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Relating energy to frequency

Activity 10E: Experiment
 
 Photons have a characteristic energy

Light of a particular colour is a stream of photons of a specific frequency. Light appears granular when seen at the finest scale. A bright light delivers lots of energy every second. If light is granular then the amount of energy must be related to the number of granules arriving each second. The energy delivered by each must also be related to this intensity. This activity relates the energy of each photon to the frequency of that photon.

You will need

  • multiple LED array
  •  
  • peering tube
  •  
  • power supply, 5 V
  •  
  • multimeter
  •  
  • five 4 mm leads
    •

    Measuring energy

    The energy released by each electron as it travels through the LED is transferred to a photon. To measure the energy released by each electron measure the potential difference across the LED when it just glows. Then we multiply this figure by the charge on the electron (1.6 ¥ 10–19 C). The quantity that characterizes the photon is the frequency so we then seek to find a connection between this frequency and the energy.

    1. Set up the circuit and check that each LED can be lit by altering the p.d.
     

    2. Calculate the frequency of the LEDs.

    3. Measure the p.d. to just light each LED. At this p.d. the energy supplied by the electrons is all transferred to photons.

    4. Look for a pattern connecting energy to frequency (plot a graph!). You should be prepared to remeasure any points that do not fit and to check your results with those from other measurements.

    5. See if you can quantify the relationship. By how much does the energy of the photon change for each hertz?
    © CERN and High School Teachers Programme at CERN

    Last modified: 28 June 2002