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[[~setup|Experiment Setup]]
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| {{phylabs:lab_courses:phys-140-wiki-home:spring-experiments:soap-bubbles-1.jpg?800}} |
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====== End of Quarter TA Evaluation ======

Before we start today's lab, we are asking all students to complete a short (<5 minute) survey in which you will have a chance to provide feedback on your TA. **Your answers are anonymous and will not affect your grade in any way.** You may access the survey from your personal computer, a lab computer, or your phone.

At the end of the quarter, TAs will receive average scores and comments (without identifying information) from their lab section(s).

Do not include any identifying information in your responses. If you have any feedback to provide to which you would like a response, please send it to Mark Chantell (mc2@uchicago.edu).

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[[https://forms.gle/Xd5PqLegLWAjGd1z5|Lab TA Feedback Survey]]
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If you cannot or do not want to complete the survey now, you may complete it at home. **The survey will remain open until Saturday, May 11 at 5:00 pm.**


====== Thin Film Interference ======

<WRAP center round important 60%>
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**Teaching Goals**
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In this lab you will:

  * Understand the basic physics of thin film interference.
  * Learn about RGB color mixing.
  * Understand how we "see" colors which do not exist in nature.
  * Use computational methods to model the spectrum produced by thin film interference and compare that with the actual observed spectrum.
  * Learn how to use a camera to record the spectrum of light produced by thin film interference.
  * Use image analysis software to decompose a color image into Red, Green and Blue channels.
  * Use image analysis software to measure the locations of interference peaks and determine the thickness profile of a thin film.
</WRAP>

For your final PHYS143 lab you will perform an experiment to measure the thickness profile of a thin film.  Your work will be based on a paper published in the American Journal of Physics titled //Investigating Thin Film Interference With A Digital Camera, Am. J. Phys. 78, 1248–1253 (2010)//.

<WRAP center round download 60%>
A copy of this paper can be {{ :phylabs:lab_courses:phys-140-wiki-home:spring-experiments:investigating_thin_film_interference.pdf |downloaded here}}.
</WRAP>

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{{phylabs:lab_courses:phys-140-wiki-home:spring-experiments:raw_0117.jpg?600}}
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//Image of thin film interference effects produced by soap bubbles.//

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{{phylabs:lab_courses:phys-140-wiki-home:spring-experiments:raw_0151.jpg?600}}
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//Image of thin film interference effects produced by a flat soap and water film.//
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===== Understanding the RGB Nature of Image Sensors =====

One of the subjects discussed in the paper is how the design of your sensor impacts the data.  In this case it is the RGB nature of most common imaging sensors, including both your typical camera sensor and your eyes.  Both are based on combining information from separate red, green and blue sensitive detector elements.  

The paper also discusses how the colors produced by thin film interference are the result of a subtractive process when certain wavelengths of light interfere destructively.

Understanding the physics of how your instruments interact with the phenomena you are measuring is crucial to scientific investigation.  The following exercise, while qualitative, is intended to illustrate how understanding your detector system informs how you will process your data to measure the thickness of a thin film.

<WRAP center round important 60%>
You will be using your cell phone camera to take images of optical spectra for analysis in the image processing program Fiji (ImageJ).  To practice using your camera for collecting data and gain insight into the nature of the spectrum you are working with do the following:

  * Use your cell phone to take a //good*// photograph of a thin film interference pattern.  Your TA will describe the apparatus you have to work with for creating and illuminating a thin film.  The paper you read discusses considerations for how to take such a photograph.
  * Transfer the photograph to the lab computer at your station and open it in Fiji (ImageJ).  Use the tools in Fiji to crop, and rotate the image along the vertical axis.
  * Find an image of the visible light spectrum, it can be from the internet or something you take with your phone camera.  Carefully examine the two images for visual differences that are related to differences in the underlying physics involved in their formation.  Keep in mind that both spectra are the result of interference effects.  Take adequate notes to be able to clearly and concisely describe the differences you observe and how they relate to the physics principles involved.
</WRAP>

<WRAP center round info 60%>
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//good*//
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So what defines a //good*// photograph?  You are using the camera in your phone to collect data which will be able to analyze.  It is easy to take a lousy photo which will be effectively useless.  You need to consider what factors are important to get optimal data, and how to setup and use your camera to get the best possible result.  Here are some tips:

  * You do not want to be so zoomed out that the spectra take up only a small fraction of the image sensor.  
  * Try to keep linear distortions to a minimum.  Ideally the camera should be positioned directly in front of the soap film, the more off center the camera is the more distorted the image of the spectra will be.  Lighting considerations will likely make it impossible to place the camera in an ideal position, so minimizing the offset should be the goal.
  * You need to be able to clearly see the patterns of colors in the spectra.  Minimize distracting back grounds and reflections.  

Note that this is not an exhaustive list and is not intended to replace careful thought on your part.  One of the goals of this lab is to give you experience assessing what are the important considerations given what you are trying to accomplish, and how can you best use the tools at your disposal to obtain the best data possible.  

