The photochemistry of vision
We have mentioned microwave ovens a few times already. Although we might employ such an oven regularly, no one has actually seen microwave radiation. The photons are wholly invisible. The phenomenon of 'sight' can be simplified to the photo-effected transformation of a pigment related to retinal within the retina at the back of the eye. Retinal is derived from vitamin A.
The outermost layer of the retina is the pigment epithelium (with supporting cells for the neural portion of the retina). The epithelium is darkened with melanin (as described above), thereby decreasing the extent of light scattering within the eye. Directly beneath this layer is the bacillary layer (also called the rod and cone layer), which comprises a layer of photoreceptor cells that respond to light. The rods are highly sensitive, and monitor the intensity of incident light; the cones are less sensitive, and relay information that is ultimately encoded within the brain as colour.
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Figure 9.20 The photo-activated process that accounts for vision: the 11-cis ^ 11-trans photo-isomerism of retinal
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Figure 9.20 The photo-activated process that accounts for vision: the 11-cis ^ 11-trans photo-isomerism of retinal
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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