Demonstrations

Inverse Square Law - Point Source

Code Number :	6B10.10
Disclaimer:	Reprinted by permission of Dick Berg, University of Maryland, for use on this website. The demonstrations contained and referenced herein are listed for the purposes of cataloging and describing physics demonstrations which should be conducted only under the direction of a trained instructional support professional or physicist. These demonstrations are not presented for the purpose of being conducted by persons unconnected to this Facility and/or persons not consulting with or being supervised by the recognized instructional support professional or physicist and his/her staff. The University is responsible only for those demonstrations carried out using its own equipment using established safety and scheduling policies, and bears no responsibility for those choosing to use this source material for their own purposes. All demonstrations described and contained herein are public domain, and can also be found in reference materials in libraries, bookstores, and electronic sources. Further information regarding legal liability in use of demonstrations and labs will be found on the web site Injuries in School/College Laboratories in USA. The University of Iowa Disclaimers: University of Iowa Disclaimer All Rights Reserved..
Condition :	Good
Principle :	Inverse Square Law
Area of Study :	Optics
Equipment :	Large inverse square boards, 100 watt light bulb and mask, masks with 1 cm holes, grid transparencies, AA Maglights, Inverse square apparatus with translation mount
Procedure :	Use a 100 W light bulb as a pinpoint source. To do this align the filament so that its long axis is perpendicular to the large white board. Once aligned set the masking tube over the light bulb to enhance viewer observation. Holding the single one foot square at distance 1 from the light bulb show that it only covers one square of the large grid boards. Move the large grid board back to 2X the distance and show the shadow now covers 4 squares. Repeat by moving the board back to 3X and 4X the distance and show that the shadow now covers 9 and 16 squares respectively. The Maglight setup is a smaller version of that mentioned above and is intended for individual viewing or lab work. Instead of 1 foot square cardboard we are using a 1 cm square mask. The Maglight set up on the translational stage and railing is a more automated version of the simple Maglight demo also using the 1 cm square mask. This is primarily intended to be used in the classroom with a video camera looking at the grid. NOTE: The camera must be positioned so that it is not directly inline with the Maglight and the masking hole. Although not shown in the pictures above the overhead projector can also be used to show inverse square. Move the projector very close to a screen and turn on and measure the area illuminated. Move the projector back twice as far and again measure the area illuminated. Continue for 3Xs the distance and 4Xs the distance and the are should increase for all of these according to the inverse square model.

References

Russell Downie, "A Data Analysis for the Inverse Square Law," TPT, Vol. 45, # 4, April 2007, p. 206.

Pietro Ferraro, "Speckle Noise or Dust Diffraction," TPT, Vol. 37, # 6, Sept. 1999, p. 324.

V. Anantha Narayanan, Radha Narayanan, "Inverse-Square Law of Light with Airy's Disk," TPT, Vol. 37, # 1, Jan 1999, p. 8.

Iona, Mario, "Proper Graph Scales," TPT, Vol. 37, # 4, April 1999, p. 195.

Simon George and Robert Doebler, "Rainbow Classes and the Inverse-Square Law", TPT, Vol. 32, # 2, Feb. 1994, p. 110.

Thomas T. Arny, "Demonstrating Atmospheric Diffraction Rings with a Tensor Lamp," AJP, 55, (1), January 1987.

F.J. Wunderlich, D. E. Shaw, and M. J. Hones, "A Light-Emitting Diode as a Point Light Source", AJP, Vol. 45, #1, Jan. 1977, p. 3.

Haym Kruglak, "Laboratory Exercise on The Inverse Square Law", AJP, Vol. 43, # 5, May 1975, p. 449.

"Perfboard Version", The Magic Wand & Other Experiments in Light and Color by The Exploratorium, p. 65.

Ron Hipschman, Rec. #175,"Inverse Square Law", Exploratorium Cookbook III, p. 175-1.

"Inverse Square Law", Science Snackbook, p. 59-1

Mail Questions and Comments to: Dale Stille