# Physics 107 - Light Intensity II

### LEARNING GOALS

• Use of log-log plots and other linearized plots to discover functions in scatter plots
• Comparing functions (models) to data using residual plots
• Reflecting on data while you take it, in order to continuously improve a scatter plot

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

### THE EXPERIMENT

Goal is to to develop a model relating intensity to distance between the detector and lamp.

A few tips to get started:

• Experiment with the zero-ing of your meter. If it reads "CALO" when you try to zero it, switch on and off.
• A light shield is in place to reduce the effect of your movements on the background light. Investigate how well this is working and how carful you must be about your movements.
• Regularly check your meter zeroing as you proceed with measurements
• Plot the data on a linear-linear plot as you go along
• Note that distance is the distance from the detector element to the lamp filament. make sure the plane of the lamp filament is perpendicular to the line between the lamp and detector.

### PART I

• Measure intensity versus distance, for distances from 50-150 cm.
• Plot the data on a linear-linear plot.
• Comment on the quality of your data plot and make a plan for any improvements to the measurements
• After collecting further data to the point where you are satisfied, move on to part II

### PART II

• Use a log-log plot to see if the data follows a power-law model
• Estimate the exponent in the power law using your log-log plot.
• Comment on the quality of your data plot and make a plan for improvements to the measurements
• After collecting further data to the point where you are satisfied with you log-log plot, move on to part III
• note that any new data can be combimed with previous data, especially if you have a few repeated data distances to chack that there have been no significant changes over time.

### PART III

• For this experiment, the power law involves a simple, integer exponent.
• Use a stright line model corresponding to this simple power law and adjust one free parameter to get the model close to your data
• Plot the residuals (difference between model and data points) and further adjust your free parameter to see if you can get residuals that look randomly scattered around zero.

### PART IV

• Perforn measurments at closer distances
• Use the log-log and residuals plot to comment on the behviour in closer.

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 critique after your first log-log plot and plan for more data-taking

3 marks for total, final data set with uncertainty
2 marks for log-log plot of data

2 marks for plot of residuals
2 marks for conclusion, including function that describes the far-field data and comments on the near-field data.