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    A laser emits blue light of wavelength 405nm and radiant power of 10mW, calculate the number of photons emitted each second. Could you also explain why you did each step?
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    (Original post by p29)
    A laser emits blue light of wavelength 405nm and radiant power of 10mW, calculate the number of photons emitted each second. Could you also explain why you did each step?
    You are given the wavelength of a blue photon, so you can find the energy of a single blue photon as  E = hf = h \dfrac{c}{\lambda} .

    Radiant power of 10mW means 10 mJ of energy per second.

    Number of photons =  \dfrac{ \text{Energy per second}}{\text{Energy of a blue photon}}
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    (Original post by Eimmanuel)
    You are given the wavelength of a blue photon, so you can find the energy of a single blue photon as  E = hf = h \dfrac{c}{\lambda} .

    Radiant power of 10mW means 10 mJ of energy per second.

    Number of photons =  \dfrac{ \text{Energy per second}}{\text{Energy of a blue photon}}
    but why does Number of photons =  \dfrac{ \text{Energy per second}}{\text{Energy of a blue photon}}
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    (Original post by p29)
    but why does Number of photons =  \dfrac{ \text{Energy per second}}{\text{Energy of a blue photon}}
    I don't quite understand the question.

    Maybe I explain the above working through another way.

    Power can be defined as the rate of transfer of energy.

    Since a photon has an amount of energy hf, when N number of photons are incident on a surface per second, the amount of energy delivered to the surface in one second, is Nhf, which is the power.

    Hope is better now.
 
 
 
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