PSL300H1 Lecture Notes - Lecture 10: Presbyopia, Far-Sightedness, Refractive Index
PSL300
Lecture 10: Optics of the Eye
• The eye is divided into 2 chambers by the lens – the lends is a
transparent disk that focuses light. It is suspended by ligaments called
zonules
o The front of the lens is the anterior chamber, filled with
aqueous humor, a plasma-like fluid
▪ Carries nutrients and nourishes the lens
o Behind the lends is the vitreous chamber, filter with the
vitreous body, a clear jelly that helps maintain the eyeball’s
shape
• Light enters the eyes through the cornea
o The cornea is a transparent bulge at the front of the eye,
continuous with the white of the eye, or sclera – the outer wall
of the eyeball
o The cornea and lens focus light on the retina, the inner lining of the eye that contains the photoreceptors
o Light passes from the cornea to the lens through a hole in the iris called the pupil
Pupil
• In bright light the pupils constrict (shrink) to 1.5 mm across – reducing the amount of light reaching the lens
• In the dark they dilate (enlarge) to 8 mm (~20X bigger in area) – increase the amount of light reaching the lens
• But the eye operates over a vast range of illumination, from starlight to sunlight, so the pupil contributes only a
little to compensating for brightness
• The pupil size is controlled by smooth muscles in the iris
o In bright light, parasympathetic signals from the brain contract the ring-shaped pupillary constrictor
muscles, shrinking the pupil
o In the dark, sympathetic signals contract the radial pupillary dilator muscle of the iris, dilating the pupil
• The pupil helps to focus light – like a pinhole camera
o Small pupil ensures that each point on the retina receives light from one direction in space → objects are
in focus (not blurry)
• The pupil also controls the depth of field
o When the pupil is tightly constricted, we have full depth of
field
▪ Everything we see is equally in focus
o When the pupil is dilated, we have a shallow depth of field
▪ Only objects near one specific distance are in
focus
• With pinhole-focusing, the retinal image is dim because the pinole doesn’t admit much light
o Enlarging the hole makes the image brighter but blurrier (because more light from different directions is
being admitted in)
o Using refraction helps to make the image brighter and less blurry
Refraction
• Light bends when it enters a medium with a different refractive index
• Out corneas are made of clear collagen – they bend light strongly because
there is a big different between the refractive indices of air and collagen
o The bending of light is called refraction
• In water the refraction is much weaker because the refractive indices of
collagen and water are similar
• Light is refracted by both the cornea and the lens
o Cornea is responsible for 2/3 of the eyes refraction, and the lens for 1/3
▪ Lens has the ability to change shape to adjust the focus
• The lens is a mesh of cells without nuclei, packed with clear proteins called crystallins, and are “zippered”
together in concentric layers for flexibility
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Document Summary
Lecture 10: optics of the eye: the eye is divided into 2 chambers by the lens the lends is a transparent disk that focuses light. In bright light the pupils constrict (shrink) to 1. 5 mm across reducing the amount of light reaching the lens. In bright light, parasympathetic signals from the brain contract the ring-shaped pupillary constrictor muscles, shrinking the pupil. It has no blood supply relies on the aqueous humor for nutrients: the lens of the eye is convex fatter in the middle and thinner on the edges. If light hits a lens at a right angle there will be no bending: changing the shape of a lens can change the angle of incidence and change how much the light bends. Accommodation: a rounder lens bends light more and has a closer focal point, for clear vision, the focal point must fall on the retina.
