Purpose:

Last week we put our ideas about inverse square laws to the test to describe the relative intensity of different light bulbs. This week it's time to explore the absorption of light and the relationship to the exponential law we have been working on in class. A particular example of this that some of you may use in the future is called a Secchi Disk. This is a tool for measuring the absorption of light by water. I have done some quick experiments for our purposes where we are going to use this technique to measure the absorption of light by coffee (and perhaps tea) we can accomplish the same task with paperclips.

• I) Process Description: Take a standard large paperclip and straighten it out except for a short right angle foot. Using a sharpie marker make a regular series of marks along the vertical portion of your secchi "stick". These do not have to be at any particular location but are just to provide reference points for your measurement.

• II) Dark Roast I: In standard room lighting (500 lux) lower the secchi stick into the coffee until you can no longer see it. With the help of your partners determine how deep it was when this happened. Each member of the group should arrive at their determination through multiple measurements. I found that it was reasonable to make this measurement in mm.

• III) Dark Roast II: Repeat the measurements except this time start with the secchi stick deep in the coffee and raise it until it just comes into view. Again, repeat for each member of the group. Your actual measurement will be the average of method I and method II for all members of the group. Compare your depth results with other groups in the class.

• IV) Coefficient of Attenuation: In the expression N=N₀e^{λ x}, λ is the coefficient of attenuation (sometimes called the attenuation coefficient). According to various sources λ = 1.7/depth (in meters). Determine λ for your dark roast coffee. Then calculate the light levels in the bottom of a standard mug from your expression.

• V) LIght Roast: Using your working knowledge of secchi sticks can you show that the absorption coefficient for a light roast coffee is measurably different than dark roast? Share your data and insights.

LAB DELIVERABLES (turn in):

An introduction and 5 (or more) paragraphs describing your data and calculations for each of the steps above. In each section present any actual measurements, explanations and descriptions of the task you are working on, calculations that you make, and comments on the results.