• Block of white packing foam from TeachSpin Optical Pumping box.
• 35mm film canister.
• Small dish for soap solution. I used the lids from the Vivitar polarizer containers which were also used for the previous lab on polarization.
• Goose neck lamp with 75W incandescent bulb. Use incandescent bulb to illuminate film with uniform blackbody spectrum.
• Sponge. For cleaning up spilt solution.

It is also helpful to have blank paper available to use to diffuse the light from the lamp, or add reflective surfaces to help with illuminating the film.

Each computer needs to be setup with FIJI (which is just ImageJ) installed. This is pretty trivial to do, just google “FIJI ImageJ” and you will find the download link. On windows put the unzipped package folder in the Documents folder. There is no installer.

The RGB Profiler plugin also needs to be installed. This is easiest to do using the Install Plugin command in the Plugin menu.

In order to keep the liquid spills to a minimum I put out one 250ml bottle of solution on a serving tray along with disposable transfer pipettes. In theory students take one dropper full of liquid for their use as opposed to pouring from the bottle.

I keep a 4L container with extra solution hidden from student view, usually in the cabinet under the bench. This is only for use in the hopefully unlikely event that the 250ml bottle is used up. The pouring cup is for transferring solution from the 4L bottle to the 500ml bottle.

The solution is approximately a 10:1 ratio of Water to Dawn dishwashing solution. Note that a weaker solution produces thinner films with more pronounced bands. More concentrated solutions will result in thicker films.

Do NOT add glycerin. Glycerin is commonly added to soap bubble mixtures because it reduces the rate of evaporation which extends the life of the bubbles. However it also produces lots of swirls in the interference pattern rather than nice straight lines.

I set out the color mixing demo for the TA's to use if they want as part of their intro. Students are very unlikely to have seen color mixing in lecture.

• Need to have incident and reflected angles as close to orthogonal to film surface as possible. Model used assumes that the path length through the film is the thickness of the film. If the camera is held an an angle to the normal of the film surface, the actual path length becomes longer than the thickness of the film. This could be worked in by trigonometry, but would then require measurement of the angle of the camera with respect to the normal to the film surface. I choose to neglect this the first time I ran the lab.
• Incorporation of using computation to compare the model with the actual data.
• Working from a journal article. Note the journal article was assigned pre-lab reading with a 5 minute quiz at the start of the lab.
• Application of concepts seen previously in lab (superposition and interference) to a new problem.
• Consideration of the impact of the detector and modeling it's performance. In this case the colors observed are due to the fact that the eye, as well as the camera sensor, uses R,G & B sensors to record color information.
• Color mixing. Interesting note to emphasize is that Magenta (red + blue) is not a color in the electromagnetic spectrum as it does not correspond to a single wavelength. Magenta is how our brain interprets the mixing of red and blue.

All True / False. The quiz should be trivially easy for students who did the pre-lab reading. Quiz is worth 4 points, one for each question, and replaces the out of lab assignment for this final lab of the year.

1) The sequence of colors produced by thin film interference is the same as what you see in a rainbow.

2) The color sensors in the human eye and a digital camera both record light as Cyan, Magenta and Yellow values.

3) Where the film is very thin (<50nm) all reflected wavelengths interfere destructively and there is no reflected light.

4) The color of the reflected light depends on the thickness of the film.