Wednesday July 29 y Chapter 2 Physics and light There are two ways to conceptualize light 1 As a wave 2 As a stream of photons Visible light waves have a wavelength between 400 and 700 nanometers As the wavelength changes within this visible spectrum the color we observe changes The color violet is at around 400 nm yellow is at 600 nm while red is at about 650 nm Some of the photons light particles will be absorbed by dust vaporized water etc and some of the photons will be scattered Some of the photons will be reflected when it strikes the surface Light reflecting off of a surface makes it lighter in appearance Light being absorbed makes it darker in appearance Light that is neither reflected nor absorbed is transmitted through the surface Anatomy Cornea 1 The window to the world because it is transparent and contains no blood vessels or blood which would absorb light A contact lens sits on the cornea The external layers of the cornea regenerate very quickly so damage usually heals within 24 hours Wednesday July 29 y Most powerful refracting occurs in the cornea Aqueous humor Fluid that fills the space behind the cornea and lens Lens Also removes waste from the cornea and lens A fluid derived from blood that supplies oxygen and nutrients from the cornea Also has no blood supply and is transparent The shape of the lens is controlled by the ciliary muscle The lens is attached to the ciliary muscle through tiny fibers called the zonules of zinn Changing the shape of the lens by contracting and dilating the ciliary muscle is called accommodation When the muscle is relaxed the zonules are stretched and the lens is flat The eye will be focused on distant objects When the muscle is contracted the tension on the zonules is reduced and the lens bulges The eye will be focused on close objects The fatter the lens the more power it has A hole in a muscular structure called the iris The iris gives the eyes its distinctive color and controls the size of the pupil thus the amount of light that reaches the retina The iris automatically contracts or expands to allow more or less light into the Pupil Iris eye 2 Choroid Vitreous humor Retina Wednesday July 29 y Pupillary light reflex whytt s reflex immediate constriction of pupil in response to bright light Red eye dim light pupil is dilated which causes too much light to enter The light is reflected back which gives the red appearance The layer of blood vessels that provide nutrition to the eye Heavily pigmented so it absorbs extraneous light entering the eye The chamber in the eye within the retina and the lens The longest part of the eyeball 80 of the internal volume of the eye Floaters are small bits of debris that can be seen when staring at a bright light Detect light and tell the brain about aspects of light that are related to objects in the world This is where seeing really begins and the light energy is transduced into neural energy that can be interpreted by the brain The transduction begins with photoreceptors When photoreceptors sense light they can stimulate neurons in other layers horizontal ganglion amacrine Only about 1 2 of the light will actually reach the retina The retina is spread across the back of the eyeball The point where the axon of ganglion cells leaves the eye via the optic nerve This portion of the retina contains no photoreceptors and is thus blind Optic disk The blind spot 3 Wednesday July 29 y Refractive errors occur when the eyeball is too long or too short relative to the power of the optic components Myopia nearsightedness occurs if the eyeball is too long for the optics The image will be focused in front of the retina and will thus be blurry Myopia can be corrected with negative lenses which diverge the rays of light before they enter the eye Hyperopia farsightedness occurs if the eyeball is too short for the optics The image will be focused behind the retina and will thus be blurry Hyperopia can be corrected with positive lenses which converge the rays of light before they enter the eye Emmetropia is no refractive error because the refractive power of the eye is perfectly matches the length of the eyeball This is good Astigmatism occurs when the cornea is not spherical With astigmatism vertical lines might be focused slightly in front of the retina while horizontal lines are focused slightly behind it The lines may appear to be out of focus with the other lines Astigmatism can be corrected with lenses that two focal points provide different focusing power in the horizontal and vertical planes Presbyopia old sight is inevitable and occurs with aging The lens becomes sclerotic harder and the capsule that encircles the lens loses its elasticity and is no longer able to change shape Bifocles are lenses that have one power at the top distance and a different power at the bottom closer l Cataracts occurs when the crystallines in the lens are no longer packed densely together The irregularity in the crystallines reduces the transparency of the lens 4 Wednesday July 29 y Cataracts can interfere with vision because they absorb and scatter more light than the normal lens does Cataracts can occur at different ages take many different forms Congenital cataracts present at birth are rare but can have devastating effects on vision development Most cataracts are discovered after the age of 50 and the prevalence increases after 70 Treatment is the removal of most of the lens except for the most posterior portion which connects to the zonules Then put in a prosthetic lens Cells in the eye Types of cells in the eye photoreceptors horizontal cells bipolar cells amacrine cells and ganglion cells Photoreceptors The retina contains two types of photoreceptors Rods and Cones The types of photo receptors differ in their shape and distribution Humans have both rods and cones are are thus duplex retinas Some animals like owls only have rods In the dark no light the sodium channels and calcium open and the cell is depolarized and releases glutamate Glutamate then inhibits the bipolar cell Because the bipolar cell is inhibited it will not send the signal to the ganglion cell etc The influx of sodium during darkness is called dark current When the rhodopsin is exposed to light the retinal changes shape and can no longer fit into the binding site within the opsin protein The retinal activates the 5 Wednesday July 29 y coupled G protein The G protein then activates a second messenger called PDE PDE then closes the sodium channel and the
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