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Chapter 9

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Department
Neuroscience
Course
NROC64H3
Professor
Niemier
Semester
Winter

Description
Neuroscience : Chapter 9 – The Eye Properties of Light Light is the electromagnetic radiation that is visible to our eyes  Wavelength: the distance between successive peaks or troughs of the wave  Frequency: the number of waves per second  Amplitude: the difference in height between a waves peak and trough The energy of a wave is proportional to its frequency; the higher the frequency of a wavelength, the more energy it has Visible light is only a small part of the complete electromagnetic spectrum; consisting of wavelengths 400-700. Of this visible light, ‘hot’ colours such as red or orange consist of longer frequencies (closer to 700) whereas ‘cool’ colours such as blue or violet consist of shorter waves (closer to 400) Optics Optics: the study of light rays and their interaction A “ray’ is a wave of electromagnetic radiation that travels in a straight line (only in a vacuum). In our environment, radiation travels in a straight line until they interact with atoms and molecules in the atmosphere or objects on the ground There are 3 types of interactions that a ray of light can have with its environment: 1. Reflection: The bouncing of light rays off a surface. The way it is reflected depends on the angle the ray strikes the surface. 2. Absorption:The transfer of light energy to a particle or surface. Black surfaces absorb energy from all visible wavelengths, white surfaces reflect all wavelengths Some compounds only absorb a limited range of wavelengths and reflect the rest; this is the basis for colour 3. Refraction: The bending of light rays as it passes from one transparent medium to another. The Structure of the Eye Gross Anatomy of the Eye Pupil: The opening that allows light to enter the eye and reach the retina; appears dark because of the light-absorbing pigments Iris: Surrounds the pupil; forms the colour of the eye. Contains two sets of muscles; one that constrict the pupil, another that relaxes the pupil Cornea: A glassy transparent surface covering the pupil and iris Sclera: The white of the eye. Contains three pairs of extraocular muscles that move the eye around in its socket Conjunctiva: A membrane that folds back from the inside of the eyelids and attaches to the sclera Optic Nerve: Carries axons from the retina, out the back of the eye, to the brain Opthalmoscopic Appearance of the Eye Optic Disk: Circular region inside of the eye where the optic nerve fibres exit the eye Macula: A pale yellowish circular region in the middle of the retina that lacks major blood vessels. It contributes to central vision Fovea: At the center of the retina, the fovea is a dark spot in the middle of the macula; also contributes to central vision There is no vision at the optic disk or underneath the blood vessels because it lacks photoreceptors. Although there is no information from these areas, the brain fills in the rest so there is no appearance of dark spots in our vision Cross-Sectional Anatomy of the Eye Aqueous Humour: a watery solution that lies behind the cornea and provides nourishment Lens: A transparent lens that lies behind the iris that helps refract light onto the retina. The lens is suspended by ligaments (called zonule fibres) Ciliary Muscles: a muscle that holds the zonule fibres; ciliary muscles are continuous with the scleraVitreous Humour: A viscous, jellylike substance that lies between the lens and retina; serves to maintain the shape of the eyeball Image Formation by the Eye In order for an image to be processes, it must pass through the eye and be focused on the retina Focal Distance: The distance from the refractive surface (the cornea) to the point where parallel light rays converge (retina) The curvature of the cornea has a major effect on the focal distance, the tighter the curve, the shorter the focal distance Diopter: The reciprocal of the focal distance (in meters); in other words, it is the refractive power of a surface The cornea has a refractive power of about 42 diopters = 0.024m = 2.4cm. The cornea has by far the strongest refractive power than any eye structure (including glasses) When an object is more than 9 meters away, the light that is emitted from it is travelling in parallel waves, therefore in order to focus these wavelengths on the retina, the light has to be bent (refraction) The cornea and the lens perform the bulk of the refraction needed to focus images The Lens in Accommodation Accommodation: The focusing power that is provided by changing the shape of the lens The lens contributes another dozen or so diopters in refracting wavelengths; however, it has a more prominent role in the formation of crisp images of objects closer than 9 metres away. Closer than 9 metres, the electromagnetic radiation emitted from objects is no longer parallel, but rather diverges outwards. This causes the need for greater refractive power in the eye Accommodation involves the ciliary muscles, and the zonule fibres that hold the lens in suspe
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