</WRAP>

===== Modeling the Phenomena =====

Using computational methods to model what you expect your data to look like is often a useful tool in scientific investigation.  In this experiment you are going to use the phenomena of thin film interference as a tool to make a measurement of the thickness profile of a thin film made from a solution of soap and water.  The paper you read develops a simplified model of this phenomena which can be used to calculate the phase shift and by extension the degree of destructive interference expected for different wavelengths of light as a function of the thickness of the film.  

To model the response of the detector we will make the simplifying assumption that the R, G, and B sensors in your camera are each sensitive to one wavelength.  By calculating the degree of interference between these three wavelengths we can predict the resulting color which will be recorded by the camera for different film thicknesses.  This information can then be used to build a simple color map that one would expect to be a reasonably good approximation of the image your camera will produce.

Run the linked notebook to model your camera's response to the light from the film.  We are providing the code in case you are interested in seeing how we approach modeling this experiment.  But for purposes of what you need to do for the lab you only need to execute the cells up through the part where it produces the color map.  The subsequent cells are for you to use to make a plot of the measurements you will make in the next section.

<WRAP center round info 80%>
[[https://colab.research.google.com/drive/1AtDGM_0uYDH5piMTPrbQuIpG0UWQY0dO#offline=true&sandboxMode=true?refresh=true|Co-Lab Notebook]]
</WRAP>

You can now compare your photograph of the thin film interference spectrum with the output of the model.  It should be the case that for both spectra, the sequence of colors and the colors themselves are in agreement.  The reason to do this is that it helps validate the simplified model you will use to calculate the film thickness in the next step.

===== Measuring the Thickness of the Film =====

You have a model of the physics describing the pattern of colors you observe from thin film interference.  You should also understand how the RGB sensors in your detector combine to produce the colors which you see.  You have done a simple numerical computation to predict the pattern of colors you should observe assuming the physics model is correct.  Finally you should have been able to compare the results of the computation to an image of the phenomena which you have taken in the lab, hopefully confirming that the model is reasonable.

From the above, and the information presented in the paper, you know all you need to in order to measure the thickness profile of the a thin film of a soap and water solution along an axis parallel to gravity when the film is held vertical.  

In order to do a quantitative analysis of the film thickness you will need to separate the full color images into their red, green and blue channels.  We give instructions for how to do this in the following section on using Fiji (ImageJ).  

The final cell of the python notebook will allow you to plot the thickness of the film along the axis parallel to gravity.

===== Apparatus =====

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{{phylabs:lab_courses:phys-140-wiki-home:spring-experiments:pxl_20240502_173641631.jpg?600}}
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===== FIJI (Also known as ImageJ) =====

Open files using File Open.

==== To obtain an RGB profile ====

  * Use the Line tool to draw a line through the section of the image you want to generate a profile along.{{ :phylabs:lab_courses:phys-140-wiki-home:spring-experiments:screenshot_2024-05-02_at_1.41.37 pm.png?400 |}}
  * From the //Plugins// menu, select //RGB Profiler//.

==== To separate out the Red, Green and Blue channels ====
 
  * The image will need to be oriented so that the interference bands run left to right instead of top to bottom.  From the //Image > Transform// you can select one of several options to rotate the image.
  * Use //Image > Color > Split Channels// to create three separate grey scale images from the R, G and B values.  
  * Use the Rectangle tool to draw a narrow rectangle across the interference fringes you want to analyze. {{ :phylabs:lab_courses:phys-140-wiki-home:spring-experiments:screenshot_2024-05-02_at_1.55.27 pm.png?400 |}}

==== To read X and Y pixel values from an image in FIJI ====
  * Unordered List Itemse the Point tool.  {{ :phylabs:lab_courses:phys-140-wiki-home:spring-experiments:screenshot_2024-05-02_at_1.49.26 pm.png?400 |}}

====== Report: Summary and Conclusions ======
----

After the lab, you will need to write up your conclusions. This should be a separate document, and it should be done //individually// (though you may talk your group members or ask questions). 

The conclusion is your interpretation and discussion of your data. What do your data tell you? How do your data match the model (or models) you were comparing against, or to your expectations in general? Your conclusions should always be based on the results of your work in the lab.  It is **not** acceptable to evaluate the results of an experiment by comparison to known values or any other form of preconceived expectation.  Your conclusions need to be supported by your data.  If your data are inconclusive or in disagreement with regard to your expectations then your conclusion should reflect that.  

<WRAP center round important 60%>
Make sure you cover the following points in your report.

  * Describe how you setup for and took your image of the thin film interference.  Note important aspects of the setup and image acquisition which which you needed to take into account.  **[EP] [SC]**
  * Describe your measured quantities and how you assessed their uncertainties.  **[EP]**
  * Present your thickness profile for your thin film.  Include data from all three wavelength measurements appropriately plotted.  **[DA] [SC]**
  * Briefly discuss your confidence in your final result.  Since you are presenting the result of a measurement of something which is unknown, why should the reader believe your result to be accurate.  **[DC] [SC]**

</WRAP>


**REMINDER**: //Your report is due 48 hours after the lab. Submit a single PDF on Canvas.//