# STUDENT PRACTICUM

## THE INVERSE SQUARE LAW OF LIGHT (TEACHER INSTRUCTION SHEET)

INTRODUCTION

The particular purpose of this exercise is for use as a practicum with students following curriculum work with the inverse square law of light. There are three parts to the exercise, and points can be awarded for each part depending upon your preferences. This is appropriately done as a whole class activity, (especially if your set-ups are limited!), and the class can be awarded points on the basis of how it performs as a whole, cooperative, unit.

The first part of the exercise asks students to explore the relationship between intensity of light detected with a light probe, and the wattage of the light bulb used. Is the relationship linear?

The second part would ask students to position the bulbs at an appropriate distance from a fixed point so that their intensities are the same.

The last portion of the exercise could be used either as an initial piece of the exercise or as an extension of the work. It asks the students to tie the previous steps together. Given a bulb of unknown wattage, the students would be asked to place it in its appropriate place in line according to increasing wattage, and at an appropriate distance to conform to an equal intensity.

MATERIALS

• 3 light bulbs, (same type, same socket). Use 15, 25, and 60 watts.
• Pasco 2.2 meter dynamics track-- (optional, but smoother running!)
• Pasco dynamics cart - (again, optional)
• CBL with light probe, motion detector, and light intensity program
• TI-82 or 83
• Black construction paper, (for bulb and probe insulation), and other materials to guard against surface reflection. (Black felt works nicely!)
• meter stick for measuring distances
• other analysis tools, such as spreadsheet and graphing programs

ORGANIZATION OF MATERIALS

Lay the dynamics track on a surface long enough to accommodate it, and wide enough for students to work with the supporting materials. Set the light bulbs along the track lengthwise, in order of decreasing wattage from left to right. Set them at the same distance from the track, approximately 30cm to avoid saturation of the light probe by the 60 W bulb. You may need to adjust this distance for higher wattage's used. The distance between light bulbs is not important, however you will want to have them close enough together so that their individual peaks can be seen on your calculator's graphing screen. With black construction paper, create a shield around each light bulb so that as the probe passes it gathers light from just that bulb.

Insert the CBL light probe into Channel 1, and wrap a paper tube around the end of the probe to shield against background light. Make sure that when you measure distances however, you account for any overlap of your shielding tube. Mount your light probe on the Pasco collision cart, and position it on the track with the probe facing your light sources.

Insert the motion detector plug into the Sonic port of the CBL and position the motion detector at the right end of the track, nearest the 15 W bulb. You may also wish to mount a Ping-Pong ball on the dynamics cart. This gives the motion detector a nice spherical surface with which to work.

HOW SHOULD THE STUDENTS USE THIS SET-UP TO ANSWER THE PRACTICUM QUESTIONS?

The students should already possess knowledge of the inverse square law of light, and the difference between brightness of a point source and its intensity. They will use this knowledge, and the apparatus to answer the practicum questions.

STEPS OF THE INITIAL PROCEDURE

***** IT IS BEST TO DO THIS PROCEDURE IN A DARKENED ROOM!!!!!! *****
1. Ask students to run your particular light intensity program for the CBL so that they can collect data for a graph of intensity vs. distance.

2. Have students turn on the light bulbs, and beginning at the left side of the track, move the cart slowly along once the motion detector begins to click.

3. The graph of intensity vs. distance that students collect should show increasingly higher peaks for smaller distances from the motion detector.

4. The student's job now is to determine the relationship between intensity and wattage, and to position the light bulbs in such a way along the track so that peaks on the intensity vs. distance graph are equal.

THE ULTIMATE OUTCOME

The entire class will work on this exercise together, solve it, and be evaluated on the outcome. A group of students will be called upon to prepare the presentation of the outcome, and all students will receive the same grade.

EXTENSIONS

As mentioned in the introduction, this exercise could be extended to unknown light bulbs. Once students have found the information for the above two activities, they could be given an unknown wattage and asked to place it in line at an appropriate distance so that it's intensity peak is equal to the others.

For an astronomy class, this application could be used to illustrate apparent and absolute magnitudes of stellar objects. The sun's luminosity curve may be able to be used to determine what type of star it would appear to be if it were removed to a distance outside of our solar system.

## PRACTICUM USING THE INVERSE SQUARE LAW OF LIGHT

### (STUDENT INSTRUCTION SHEET)

Given the apparatus set up for you in the lab, you must :

1. Determine the relationship between the intensity as read from the CBL, and the wattage of the light bulbs used in this exercise.

2. You must graph intensity vs. distance for the arrangement of the light bulbs to determine this relationship.

3. When you have determined that relationship, you must use it, and your knowledge of physics to determine the distance at which light bulbs must be placed from the probe in order to make the peaks of your graph be of equal heights.

Written by Tina Morin, July 1997