On weight of photon

  1. hi pf, my friends asked me a strange question to me today,
    "if in a vacuum chamber if light is somehow focused on a suitable weighing machine then will there be any deflection?
    if yes does that mean light has mass? "

    sounds strange, isnt? that why i choose to ask here?
  2. jcsd
  3. mfb

    Staff: Mentor

    Focused in which way? Coming from above, reflecting light upwards? This will transfer momentum, and the scale will change.
    Focused to have a high and constant energy density close to some (hypothetical) very sensitive scale? The scale should see an attraction.
    Something else?
  4. coming from above, and focused on a sensitive scale..
  5. mfb

    Staff: Mentor

    Light has a momentum, and momentum times intensity is a pressure. Pressure times surface area (of your scale, or focus) is a force, and the scale can measure this.
    The Nichols radiometer is not a scale, but it operates with light pressure.
  6. wow..beautiful explanation. i think i got it... i need some time for review..thanks by the way
  7. i guess it doesnt have anything to do with mass of photon?
  8. mfb

    Staff: Mentor

    A photon has no mass.
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