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.
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
STEPS OF THE INITIAL PROCEDURE
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.
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.
Given the apparatus set up for you in the lab, you must :
Written by Tina Morin, July 1997