both your blue sine-wave template and the smaller cards over on a horizontal axis (from top to bottom) and draw a sine wave on the back that starts at the center of the left edge of the card and move downwards. (If you hold the card up to a light, the wave on the front and the wave on the back will coincide.)
   
8. Using transparent tape, tape the smaller cards together into two straight rows of twelve cards each, making sure that all the sine waves move upwards from the left side (following the original pattern of the template before you flipped the cards over). It’s important that the sides of the cards touch but don’t overlap.
9. Once you've finished making your two sets of blue sine-wave cards, follow all of the steps above again with the larger cards and a red marker to make two sets of red sine-wave cards with eight cards each.

Compare the blue and red sine waves. Notice the difference in length between the blue and red waves. The red light waves will be longer than the blue. You have made a good model of red and blue light waves.

Using masking tape, make two parallel lines on the floor to model the front and back surfaces of the soap film. These parallel tape pieces need to be one wavelength of blue light apart: align the outside edges of the tape so they are five inches apart—the width of one of the blue wavelength cards. The left edge of the left piece of masking tape represents the front of the soap film and the right edge of the right piece of masking tape represents the back of the soap film (for those who completed Soap Film on a Can: this is your water sandwich). 

Lay out the two rows of blue waves across the parallel lines, starting with two maxima to the left of the front soap film surface and extending all the way through the soap film (click to enlarge diagram). This should mean the waves cross both the front and back soap film surfaces—the masking tape lines—at the maximum point of each wave.

To see what would happen when the wavelength hits the back of the soap film—the line on the right—fold the bottom wave back on itself at the point that it hits the back surface. The drawing on the back of the index card is then displayed and shows the reflected wave. In this experiment, the outgoing and incoming wave lay exactly on top of each other. The exact alignment of incoming and outgoing waves is only true when you position the maximum or minimum point of the wave at the reflecting surface—in this case, the back surface of the soap film, represented by the right side of the right piece of tape. These two waves are in phase, so blue light is reflected and strengthened.

To see what would happen when the wavelength hits the front of the soap film—the left edge of the tape on the left—fold the wave back on itself at the point that it hits the front of the soap film, then flip the whole wave about a horizontal axis (top to bottom), so that the maximum becomes a minimum. This process is called inversion.  

Inversion occurs when a light wave reflects as it goes from a high speed of light material—air—to a lower speed of light material—soap (click to enlarge diagram). 

After you invert the wave, unfold it so there is no double thickness. The maxima that were to the left of the front surface originally will get unfolded so they are to the right. The two waves are now out of phase and cancel each other, so no blue light is reflected.

There are two things occurring here that contribute to the phases of the reflected waves. As the top wave of light hits the soap film, it reflects back off the front of the soap film and flips over—showing the forces after inversion. The bottom wave travels into the soap and then reflects off the back of the soap film, travelling back to join the top wave.

There are three important points to notice on the sine waves: the maximum (the highest point), zero crossing (the point where the wave crossed the middle line of the card), and the minimum (the lowest point). Scientists describe these points as the phase. When two waves add up “out of phase,” this means the highest point of one wave lines up with the lowest point of the other, canceling out the light. This is called destructive interference. When waves add up “in phase,” this means the highest point of one wave lines up with the highest point of the other, strengthening the light. This is called constructive interference.

There are many other experiments you can do to explore how light waves behave when they are reflected off soap film. We've included some of them in the video demonstration above.

For example, using the larger cards, try adding red wavelengths to the experiment. You can also adjust the thickness of the soap film by moving the masking tape lines closer together. Try soap films that are one blue wavelength apart, half a blue wavelength apart, a quarter of a blue wavelength apart, and a very thin soap film. To make the very thin soap film, you can use one strip of masking tape and draw a line down the center to delineate the front and back of the very thin soap film. Alternatively, you can use a single piece of string, as we do in the video.

When experimenting with the thickness of a soap film, always arrange the incoming wavelengths so that a maximum of each wave hits the front surface of the soap film.

To model how green light would behave, make green wavelengths out of 4 x 6-inch cards (roughly 102 x 152 mm; closest metric equivalent is A6) and conduct the experiment again.

Soap films that are very thin compared to the wavelengths of light moving through them reflect no light at all, making them invisible. You see this at the top of the soap film in the Soap Film on a Can activity. Moving down the film, when it is half of a wavelength of blue light thick, the blue waves add up out of phase and cancel. At this point, the soap film is now a quarter of a wavelength of red light thick and the red waves add up in phase, resulting in a reddish color band.

Blue light is canceled at soap-film thicknesses that are even multiples of one-half blue wavelength, and strengthened at odd multiples of a quarter blue wavelength. Red light is strengthened at multiples of a quarter red wavelength. The result is alternating bands of bluish and reddish light as the film grows thicker—like contour lines on a topographic map.

In reality, the wavelength of light actually changes when it enters the soap film—we have not added this additional complication to our model. To factor this in, think of the thickness of the soap film in terms of the number of wavelengths measured in the soap film. Since the index of refraction of soap is a little higher than water, for soap use n = 1.4. To calculate the wavelength of light in soap, divide the wavelength in air by 1.4.