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Chapter 2

Chapter 2

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University of Toronto St. George

Chapter #2 – Lecture Notes Light: a wave, a stream of photons, tiny particles that each consist of one quantum of energy • 2 ways to conceptualize light: o Wave o Stream of photons, tiny particles that each consist of one quantum of energy o Light – made up of waves as it moves around the world and of photons as it is absorbed o Gamma rays  ▯short wavelengths o Radio and television waves  ▯long wavelengths o Visible light  ▯400­700 nanometers  o As light reaches the atmosphere from space, some of the starlight’s photons are absorbed, while  some light is scattered by different particles. Most photons make it through the atmosphere and  hit the surface of an object. • Light can be ATRRD absorbed, transmitted, reflected, refracted and diffracted 1. Absorbed - energy is taken up, transformed into another type of energy, different from original source (ex) Solar panel 2. Transmitted – convey light from one place to another 3. Reflected – to redirect something that strikes a surface, usually back to its original point 4. Refracted - To alter the course of a wave of energy that passes into something from another medium, as water does to light entering from the air. 5. Diffracted – bent or having waves that are spread out (ex) waves of sound or light as they encounter an obstacle or pass through a narrow aperture (ex) Sun shinning through a tree • Circadian Rhythm – we are supposed to sleep at night, we renew our sleep patterns by using light (ex) If we are jetlagged, we follow the day/ and night to ensure we fall back into a regular sleep pattern Evolution of the Eye • Opsin (a light sensitive protein) develops o Becomes more selective because there is more light entering the eye cavity o Eye cavity becomes filled with water o Then becomes enclosed o Then lens develops o Iris and cornea develop – The eye as we know it today Accommodation – the process by which the eye changes it’s focus, in which the lens gets fatter as gaze is directed toward nearer objects. It is accomplished by contraction of the ciliary muscles. Presbyopia – literally “old sight.” The loss of near vision because of insufficient accommodation. Cataracts – An opacity of the crystalline lens. A. Emmetropia – the condition in which there is no refractive error, because the refractive power of the eye is perfectly matched to the length of the eyeball. B. Myopia – near-sightedness (you see near objects well), a common condition in which light entering the eye is focused in front of the retina and accommodation is required in order to see objects clearly. C. Hyperopia – farsightedness, a common condition in which light entering the eye is focused behind the retina and accommodation is required in order to see objects clearly D. Astigmatism – a visual defect caused by the unequal curving of one or more of the refractive surfaces of the eye, usually the cornea. Vertical lines might be focused in front of the retina while horizontal ones might be focused behind it. • The process of seeing begins with the retina, were the light energy from our star is transduced into neural energy that can be interpreted by the brain Fundus – the back layer of the retina- what the eye doctor sees through an ophthalmoscope Optic Disc - the point where the arteries and veins that feed the retina enter the eye, and where the axons of ganglion cells leave the eye via the optic nerve - has no cones or rods • The retina is a clear layered sheet of clear neurons, about half the thickness of a credit card, with another layer of darker cells the pigment epithelium, lying behind the final layher • The transduction of light energy into neural energy begins in the backmost layer of the retina, which is made up of cells called photoreceptors • If photoreceptors stimulate our sight, why are they at the back of our retina? Retinal Information Processing • The retina contains five major classes of neurons: photorec
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