Waveguide Modes and Refractive Index in Photoreceptors of Invertebrates

The refractive index of visual photoreceptors, if estimated by utilizing waveguide propagation, has to be corrected by a factor depending on the occurring mode. The correction factor is presented graphically for a number of relevant modes. Applied to the honeybee rhabdoms, it is shown that the correction for the mode propagation shifts the refractive index value of the rhabdom n = 1.347 as reported previously, to much higher levels presumably in the range of n = 1.365 ± 0.006. It is concluded that this value represents an acceptable estimate for the refractive index of invertebrate photoreceptors. Observed mode patterns now are in accordance with the theory. The consequences of the deduced data for the literature concerning waveguide optical studies of invertebrate photoreceptors are reviewed.