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Does greater light intensity imply greater energy?

  1. Oct 12, 2011 #1
    Since light intensity increases with the number of photons, and the photon can be viewed as energy, would energy be proportional to intensity of light? Also since energy is inversely proportional to the wavelength of light, this would mean that the intensity of light is also inversely proportional to the wavelength of light.

    The reason I'm asking this is because I saw this problem:

    Star A is radiating two times as much energy as Star B, but it
    is two times the distance from us. Which star will appear
    brighter, and by how much?

    a. Star A will be 2 times brighter
    b. Star B will be 2 times brighter
    c. Star A will be 4 times brighter
    d. Star B will be 4 times brighter
    e. they will both have the same observed brightness

    Since brightness increases with intensity of light, and intensity is proportional to energy, brightness should also be proportional to energy, which means that Star A would be 2 times brighter (if they are at equal distances). Since brightness is inversely proportional to distance, wouldn't both stars have the same observed brightness? But the answer is b., which I don't understand...I would really appreciate it if someone could clear this up for me. Thank you so much!!!
  2. jcsd
  3. Oct 12, 2011 #2


    User Avatar
    Gold Member

    Hint. Re-examine your assumptions.
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