Article citations

    Shu, F.H. The Physical Universe. And Introduction to Astronomy, 108.
    https://books.google.es/books?id=v_6PbAfapSAC&pg=PA108&lpg=PA108&dq=range+wavelength+virtual
    +photon&source=bl&ots=oqrksX7gjD&sig=9nd3Cr5gK7MWDVuYGnLxL1jZ2Ek&hl=es&sa=X&ved=0ah
    UKEwil29iSsYXPAhVBnBQKHcuiCkcQ6AEIMTAC#v=onepage&q=range%20wavelength%20virtual%20
    photon&f=false

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