For polychromatic light having a broad spectrum width the effective Faraday rotation angle <θF> is defined operationally as <θF> = ∫θFλSλPλdλ/∫SλPλdλ・・・(1). Here θλ, Sλ, and Pλ are Faraday rotation angle, sensitivity of photodetector, and light intensity, respectively, at wavelength λ. Integration is performed over the spectrum width of the polychromatic light. By classical electromagnetic-wave theory we derived Eq. (1), to which we gave a consistent quantum description of photon image. Experiments on an FR-5 glass sample using a halogen lamp as a polychromatic-light source supported our calculations.