Light Flux received by a Spectrograph from a Spatially Extended Refracting Source

The geometrical-optical treatment of Nielsen, which referred to a spatially extended source of refractive index unity, is extended to sources of any refractive index. Provided that the direction of emergence of the light from the source is nearly normal, it is shown that a change of refractive index from 1 to n modifies the light flux by the factor 1/ n 2 . This result is independent of whether or not a condensing lens is used between source and spectrograph. Its relevance in connection with observed intensities in Raman spectra is discussed.