# PHGY 210 Lecture Notes - Lecture 16: Aqueous Humour, Vitreous Body, Extraocular Muscles

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Published on 31 Jan 2013
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PROPERTIES OF LIGHT
Light
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 iris and provides the eye’s 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 eye’s 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.