Light Intensity vs. Distance


You will need: The EA-100 with the light probe attachment, the CASIO CFX-9850G calculator, a working flashlight, a tape measure, a wooden block, and masking tape.

Procedure: Turn the flashlight on and place it on the floor. Darken the room. Using the light probe and the tape measure, sample the light intensity at different distances from the light source. Keep a record of these data. It is useful to keep the light sensor at the same height, pointing towards the center of the light source; to do this tape the light sensor to a wooden block or similar object. It is also useful to take the average reading, say the average of 10 readings, at the different distances, to minimize variation. Use the program EZEPERI to sample the light intensity by entering 2 for new experiment, 1 for the channel, 1 or 9 for the operation, 3 (STATS) for post-processing, 10 for statistic sample size, 1 second for time, and 1 for the sample size. Keep track of the entries in List 2, which will be the average light intensity of the 10 readings. Also for each of the light readings keep track of the distance to the light source.

Enter the distance data into List 1, and the light intensity into List 2. "Normalize" the light intensity data (i.e., divide by the largest intensity value); this will allows the use of a smaller viewing window for easier comparisons between samples. To enter the normalized data into List 3, highlight the List3 title using the cursor arrows. Assuming the largest value of intensity is on cell 1 of List 2, press OPTN F1 (LIST) F1 (List) 2 F1 (List) 2 [1] EXE. The normalized data is saved into List 3.

To draw the scatterplot press MENU, use the cursor arrows to highlight the STAT icon, and press EXE.

Press F1 (GRPH) F6 (SET) and set the parameters as follows:


Graph Type :Scatter

XList         :List1

Ylist                  :List3 (This is the normalized data)

Frequency :1

Mark Type : (This is your choice)

Graph Color : (This is your choice)

Press EXIT F1 (GPH1).

Many questions can be asked of the data. For example:

  1. Is the intensity increasing or decreasing as the light source is farther away? What is the sign of the derivative of this function?
  2. Approximate numerically the derivative of the light intensity function with respect to distance. Use the numerical derivative to approximate the light intensity at different distance.
  3. Can you match a function to the data?