Anonymous
Anonymous asked in Science & MathematicsPhysics · 1 decade ago

How many photons per second?

The human eye can barley detect a star whose intensity at the earth’s surface is 1.6 x 10 -11 W/m2. If the dark adapted eye has a pupil diameter of 7.0 mm, how many photons per second enter the eye from the star? Assume the starlight has a wavelength of 550 nm.

Please show equations and answers

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  • knr
    Lv 6
    1 decade ago
    Favorite Answer

    with the given intensity of radiation ,we can have

    energy at the eye E1 = I x A x t = I x pi r^2 x t

    = 1.6x10^-11x 3.14x (3.5x10^-3)^2 x 1

    = 6.1544 x10^-16 J

    now energy of photon E2 = 12400 / wavelngth(in angstrom) ev

    = 12400 / 5500 = 2.25 ev = 2.25x1.6x10^-19 J

    = 3.6 x 10^-19 J

    then number of photons N = E1/E2 = 6.1544x10^-16 / 3.6x10^-19 = 1709 (nearly)

  • Al P
    Lv 7
    1 decade ago

    h = 6.6260681 * 10 ^ -34 m2 kg / s

    c = 3 * 10 ^ 8 m/s

    P = 1.6 * 10 ^ -11 W/m^2

    w = 550 * 10 ^ -9 m

    r = 0.007 / 2 m

    A = pi * r ^ 2

    The total energy arriving in one second is:

    E = P * A * 1 = 6.1575164E-16 J

    Energy of one photon

    Ep = h * c / w

    Photon_Number = E / Ep

    Photon_Number = 1703.6921287704

    With rounding:

    Photon_Number = 1704 photons /s

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