# Using Bubbles to Learn about Light Interference

## Objective

To understand the wave properties of light, especially the phenomenon of interference.

### Concepts:

• Light travels in waves.

• Sometimes, two or more waves join together.

• Interference is the addition, or coming together, of several waves.

• Constructive Interference happens when two or more waves come together to form a larger and stronger wave, matching their crests and troughs.

• Destructive Interference is when two or more waves come together and cancel each other out to make a weaker wave.

### Principles:

• Since light has wave properties, it will experience interference (the addition of waves).

• This interference is like that seen with water waves.

• Whether you get constructive or destructive interference depends on the wavelength (color) of the light.

• White light is made up of many colors. These different colors have different wavelengths.

• We can see the many colors of light on bubbles.

• The separation of white light into many colors on a bubble happens because of interference.

### Skills:

• Hypothesizing, understanding cause-and-effect relationships.
• Following directions
• Observation
• Drawing conclusions

### Materials:

1. Soap solution (1 part dish soap, 10 parts water)
2. Plastic straw
3. 4 x 4 inch piece of a black plastic bag
4. 4 x 8-1/2 inch strip of white paper
5. Scotch tape
6. Plastic plate (one with sections, or separate food compartments)
7. Water

### Room preparation:

• Provide a large table area.
• Keep paper towels handy. A sink is handy.

### Safety and Precautions:

Soap solution may cause irritation if it gets into eyes. Rinse eyes with cool water. Don’t suck on straw.

## Procedures and Activities

### Introduction:

1. What is light?
Light is electromagnetic radiation we can see with our eyes.

2. How does light travel?
Light has the properties of a wave. One of these properties is interference, caused by the addition of waves.

3. What other waves are we familiar with?
Sound and water waves.

4. You can give an example of wave interference with water waves. Put a clear dish of water on an overhead projector. Use two spoons; tap the spoons in the water on opposite sides. Notice the pattern in the water made up of light and dark bands. This is interference. When two waves add (crest with crest), they make a stronger wave (constructive interference). When two waves cancel (crest adds with a trough), it is destructive interference.

### Activity

1. Pass out to each student a plastic plate, straw, a piece of black plastic, and a piece of paper.

2. With some water, wet the larger section of the plate and stick the black plastic to it. Pour soap solution in another section of the plate (put in a quarter-inch or more).

3. Instruct students to dip straw in soap solution, enough to blow a bubble. Have students blow bubble onto black plastic piece.

4. Have students observe bubble, looking at the colored fringes. The white strip of paper can be bent around the bubble to better see the fringes. We call this making a “bubble home.”

5. Keep making bubbles. Watch the color patterns on the bubble. The colors will slowly go around the bubble and form rings, then a black dot will form in the middle of the bubble and it will pop.

a) Do you see the fringes? What do they look like?
b) Why do you think there are many colors? What makes the dark bands?
c) Can you see evidence of constructive interference? Destructive interference?
d) What are the fringes doing?
e) What determines what the fringes look like?
f) What happens when the bubble gets old (before it pops)?
g) Do you think this interference is useful for anything?

7. As students experiment, share that the colors we see on the bubble are the reflection of white light shining on the bubble film. White light has many colors—each color has a different wavelength.

8. If the crests of two sets of waves meet, the colors will intensify and we see evidence of constructive interference

9. When the crests of one wave meets the trough of another wave, the waves cancel each other out and we see destructive interference.

10. As a bubble film thins out, the colors cancel each other out until all we see is a black dot in the middle of the bubble.

### Discussion

Review with students what intereference is. We get interference from a thin film like a bubble, because the light waves which bounce off the front and back surfaces of the bubble wall add constructively or destructively. Whether you get destructive or constructive interference depends on the color of the light and the thickness of the bubble.

The bubble’s thickness is not uniform, that is why you see fringes or bands of different colors, and dark bands where the light interferes destructively. Over time, the thickness of the bubble changes, leading to a changing and swirling of the colors. As the bubble ages, it gets thinner on top and thicker on the bottom. Eventually the top gets so thin that light interferes only destructively, all the color waves cancel each other out, and we get a dark spot.

How is this thin film interference useful? Well, since the color of the fringes (and also their spacing) is related to the thickness of the film, we can use this to measure the thickness of films, which are too thin to measure with, say, a ruler. Scientists actually use this to measure very thin objects! Also, you can use this property to create light filters that only let through certain colors of light (the other colors interfere destructively).

## Evaluation:

1. Have students review the concepts of light, waves, and intereference.

2. Have students describe the two kinds of interference, constructive and destructive.

3. Ask students where else they have seen this type of phenomenon (e.g, a compact disk, a hologram, oil on water, a mallard’s head). A mallard’s head? Yes! This duck’s head looks green, but the feathers are actually black! The feathers have grooves such that reflected green-blue light is enhanced by constructive interference.

## Extension Ideas:

1. If you want to get fancier, think about an experiment in which you can actually measure the thickness of a thin film. Consult a physics text for the proper equations. The reference is about thin film interference.

2. Repeat experiment by trying different colored backgrounds or strips of paper for the bubble home. What happens? Why?

3. Do some on-line experiments with light at The Teachers’ Lab.

## Prerequisite Vocabulary

Fringe
A light or dark band made by the interference of light.

Interference
The addition or coming together of several waves (can be light, sound, or water waves).

Light
Electromagnetic radiation visible to the human eye.

Wave Crest
The top or high point of a wave.

Wave Trough
The bottom or lowest point of a wave.

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