This is the fourth paper in our Light-Emitting Diodes series. The series aims to create a systematic library of LED-based materials and to provide readers with the description of experiments and the pedagogical treatment that would help their students construct, test, and apply physics concepts and mathematical relations. The first paper1 provided an overview of possible uses of LEDs in physics courses. The second paper2 discussed how one could help students learn the foundational aspects of LED physics through a scaf-folded inquiry approach, specifically the ISLE cycle. The third paper3 showed how the physics inherent in the functioning of LEDs could help students deepen their understanding of sources of electric power and the temperature dependence of resistivity, and explore the phenomenon of fluorescence also using the ISLE cycle.4 The goal of this fourth paper is to use LEDs as black boxes that allow students to study certain properties of a system of interest, specifically mechanical, electric, electromagnetic, and light properties. The term “black box” means that we use a device without knowing the mechanism behind its operation.

Topics
Light emitting diodes, Batteries, Electric lamps, Measuring instruments, Black-box, Electric power, Magnetic materials, Educational assessment, Teaching, Libraries
1.
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5.
White and magenta LEDs employ blue LEDs covered with phosphor coating. Mixing blue light with the light emitted by the phosphor produces light that appears white or magenta. The activity in which students discover this mechanism is described in paper #3 from our series.
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8.
Students should realize that the uncertainty of the period is smaller if exposure time is larger and that the cart need not move with constant speed in order to determine the period.
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10.
A comparison of efficency of light bulbs and LEDs was published before:
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11.
We used OptoSupply white LED OSPW5111P.
12.
We used a conventional flashlight bulb with the following data: 3.8 V, 0.3 A.
13.
We used Vernier Light Sensor LS-BTA.
14.
Using a hacksaw, carefully saw off the part of the LED body that makes the lens, then brush and polish the sawed surface using a fine water sandpaper and finally a white toothpaste until the surface looks clearly transparent [see also
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15.
The Ping-Pong ball scatters light off the interior surface of the ball with equal intensity regardless of viewing direction, making the ball appear equally bright from all directions. However, note that some light is absorbed by the ball (i.e., light energy is converted into thermal energy).
16.
Here we define transmittance as a fraction of the energy of incident light at a specified wavelength range that passes through a sample.
17.
We used LD 271, which has the peak wavelength at 950 nm and can stand maximal forward current 130 mA.
18.
We used Optosupply LEDs: red OSHR5111P, green OSPG5111P, and blue OSUB5111P.
19.
The spectrum of our UV LED had the peak wavelength at 400 nm (visible range) and extended to about 380 nm in UV region. These are typical data for commonly available UV LEDs.
20.
We used Vernier Light Sensor LS-BTA.
21.
Sun protection factor (SPF) X means that using this sunscreen you can stay X times longer in the Sun to burn the same way as without the sunscreen.
22.
We used Vernier UVA Sensor UVA-BTA.
23.
Using the LED as a detector in this case would not give useful measurements because of the lens, which modifies the intensity of light that scatters on the sunscreen layer.
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25.
We used Optosupply LEDs: red OSHR5111P and yellow OS5YKA5111P.
26.
We made videos at 1200 frames per second using a Casio Exilim camera.
27.
Some students may realize that due to the shape of the dipolar field of the magnet, there is also a small change of the magnetic flux through the coil (and consequential induced voltage) when the magnet is outside the coil and approaching the coil region. This is true and it is a sign of deeper understanding of the topic. However, the induced voltage in this case is far too small to turn on the LED, but it can be measured using an oscilloscope.
© 2015 American Association of Physics Teachers.
2015
American Association of Physics Teachers
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