# Physics 107 - Light Intensity I

### LEARNING GOALS

• Use of semi-log plots to reveal exponentail behaviour in scatter plots
• Refelecting on data while you take it, in order to continuously improve a scatter plot

First, go through worked example I and answer all of the questions. Work in pairs on this and hand in when you are done.

### THE EXPERIMENT

To develop a model relating intensity to the number of plastic sheets blocking the light, make measurements using a wide range of number of sheets. To get started

• Write  down a plan for your first 10 measurments.
• Make measurements with several different numbers of sheets (about 10 measurments) in front and remember to:
• adjust the light's distance so that the intensity reading is 500 lux without any sheets in front of it
• regularly remeasure the intensity with no sheets in the way is near 500 lux. Adjust distance slightly if it is has changed more than 1%.
• check your meter zeroing occasionally
• Plot the data on a linear-linear plot as you go along

### REFLECTING ON YOUR DATA

Once you have an initial set of data, consult with another group and decide what to do next to improve the quality of the data set. This could include, but are not limited to:
•  trying more thicknesses
•  trying more extreme thicknesses
•  retaking some data points (but do not throw data out)
Based on your discussion, write down what you think is wrong with your data so far, and what you plan to do next, and then take more data.

### THE SEMI-LOG PLOT

Produce a semi-log plot of your data so far to qualitatively confirm whether or not it shows exponential decrease.

Again, confer with other about what extra data you might need to improve the semi-log scatter plot.
Again, write down what you think needs improvement and what you measurments you need to do to improve things.
Continue until you are satisfied with your final result.

You need to include uncertainty in these plots and we will show you how to handle that when rescaling graph axes.

Once you are satisfied with the data, and are satisfied with your semilog plot, write down the model that describes intensity versus number of sheets.

Marking Scheme

2 marks for worked examples at the start

2 marks for description of experimental technique, for your first measurements

2 marks for critique of first data and plan for more data-taking

2 marks for critiqe after your second set of data and plan for more data-taking

3 marks for total, final data set with uncertainty
2 marks for semi-log plot of data
2 marks for conclusion, including estimate of the coefficient in the exponential (what model describes the data, including estamates of the coefficients)