Advanced Laboratory and/or Lecture Demonstration Apparatus and Low-Cost
Apparatus Title: Handy Self-Contained E=hf Demonstrator
Abstract. This apparatus uses a set of different colored LED's and a strip of phosphorescent tape to demonstrate that photons of different wavelength also have different energies. Only the blue LED can excite the phosphorescence. Light of long wavelength quenches the phosphorescence.
Support required for apparatus:
None other than set-up, except that this apparatus needs to be operated in a darkened room. For the purposes of the apparatus competition, the apparatus can be operated inside a medium-sized cardboard box with viewing holes, which I shall send along with the apparatus.
Approximate size: 12 cm x 6 cm x 7.5 cm, not including a flashlight and a medium size cardboard box with a couple of holes in it for display (to provide a dark enclosure). With surrounding box to provide a dark environment, <= 50 cm in any dimension. I am contemplating making another version of the apparatus, which will be comparable in overall size to the original but which may have somewhat different linear dimensions.
Does this apparatus require Electrical Power? yes___ no _X__
Will you be present to set up your apparatus? yes ___ no _X__
When do you plan to set up your apparatus?
Sunday July 22, 9-12 AM_____ 1-5 PM_____
Other support needed for the proper operation of this apparatus: none
Modern physics demonstrations are usually costly and difficult to set up. This apparatus provides a simple demonstration of the relationship between photon energy and the frequency, or color, of light. The E=hf demonstrator consists of a small box with a hinged lid. When the lid is closed, a strip of glow-in-the-dark (GITD) tape is put into contact with a row of different colored light-emitting diodes (LEDs). Two demonstrations can be performed with this apparatus. Both demonstrations are performed in a darkened room with a small audience.
As seen in Fig. 1, the construction of the apparatus is very simple. The LED's were all selected to have clear lenses, so that the color of the emitted light was due entirely to the characteristics of the semiconductors from which the LEDs were made. The LEDs are arranged in a single row on top of a small electronics project box. The box is provided with a hinged lid. A strip of GITD tape is mounted on the underside of the lid so that it contacts the LEDs when the lid is closed. A resistor chain voltage divider attached to a 9V transistor radio battery is used to provide the voltage and current to each LED which is suggested by its manufacturer. A single momentary contact pushbutton switch completes the electronics.
The inside of the lid is also fitted with a chart showing the operating voltages, nominal wavelengths, and nominal luminosities of the LEDs. These data are available on the packages in which the LEDs were purchased. The students can calculate the photon energies from these data, and discuss the lack of effectiveness of luminosity (the red is brightest, the blue dimmest) on exciting the phosphorescence, as well as the reasons why the blue LED requires a higher operating voltage than the red.
Fig. 2: This is a photo of the apparatus with its lid closed. The switch is mounted on top of the box in this version.
Fig. 3: This is a photo of the box with its lid open and its LEDs activated. The chart shows the peak wavelength, nominal luminosity, and operating voltage of each LED. The data were obtained from the manufacturer's LED packaging.
Other uses for this apparatus include applying the lights to other light-sensitive devices, such as solar cells, photocells, photoresistors, etc. to test for wavelength dependences in their functioning.
Reference. 1. Lisensky, G. C., Patel, M.D., Reich, M., J. Chem. Ed. 73, 1048 (Nov. 1996).