A sensor is exposed for time $t$ to a lamp of power $P$ placed at a distance $l$. The sensor has a circular opening that is $4d$ in diameter. Assuming all energy of the lamp is given off as light, the number of photons entering the sensor if the wavelength of light is $\lambda $ is $(l >> d)$

  • A

    $\frac{{P\lambda {d^2}t}}{{hc{l^2}}}$

  • B

    $\frac{{4P\lambda {d^2}t}}{{hc{l^2}}}$

  • C

    $\frac{{P\lambda {d^2}t}}{{4hc{l^2}}}$

  • D

    $\frac{{P\lambda {d^2}t}}{{16hc{l^2}}}$

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