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Relating energy to frequency - Teachers Notes

Practical Advice

We suggest setting up several competing research groups, and actively encouraging students to form a consensus about the relationship between frequency and energy. An appropriate degree of collaboration gets the correct answer; inappropriate degrees yield a work of fiction or no consensus. Thus can physics progress.

Students will know about the existence of an LED from previous work on electricity and will know that it conducts in one direction only. Thus electrons, simply introduced as what moves when electricity is conducted, can be presented as meeting an electrical barrier when the LED is reverse biased and falling down that barrier when forward biased. This simple model of the action of an LED is enough for this purpose. Connecting this electrical model to an energetic model requires the notion of potential difference to be reviewed as being likely to be the sensible way of determining the height of the barrier and the energy as being the potential difference times the charge on the electron. Analogies with the energy released in falling down a hill can reinforce this idea. So as to make sure that none of the electrical energy is dissipated (actually as phonons) we need to insist that we require the smallest p.d. across the photodiode. This energy, plotted against the frequency of emitted light (taken from the manufacturer's specifications), can then be used. Experience shows that the measurements made by the students may not be so accurate, and that encouragement to settle on a simple pattern, together with the consensual approach suggested above and the ability to make several measurements before deciding on the accurate answer, will be necessary. A class using a graph-plotting package may make this review and interaction more likely.

Students should easily establish E = h f. A consensus on the value of h should give a value that is far from embarrassing.

Sample results:

Alternative Approaches

To establish the connection between the energy associated with each click – or the arrival of each photon – we suggest measuring the energy required to release one photon of light in a light-emitting diode. This has the advantage that we can cheaply try out several frequencies and rapidly obtain a picture of how energy varies with frequency. It is, of course, not the only technique for establishing a link between frequency and energy. You may substitute others. The photoelectric effect has been used for this for many years, as has appeal to the evidence of spectra.

Social and Human Context

Engineering barriers of different heights to release different amounts of energy takes us into the structures of semiconductor materials and engineering bandgaps, which, whilst fascinating, does not contribute to the central understanding sought here. It is, however, central to quantum engineering. This can be used to start a discussion linking the potential barrier to an energetic understanding of the situation.

Level

Adaptable

Last modified: 28 June 2002