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KEY QUESTION
What causes the colors in bubbles?

KEY CONCEPT
Light rays travel in waves that can interfere with each other.

GENERAL INFORMATION
 Target Grades:  All ages
 Participant Size: Whole class divided into groups of five or six
 Length of Activity: One class period
 Where:   Indoors (in a darkened room)
 Method:   Demonstration or teacher guided experiment
 Focus:   Exploration
 Skills:   Observation and construction

Materials List
Small clear plastic lid                                              Clear plastic tape 
 (from margarine or yogurt)                                     Flashlight that works
Bubble juice (see below)                                         Spoon
Straw                                                                     A room that can be made dark

PROCEDURE
Divide your students into groups of five or six and provide a flashlight for each group.  Waterproof or inexpensive flashlights are best.  This activity can be messy.  You’ll need flashlights that are fairly stable when set on their ends.

Mix the ingredients together in a big bucket or a dishpan.  You should make your bubble juice the day before you intend to use it.

DOING THE ACTIVITY
1. Tape the plastic lid over the beam end of the flashlight with the rim up.

2. Set the flashlight for each group on end on a flat table or desk and have each group seated     around the table.  You may want to tape the flashlight to the table.

3. Dip your fingers in the bubble juice and wet the tops of all the lids.  Then put a spoonful of bubble juice in the plastic lid of each flashlight.

4. Turn on all the flashlights and turn off the room lights.  (The room needs to be as dark as possible.)

5. Use the straw to blow one big bubble to make a bubble dome that covers the whole lid of one flashlight and have one of the students in each group do the same for the rest.

6. Instruct the students to look at the bubble dome at eye level or from slightly below.

7. Watch the colors change over time.  You can blow gently on the dome with the straw and swirl the colors.

CLOSURE
Have each group discuss what they think causes the amazing colors.

EXTENSIONS
For the younger grades, just watching the pretty colors is probably enough.  All kids like bubbles.  This activity can be used to introduce the concept of light waves and interference patterns to older students.  You can also have them compare different brands of soap and decide which one makes the best bubbles.

BACKGROUND
Soap bubble colors come from white light, which contains all colors.  Light is made of waves like the waves in the ocean.  Light waves, like water waves can interfere with each other.  The light dome inside the bubble is being reflected from the inside of the bubble.

A bubble film is like a sandwich, a layer of soap molecules, a filling of water molecules and another layer of soap.  When the light reflected from one soap layer meets the light reflected from the other, they interfere with each other.  Some waves add and make other bright colors; some waves cancel each other and remove a color from the mix.

The very thinnest film looks black because all the reflecting light waves cancel each other.  If the soap bubble looks black or has black and white polka dots, the bubble is about to pop.