Mechanism of vision:
Light rays (of the visible spectrum) are focussed on the retina via passing through the cornea and lens $\to$ Light-induced dissociation of photosensitive compounds [(photopigments composed of opsin (a protein) and retinal (an aldehyde of vitamin A)] into retinal and opsin $\to$ Structural changes in opsin $\to$ Changes in membrane permeability $\to$ Potentials are generated in photoreceptor cells (rods and cones) $\to$ Production of a signal crosses the bipolar cells and reaches the ganglion cells $\to$ Generation of action potentials in the ganglion cells $\to$ Transmission of these action potentials (impulses) by the optic nerves up to the visual cortex area of the brain for the analysis.