Thin Film Interference in a Plastic Bottle

PIRA 6D30.20

The 2 l plastic pop bottle shown at left has a small hole with a straw through it.  In the bottom of the bottle is apx 20 ml of super-bubble soap.  We have some commercial super soap bubble concentrate from Sargent Welch, and some solutions made from this.  The following recipe also works well: 2/3 cup Joy, 1 gal. distilled water, 3 tablespoons glycerine - mix slowly & allow to sit in an open container overnight.

Wet the inside of the bottle with the soap solution.  With the end of the straw submerged in the solution at the bottom of the bottle, gently and slowly blow a bubble.  When it forms a film all the way across the diameter of the bottle, gently remove the straw and cover the hole with tape.  With no evaporation, these films will last a long time.  With the film held vertically as shown in the picture above at right, the film will thin at the top and thicken at the bottom.  Alternating color bands are clearly visible due to thin film interference on reflection.  If a single bright source of light is provided in an otherwise dark room, interference on transmission can be observed.  The film is usually sturdy enough to pass around to the students in a class for closer observation.