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Chapter 9


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INTRODUCTION Light is electromagnetic energy that is emitted in the form of waves. The visual system begins with the eyes. At the back of the eye is the retina which contains photoreceptors specialized to covert light energy into neural activity. The output of the retina is not a reproduction of the intensity of the light falling on it but is specialized to detect differences in the intensity of light falling on different parts of it. The first synaptic relay in the pathway that serves visual perception occurs in a cell group of the dorsal thalamus called the lateral geniculate nucleus (LGN). From the LGN, visual information ascends to the cerebral cortex where it is interpreted and remembered. PROPERTIES OF LIGHT Light Electromagnetic radiation: Wavelength: the distance between successive peaks or troughs Frequency: the number of waves per second Amplitude: the difference between wave trough and peak The energy content of electromagnetic radiation is proportional to its frequency [i.e. high frequency (short wavelengths) has the highest energy content]. Only a small part of the electromagnetic spectrum is detectable by our visual system. This visible light consists of wavelengths of 400-700 nm. (see Fig. 9.2) Hot colours (e.g. red or orange) have long wavelength light and less energy than cool colours (e.g. blue or violet). Optics In a vacuum, a wave of electromagnetic radiation will travel in a straight line. This is a ray. Reflection: the bouncing of light rays off a surface. This depends on the angle at which it strikes the surface. A ray striking a mirror perpendicularly is reflected 180 back upon itself and a ray striking a mirror at 45 angle is reflected 90. Absorption: the transfer of light energy to a particle or surface. Black surfaces absorb the energy of all visible wavelengths. Some absorb light energy only in a limited range of wavelengths and reflect the remaining wavelengths. For example, a blue pigment absorbs long wavelengths but reflects a range of short wavelengths centered on 430 nm that are perceived as blue. Refraction: the bending of light rays that occur when they travel from one transparent medium to another. It occurs because the speed of light differs in two media (e.g. light passes through air more rapidly than through water). The greater the difference between the speeds of lights in the two media, the greater the angle of refraction. (see Fig. 9.3). THE STRUCTURE OF THE EYE Gross Anatomy of the Eye Pupil: the opening that allows light to enter the eye and reach the retina. It appears dark due to the light-absorbing pigments in the retina. Iris: surrounds the pupil and provides the eyes colour. It contains two muscles: one to make the pupils smaller when it contracts and the other to make the pupils larger. Cornea: a glassy transparent external surface of the eye that covers the pupil and iris. Sclera: the white of the eye which is continuous with the cornea. It forms the tough wall of the eyeball. The eyeball sits in the eyes orbit. Into the sclera are three pairs of extraocular muscles lying behind the conjunctiva and move the eyeball in the orbit. Optic nerve: carries axons from the retina to the base of the brain near the pituitary. Ophthalamoscopic Appearance of the Eye Optic disk: a pale circular region that gives rise to the retinal vessels and also where the optic nerve fibers exit the retina. Sensation of light cannot occur here because there are NO photoreceptors. Also sensation of light cannot occur where the large blood vessels exist because the vessels cast shadows on the retina. Macula: located at the middle of the retina. It is responsible for central vision and lacks large blood vessels which improve the quality of central vision. Fovea: a dark spot about 2mm in diameter. Cross-Sectional Anatomy of the Eye Aqueous Humor: it is the fluid behind the cornea which nourishes it Lens: located behind the iris and is suspended by zonule fibers attached to the ciliary muscles. Changes in the shape of the lens allow the eyes to adjust their focus to different viewing distances. o The ciliary muscles are attached to the sclera and form a ring inside the eye. They also divides the interior of the eye into two compartments containing slightly different fluids: The aqueous humor which lies between the cornea and lens, and The more viscous, jellylike vitreous humor which lies between the lens and the retina and serve to keep the eyeball spherical. IMAGE FORMATION BY THE EYE Cornea, rather than the lens, is the site of most of the refractive power of the eyes Refraction by the Cornea Light strikes the cornea and passes from the air into the aqueous humor. Then light rays bend and converge on the back of the eye (see Fig. 9.7) Focal distance: the distance from the refractive surface to the point where parallel light rays converge and depends on the curvature of the cornea (i.e. tight the curve, the shorter the focal distance). Diopter: reciprocal of the focal distance in meters. The cornea has a refractive power of 42 diopters which means that parallel light rays striking the corneal surface will be focused 0.024 m (2.4 cm). Refractive power depends on the slowing of light at the air-cornea interface. If we replace air with a medium that passes light at about the same speed as the eye, the refractive power will eliminated. This is why ones vision is blurry when you open your eyes underwater.Accommodation by the Lens The lens contributes another dozen or so diopters to the formation of a sharp image at a distance. It is, however, more importantly involved in forming crisp images of objects located closer than about 9m from the eye. As objects approach, the light rays originating at a point can no longer be consider to be parallel. These rays diverge and greater refractive power is required to bring them into focus on the retina. This focusing power, called accommodation, is provided by changing the shape of the lens (see Fig. 9.8). During accommodation, the ciliary muscles contract and swells in. The lens becomes rounder and thicker, increasing the curvature of the lens surfaces and increases the refractive power. The ability to accommodate changes with age. The Pupillary Light Reflex The pupil continuously adjusts for different ambient light levels. Pupillary light reflex involves connections between the retina and neurons in the brain stem that control the muscles that constrict the pupils. o This reflex is consensual. Shining a light into only one eye cause the constriction of the pupils of both eyes. Constriction of the pupil increases the depth of focus. The Visual Field It is the points where one can no longer see an object while staring straight ahe
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