A new formula on the Fresnel reflectance is derived based on our understanding of the complex index of refraction and continuity condition of the E-M waves across the interface between two media. The imaginary component of the complex index of refraction is related to the attenuation of the E-M waves propagating in a medium. It does not denote the phase of the E-M waves. The traditional derivation confuses the physical meaning of the imaginary component of the complex index of refraction. The derived formula on the Fresnel reflectance can be used to obtain the emissivity of sea surface, which is useful to retrieve sea surface temperature and sea surface salinity in the microwave remote sensing. A new formula on the Fresnel reflectance is derived based on our understanding of the complex index of refraction and continuity condition of the EM waves across the interface between two media. The imaginary component of the complex index of refraction is related to the attenuation of the EM The current derivation on the Fraction of the EM waves. The traditional derivation confuses the physical meaning of the imaginary component of the complex index of refraction. The derived formula on the Fresnel reflectance can be used to obtain the emissivity of sea ​​surface, which is useful to retrieve sea surface temperature and sea surface salinity in the microwave remote sensing.