Interaction of Light and Matter

 

This experiment is in several parts, but we have one over-riding purpose – to begin to understand how light and matter interact.  Please work in groups of two.  Submit one report for the group.  We will be asking you to make a few predictions throughout this lab.  Make those predictions based on the observations you have made up to that point.  We will not be as concerned about whether your predictions turn out to be “correct”, we simply want you to base your predictions on your observations.  That’s what any good scientist would do.  Our hope is through your observations, predictions, and testing your predictions, you will discover (learn) a bit about how matter and light interact.  

 

Part 1.  The first part of the experiment is to determine the different colors of light that combine to form white light.  To do this, your lab instructor will set up a “white” light source in lab and you will look at that light through special glasses that are made with a diffraction grating.  Observe and record the order of the color as you see them through the diffraction grating glasses. 

 

Part 2.  In the next part of the experiment, we will try to quantify the wavelength of the various colors you observed in the previous part.  Your instructor will show you how to use a spectrometer and a special prism to view light of specific wavelength.  Set the spectrometer at 350 nm and observe/record the light coming from the prism.  Then increase the spectrometer to 400 nm and observe/record the light from the prism again.  Continue increasing the wavelength by 50 nm and observe/record the color observed until you reach 700 nm.  Prepare a neat table of your observations. 

 

Part 3.  Next, your lab instructor will assign a food coloring solution for you to prepare – either red, blue, or yellow.  Prepare your solution using the amounts shown on the table below.  Do not throw this solution away.  We will use it again later in the experiment. 

 

Solution Color	Milliliters of Distilled Water	Drops of Food Coloring
Red	50 mL	One Drop
Yellow	100  mL	One Drop
Blue	100 mL	One Drop

 

Using the table you prepared from part 2 of the experiment, predict the wavelength of light that is absorbed by your solution.  Your prediction should appear in a data table of your results from this part of the experiment.  Now, test your prediction.  Place your solution into a cuvette and record an absorbance spectrum.  Don’t panic, your lab instructor will show you how to do this.  Record the wavelength of maximum absorbance (sometimes given as the symbol l_max) and compare that wavelength to your predicted wavelength.  Also record the absorbance of your solution.  (Absorbance is different from wavelength.  Again, your lab instructor will help you.)  Also record the l_max for the other color solutions (consult with other groups).  Record data on a neatly organized data table.  Be sure you understand the relationship between color observed and wavelength absorbed before going any further. 

 

Part 4.  Next, predict the wavelength of light that will be absorbed by green food coloring.  Test your prediction by mixing one drop of green food coloring with 50 mL of distilled water, placing the resulting solution into a cuvette, and running a spectrum - just as you did in part 3.  Record your prediction and your results on the data table from part 3.  You may work with another group for this part of the experiment, if you wish.  Again, be sure you understand the relationship between color observed and wavelength absorbed before going any further.  Predict the wavelength absorbed by an orange food coloring solution and a purple food coloring solution.  Include your predictions on your data table.  You do not need to test your predictions for orange and purple. 

 

Part 5.  Pipet 25 mL of your red (or yellow or blue) solution into a 50 mL volumetric flask and dilute to the mark with distilled water.  Predict what will happen to the absorbtion spectrum of this diluted solution.  Test your prediction by running the spectrum.  Pipet 25 mL of your diluted solution into a 50 mL volumetric flask.  Predict what will happen to the absorbtion spectrum of this newly diluted solution.  Test your prediction by running the spectrum again.  Organize these data on a neat data table and prepare an appropriate graph.  Predict what would happen if your most dilute solution were diluted in the same way one more time.  Include your prediction on your data table and on your graph. 

 

For the sake of this lab, assume the concentration of your original solution to be 100 parts per million.  Based on this starting point, what will be the concentrations for the two diluted solutions you prepare?  Use these numbers on your data table and graph.  Also include the concentration of your “predicted” solution. 

 

Prepare a single lab report for this experiment.  Include a purpose, brief procedure, results/data, and conclusion.  Be sure your conclusion includes a brief explanation of the relationship between color observed and wavelength (color) absorbed.  You will prepare several data tables with predictions shown on the data tables.  Label these tables according to the part of the experiment they correspond with.  We will be looking carefully at the organization of these tables.  If you print any spectra, attach them to your report.