Using Bubbles to Learn about Light Interference
This lesson developed by John Nees,
U of M Center for Ultrafast Optical Science
Guiding Question: What is interference?
To understand the wave properties of light, especially the phenomenon of
- 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.
- 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
- 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.
- Hypothesizing, understanding cause-and-effect relationships.
- Following directions
- Drawing conclusions
- Soap solution (1 part dish soap, 10 parts water)
- Plastic straw
- 4 x 4 inch piece of a black plastic bag
- 4 x 8-1/2 inch strip of white paper
- Scotch tape
- Plastic plate (one with sections, or separate food compartments)
- 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
- What is light?
Light is electromagnetic radiation we can see with our eyes.
- How does light travel?
Light has the properties of a wave. One of these properties is
interference, caused by the addition of waves.
- What other waves are we familiar with?
Sound and water waves.
- 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.
- Pass out to each student a plastic plate, straw, a piece of black
plastic, and a piece of paper.
- 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).
- Instruct students to dip straw in soap solution, enough to blow a
bubble. Have students blow bubble onto black plastic piece.
- 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
- 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.
- Ask the students:
a) Do you see the fringes? What do they look like?
b) Why do you think there are many colors? What makes the dark
c) Can you see evidence of constructive interference? Destructive
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?
- 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.
- If the crests of two sets of waves meet, the colors will intensify and
we see evidence of constructive interference
- When the crests of one wave meets the trough of another wave, the
waves cancel each other out and we see destructive interference.
- 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.
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
Closing - Original Question
Ask again, "What is interference?"
- Have students review the concepts of light, waves, and intereference.
- Have students describe the two kinds of interference, constructive and
- 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.
- 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
- Repeat experiment by trying different colored backgrounds or strips of
paper for the bubble home. What happens? Why?
- Do some on-line experiments with light at The Teachers’
Careers Related to Lesson Topic:
- A light or dark band made by the interference of light.
- The addition or coming together of several waves (can be light, sound,
or water waves).
- 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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