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Course: Exploratorium > Unit 2
Lesson 3: Soap Film Interference ModelComplete activity guide: Soap Film Interference Model
Soap-Film Interference Model
Introduction
Get on our wavelength! Soap films allow you to see the colorful appearance of difference wavelengths of light. This activity takes you further. By experimenting with this model of light-wave addition, you can understand the behavior of light as it is reflected off the front and back surfaces of soap films. Why do you see blue or red? It’s all a matter of phases.
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 move you from observing the soap-film phenomena to understanding the science behind them.
Tools and Materials
- Seventeen 5 x 7 in cards (approximately 12.5 x 15.5 cm)—we’ll call these “larger cards”
- Masking tape that's 1/2 in (1 cm) wide; the narrower the better
- Transparent tape
- Blue and red marker pens
- Scissors
- Pencil
- Optional: String
Assembly
Making your sine-wave template: Use the following trick to draw more accurate sine waves.
To make your blue sine-wave template, draw a line horizontally across the center of one of the smaller cards and mark the midpoint.
Along the top of the card, mark a point a quarter of the way in from the left edge and 1/2 in (1 cm) down from the top—your maximum.
Along the bottom of the card, mark a point a quarter of the way in from the right edge and 1/2 in (1 cm) up from the bottom—your minimum.
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:
Using scissors, cleanly cut along the sine wave that you’ve drawn. The top half and the bottom half give you two sine-wave templates—share one with a friend. Do the same with one of the larger cards to make red sine-wave templates.
To use this template, line up the bottom of the template with the bottom of the card and then draw along the wave edge. Hint: Watch the Soap Film Interference Model video tutorial to discover another trick for making a sine-wave template.
Making your wavelengths on the card backs:
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.)
Use the red sine-wave template and red marker pen to draw similar sine waves on the larger cards. These are your red wavelengths.
Flip both your red sine-wave template and the larger 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 moves downwards.
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. It’s important that the sides of the cards touch but don’t overlap.
As with the smaller cards, tape the larger cards together in two straight rows of eight cards each.
To Do and Notice
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.
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. 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.
Using masking tape, make two parallel lines on the floor. The lines should be one blue wavelength apart—5 in (12.5 cm) for index cards. The line on the left represents the front of a soap film and the line on the right represents the back of a soap film (for those who completed Soap Film on a Can: this is your water sandwich). Notice that the masking tape has a width. To account for this, line the start of the wavelength up with the left side of the front line of masking tape and line the end of the wavelength up with the right side of the back line of masking tape.
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. 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.
To see what would happen when the wavelength hits the front of the soap film—the line 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.
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.
Going Further
Using the larger cards, try adding red wavelengths to the experiment.
To adjust the thickness of the soap film, move 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 of it line to delineate the front and back of the very thin soap film. Alternatively, you can use a single piece of string.
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.
Experimenting with one-blue-wavelength-thick soap film:
You already have the soap film set at a thickness of one full blue wavelength. Try adding red waves to this. Notice that the soap-film reflections add up out of phase for blue light and therefore cancel. The red light also adds up out of phase, but not perfectly out of phase, so the reflected light has some red in it.
Experimenting with half-a-blue-wavelength-thick soap film:
Explore what happens when the soap films are half a blue wavelength apart. For the blue light, the front surface is at a wave maximum while the back is at a minimum. The extra path length is one full wave, so the waves cancel. Notice that while the soap-film reflections cancel for blue light, they add up in phase for red light, making the reflection red.
Experimenting with a very thin soap film:
The diagram below demonstrates what happens to the blue wavelengths when the soap film is very thin. For the blue light, the top wave flips over and there is no additional phase shift because there is no extra path length, so the waves cancel. Since the soap film is thinner than the wavelength of all colors of light, no wavelengths are reflected and the soap film looks completely transparent.
Experimenting with a quarter-blue-wavelength-thick soap film:
In these variations, the blue waves all cancel, which might lead you to think blue light is never reflected from soap films—however, try a soap film thickness of a quarter wavelength (see diagram below):
- Align the front of the soap film with the maximum of the waves. The back of the soap film will now be at a zero crossing.
- Top wave: Fold the wave back on itself as it hits the front of the soap film before and then flip it over (top to bottom).
- Bottom wave: Fold the wave that reflects from the back of the soap film back on itself.
You’ll notice that the reflected wave from the back will add with the reflection from the front; these are now in phase and result in a stronger blue light reflection.
Understanding the colors of the soap film from top to bottom:
We recommend doing the Soap Film on a Can video tutorial so that you can see what the colors on a soap film look like.
Soap films that are thin compared to the wavelengths of light moving through them reflect no light at all, making them invisible. If you try Soap Film on a Can, you can see this happening at the top of your soap film.
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.
Every integral multiple of a half blue wavelength in thickness, blue light is canceled; every odd multiple of a quarter blue wavelength, blue light is strengthened. Every multiple of a half red wavelength, the red light is strengthened. The result is alternating bands of bluish and reddish light as the film grows thicker—like contour lines on a topographic map.
Experimenting with green light:
To model how green light would behave, make green wavelengths out of 4 x 6 in (10 x 15 cm) cards and conduct the experiment again.
Going Even Further
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.
Want to join the conversation?
If white light is used for thin film interference than colors are formed due to interference or dispersion or both?(2 votes)