Vision begins when light energy reflected by an object enters the eye through the pupil (the black opening in the eye). The muscles attached to the lens of the eye controls how the eye adjusts its focus on the object (the muscles relax to make the lens thin when object is far away or contract to make the lens thick when object is near). The Iris (the colored area surrounding the pupil) controls the amount of light that enters the pupil by opening the pupil wide when light is dim or closing it partially when light is bright. The light that enters the eye is refracted and focused on the retina (a curved surface at the rear of the eye). The cornea (the protective outer covering of the eye) refracts most of the light. An inverted and smaller image of the object is then formed on the retina.
The retina contains photoreceptor cells called rods and cones (There are about 3 million cones and about 100 million rods contained in the human retina). The rods are sentive to dim light and are used in night vision while the cones are color-sensitive. The upper ends of the rods and cones contain the molecule rhodopsin (figure 1) which is made up of opsin (a protein) and retinal (a reddish purple pigment). On absorption of light energy, the π bond of one of rhodopsin's double bond (indicated in red in the retinal portion in figure 1a) breaks thereby allowing the flexibility needed by the molecule to rotate about an otherwise rigid double bond. This rotation changes the molecular geometry of the molecule. Retinal separates from opsin and the chemical reactions triggered, produce the nerve impulse that the brain interprets as the sensation of vision.
The light energy; the motion of the reflected light from object to the retina; the adjustment of the focus of the eye's lens; the opening or partial closing of the pupil depending on the brightness of light; the containership of rods and cones in the retina; the containership of rhodopsin in the rods and cones; the breaking of the π bond of one of the double bonds of rhodopsin; the rotation of rhodopsin; the separation of retinal from opsin; the chemical reactions that produce the nerve impulse that the brain interprets; the transmission of the nerve impulse to the brain and the interpretation of the nerve impulse by the brain are all expressions of Pj Problems.