Bacillary layer

The active compound within the bacillary layer is retinal. To simplify the photo-physics within the rods and cones hugely, absorption of a photon initiates a series of conformational changes that lead ultimately to photo-isomerization of retinal from the 11-cis isomer to the 11-trans isomer see Figure 9.20. The uncoiling of the molecule following photo-excitation triggers a neural impulse, which is detected and deconvoluted by the brain. The photochemical reaction is breakage and, after rotation, re-formation of the C=C bond. 

Photons of microwave light do not possess sufficient energy to break the bond in retinal, so the photochemical reactions in Figure 9.20 do not occur in the eye when looking at microwaves. Without the photochemical reaction in the bacillary layer, no electrical impulse is formed, and the brain does not see the microwave radiation, so it is invisible light. 

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