User:Steven J. Koch/080328 Physics 102 Soap Bubble Homework

Introduction

Picture of my coffee mug experiment. See video on youtube

I performed the experiment on page 570 of the book (Conceptual Physics, 10th edition, Paul Hewitt). To the right is a picture of a soap bubble made on the mouth of a black coffee cup, illuminated by sunlight through my window, about 5 pm Wednesday Oct. 18 2006. (You can also watch the video on youtube.) Though the perspective is a little confusing, I am holding the cup so that gravity points to the right (and into the page), so the film is thinnest on the left, and thickest on the right.

Part of coffee mug picture compared with simulation used on wikipedia. Look how close the simulation is to the real image!

Below that image on the right is a magnification of the center of the above image, compared with a simulation from wikimedia. From the left, I see: Black, blue, white, yellow, red, (black?), blue, cyan, (white?), yellow, magenta, purple, blue, cyan, green, magenta, blue, green, magenta, green, white? The simulation is pretty close, huh? Amazing physics!

Homework

Please turn these answers in via WebCT or facebook by 11:00 am Tuesday October 30.

1. Try to repeat this experiment!!! If you are able, upload a photo or video to facebook or WebCT with your assignment. Alternatively, see if you can find other interference patterns like this around your home or apartment and submit either a photo or description. If you want to do the coffee cup experiment, I will give some hints below.
2. As marked on the first photo above, the thinnest part of the soap bubble is on the left, and appears black. It is black because very little sunlight is reflected, due to destructive interference. The film there is below 50 nanometers, and the wavelength of visible light is between 400 and 750 nanometers. Can you explain why all of the colors of light would undergo destructive interference at the thin part of the film? (Hint: Do either the top or bottom waves undergo a phase shift? See wikipedia soap bubble article)
3. The author says on page 569, the middle paragraph, that “You’ll notice these colors are predominantly cyan, magenta and yellow, due to the subtraction of primary red, green, and blue.” Looking at my pictures above (or really any you can find on the web), do you agree with this statement? Any comment?

Tips for the coffee cup experiment

• Use a coffee cup that is black on the inside. This will give you good contrast to see the colors.
• Mix up a little Tupperware dish with some water and some dish soap. I used about 1 cup of water and then “some” dish soap…basically like 4 doses from one of those pre-foamed soap dispensers (did you know when I was your age we had to lather the soap up ourselves?).
• Stir up the soap and dip the coffee cup upside down and lift out, to form a bubble near the top. Make sure you mix the soap each time you try. For me the bubble lasted about 30 seconds or so.
• Hold the cup so you can see reflections from bright light…Try different sources of light.