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Homework Help: Quantum Physics Questions Very Very please respond

  1. Nov 10, 2004 #1
    Quantum Physics Questions!!!Very Very urgent please plz respond

    a) A student obserbs the LED from a distance of 0.20 m.The pupil of her eye has a diameter of 0.6mm.Calculate the no of photons which enter her eye /sec??
    Plz give me some hint and i'll try to solve it.

    b)Explain in terms of photon why light intensity decreases with increasing with distance from LED?
    ans) as the photons travel along,it gets absorbed by air molecules ,so intensity decreases with distance .(plz commet on my answer)

    c)If the elcetron was confined within the nucleus,it would have very high energy?
    I absolutely have no idea.sorry ;(
  2. jcsd
  3. Nov 10, 2004 #2
    Can help you with your 3rd question
    First apply the uncertainty principle to an electron confined inside a nucleus
    ^x^p>h(bar)/2 where ^x uncertainty in position ^p uncertianty in momentum. Using this you get uncertainty in momentum is 0.527E-20kgms-1 (^x is approx 10fm, 1E-14 for an electron inside the nucleus)

    Using the relativistic equation for energy

    E^2=(pc)^2 + (mc^2)^2 where p - momentum of electron
    m - electron rest energy
    c - speed of light
    using the above value for momentum of the electron and rest mass of the electron and substituting into the energy equation gives
    E=1.58E-12 which is 9.88MeV which is far far greater than the Coulomb Force which is around 0.51MeV

    So the electron cannot exist as a seperate particle inside nuclei.
  4. Nov 10, 2004 #3

    Doc Al

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    Staff: Mentor

    You need additional information. You need to know how powerful (light energy emitted per unit time) the LED is and the frequency of the light emitted. Then you can figure out the energy per photon, thus how many photons are emitted. Assume they spread out uniformly.

    No. Think of the photons as bullets of light energy that spread out as they leave the source. The intensity (photons per unit area) gets weaker as you get further from the source.
  5. Nov 10, 2004 #4
    Quantam physics question

  6. Nov 10, 2004 #5

    Doc Al

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    Staff: Mentor

    If you are given the intensity, then you can calculate power by realizing that: Intensity = Power/area. You need to calculate the power entering the eye.

    Yes, that will give you the number of photons per second.

    As I explain above, start by calculating the power entering the eye.
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