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1. Vision

Module 8 - HSC Biology
This Lesson
● Structure of the eye
● Normal vision process
Refracting light
The Retina
Syllabus Links
IQ 8.5: How can technologies be used to assist people who experience disorders?
● explain a range of causes of disorders by investigating the structures and functions of the
relevant organs, for example:
hearing loss
visual disorders
loss of kidney function
● investigate technologies that are used to assist with the effects of a disorder, including but not
limited to:
hearing loss: cochlear implants, bone conduction implants, hearing aids
visual disorders: spectacles, laser surgery
loss of kidney function: dialysis
evaluate the effectiveness of a technology that is used to manage and assist with the effects
of a disorder
Starter Questions
Why is vision so important to animals, including humans?
What are some disorders of the eye/vision?
Extraocular Muscles
Ciliary Body
Aqueous Humor
Blood vessels
Blind spot
Optic Nerve
How does the eye allow vision?
1. Light Enters the Eye
2. Lens Focuses Light
3. Retina creates an
electrochemical signal
Light enters the eye through
the Cornea, is refracted and
then passes through the
Aqueous Humor and Iris/Pupil
The Lens ‘accommodates’ to
focus the light/image on the
When light/image hits the
retina specific cells are
activated (rods and cones).
When these cells are
stimulated they create an
electrochemical signals that
are sent via the optic nerve to
the brain (occipital lobe)
How does the eye work?
Step 1: Light enters the eye
But first, some physics revision…
Refraction is the bending of light. It is due to a
change in its speed of travel, because of a
change in the density of the medium it is
travelling through.
If it enters a medium where it travels more slowly
(such as entering water, from air) then it bends
towards the normal.
How does the eye work?
When light is passed through a biconvex lens
(rounded on both sides), the rays are refracted
towards a center point known as the focal point
The rays then diverge (spread out) and cross
over from that point
This is the basic principle for how the lens in the
eye works
Refractive Media
● To be refractive a medium must be transparent.
● The eye has four refractive media: the cornea, the aqueous humor, the lens
and the vitreous humor.
● Of these, most refraction occurs when light enters the cornea.
This is because this is where the greatest difference in speed of travel occurs.
Because the cornea, aqueous humor, lens and vitreous humor are all similar in the speed at
which they let light travel, there is little refraction between them.
● This information is particularly important when considering vision disorder
How Vision Works
Step 2: Lens focuses light (accomodation)
● Accommodation is the term used to describe the focusing of objects at
different distances by changing the shape (convexity) of the lens
○ The result of this is to change the refractive power
● This change in shape is brought about by the ciliary muscles which affect the
tension of the suspensory ligaments that hold the lens
Accomodation in the Eye
Activity: Draw a table to compare the shape of the lens; action of the ciliary
muscles and refractive power for distant and nearby objects
Accomodation in the Eye
Distant Objects
(distant vision)
Close Objects
(near vision)
Lens shape
More curved or rounded
(thinker/ fatter)
Ciliary Muscles
Refractive Power
How does the eye work?
Step 3: Retina creates an electrochemical signal
More physics!
Humans vision is able to see anything within the
visible light part of the EM spectrum
Objects absorb some wavelengths and reflect others
White light is a mixture of all the colours in the
EM spectrum
Object appear coloured because of the
wavelengths they reflect
The Retina
● The retina is the innermost layer of the eye and is a thin sheet of cells
about 0.1mm thick
● The retina is made up of several layers, one of which contains the visual
● These cells are called photoreceptor cells and are special types of
neurons that are sensitive to light.
In a human eye there are two types of photoreceptors:
■ rods which contain the light sensitive pigment rhodopsin
■ cones which contain the light sensitive pigment photopsin
The Retina
The Retina
● Photoreceptor cells contain light sensitive pigments.
● These cells convert light into electrochemical signals that the brain can
● An electrochemical signal consists of a wave of sodium and potassium ions
which move across the cell membrane of the neuron
● Rods are responsible for vision in low light
● Rods have only one type of pigment
(rhodopsin), thus they are not sensitive to
different colours
● There are approximately 90 million rods in
each retina, concentrated in the outer edges
● Vision involving rhodopsin allows humans to
see in shapes of black, white and grey
Cone cells are responsible for colour vision and contain
pigments of iodopsins
There are approximate 6-7 million cones in the retina and
these are concentrated in and near the fovea
There are three types of cones found in the human eye which
are all sensitive to different wavelengths of the visible
○ Blue region: short wavelengths peak at 455nm
○ Green region: medium wavelengths, peak at 530nm
○ Red region: long wavelengths, peak at 625nm
The Retina
The sensitivity of cone cells allow them to detect light on either side of the
peak sensitivities, giving an overlap in some of the colours detected
Thus, a particular wavelength may stimulate more than one cone
By comparing the rate at which various receptors respond, as well as the
overlap in colours, the brain is able to interpret these signals as intermediate
colours, thus we can see a variety of colours
Putting it all together
Activity: Complete Blitzing Biology 21.1 ‘Structure and function of the human eye’