Photon Energy and the Electronvolt · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Packets of light, priced per photon

Science · Waves, Light & Sound · ages 17-18
Name ______________________   Date ____________

  1. A photon carries 2.5 eV. Using 1 eV = 1.6e-19 J, what is its energy in joules?

    • 4.0e-19 J
    • 1.6e-19 J
    • 2.5e-19 J
    • 4.0e-18 J

  2. A photon carries 2.5 eV. Using 1 eV equals 1.6 times ten to the minus 19 J, what is its energy in joules?

    • 4.0 times ten to the minus 19 J
    • 1.6 times ten to the minus 19 J
    • 2.5 times ten to the minus 19 J

  3. Light has frequency 6.0 times ten to the 14 Hz. Use c equals 3.00 times ten to the 8 with c equals f lambda. What is its wavelength in nanometres?

    • 600 nm
    • 500 nm
    • 400 nm

  4. One electronvolt is bigger than one joule.

    Circle one:   True   False

  5. A 1.0 W beam is made of photons that each carry 2.0e-16 J. How many photons arrive per second?

    Answer: ______________

  6. Frequency doubles while brightness stays level. What happens to each photon's energy?

    • It halves
    • It stays level
    • It doubles

  7. A photon has frequency 5.0 times ten to the 14 Hz. Use h equals 6.63 times ten to the minus 34 and E equals h f. What is its energy in joules?

    Answer: ______________

  8. A photon has frequency 5.0e14 Hz. Use h = 6.63e-34 J s and E = h f. What is its energy in joules?

    Answer: ______________

  9. A 2.0 W beam uses photons carrying 2.0 times ten to the minus 16 J each. How many photons arrive per second?

    Answer: ______________

  10. Zac says bright red light beats dim UV photon for photon. What is his error?

    • UV photons each carry more punch
    • Red photons travel faster
    • Brightness changes frequency
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Answer key

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Packets of light, priced per photon W1-mt_UfmDhGRaYT-s1

  1. 4.0e-19 J · One eV is 1.6e-19 J, so 2.5 eV is 2.5 x 1.6e-19 = 4.0e-19 J.
  2. 4.0 times ten to the minus 19 J · Multiply 2.5 by 1.6 times ten to the minus 19 to get 4.0 times ten to the minus 19 joules.
  3. 500 nm · Divide c by f: 3.00 times ten to the 8 over 6.0 times ten to the 14 is 500 nm.
  4. False · One eV is a sliver of a joule: 1.6 times ten to the minus 19.
  5. 5000000000000000 · Divide power by energy per photon: 1.0 / 2.0e-16 = 5e15 photons each second.
  6. It doubles · Energy follows frequency photon by photon, so doubling f doubles each packet.
  7. 3.315e-19 · Multiply h by f: 6.63 times ten to the minus 34 times 5.0 times ten to the 14 is 3.315 times ten to the minus 19 J.
  8. 3.315e-19 · Multiply h by f: 6.63e-34 x 5.0e14 = 3.315e-19 J.
  9. 10000000000000000 · Divide 2.0 by 2.0 times ten to the minus 16 to get 1 times ten to the 16 per second.
  10. UV photons each carry more punch · Punch per photon comes from frequency alone, and UV outranks red however bright.
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