Communication
Topic 4: Accommodation
Biology in Focus, HSC Course
Glenda Childrawi, Margaret Robson and Stephanie Hollis
DOT Point(s)
 identify the conditions under which refraction of light occurs
 identify the cornea, aqueous humor, lens and vitreous humor as
refractive media
 identify accommodation as the focusing on objects at different
distances, describe its achievement through the change in
curvature of the lens and explain its importance
 compare the change in the refractive power of the lens from
rest to maximum accommodation
 analyse information from secondary sources to describe
changes in the shape of the eye’s lens when focusing on near and
far objects (you will get this on your homework)
Introduction
In order for the visual signal to be interpreted it must reach the
photoreceptor cells at the back of the eye in a clear and precise
form. It is transferred in the form of reflected, transmitted or
generated light from the environment outside the eye, to the retina
at the back of the eye, where a focused image should fall on the
rods and cones.
www.clipart.dk.co.uk
Introduction
To achieve this, the eye has
transparent structures that must
be maintained to perform their
function of refracting light to the
correct degree so that the image
that ultimately reaches the retina
is clear and in focus. In this
section of the Unit we are going
to explore in detail how this
happens.
www.scripps.org
The Refraction of Light
In order to be seen, an object
either reflects, generates its
own light or transmits light to
our eyes. When looking at this
pencil in a cup of water, the
image that is seen is light that
has been reflected through
water, glass, air and then
through your eyes.
www.teachengineering.org
The Refraction of Light
When light moves from one substance or medium to another
medium, it is bent or refracted. This leads to a distortion of the
image that you see, making it difficult to judge the exact location
of the object at which you are looking. Like the pencil in the
previous image.
images.yourdictionary.com
Refractive Media in the Mammalian Eye
The density of the cornea, aqueous humor, lens and vitreous
humor are similar to each other and close to that of water.
 All of these structures refract light that passes through the eye,
to greater or lesser degrees.
www.visionweb.com
Refractive Media in the Mammalian Eye
The refractive power of air, however, through which light travels
to reach the eyes of terrestrial mammals, is quite different (much
lower) from the refractive power of the parts of the eye.
 The greatest degree of refraction in the human eye occurs when
light moves from the surrounding ait into the cornea of the eye.
www.svcmd.com
Refractive Media in the Mammalian Eye
 The lens is able to refract light to a greater or lesser degree by
altering its shape.
 This is termed accommodation and is useful in allowing the eye
to adjust for near or distant vision.
www.rci.rutgers.edu
Refractive Media in the Mammalian Eye
The lens consists mainly of living protein fibres called crystallins,
housed in a lens capsule. These proteins are folded in a particular
way to make them transparent. The overall shape of the lens
determines the degree to which light can be refracted.
www.peposevision.com
Accommodation
Accommodation is the term used to describe the focusing of
objects at different distances, brought about by changing the
convexity of the lens and, as a result, it refractive power.
This change in shape of the lens results from the action of ciliary
muscles, which in turn affect the tension of the suspensory
ligaments that hold the lens.
www.takechargemama.com
Accommodation
 If light travels towards the eye from a distant source, the light
rays tend to be parallel. This means that they do not have to be
refracted much, in order to pass through the pupil and fall onto
the retina. The lens of the eye can remain in its relaxed and fairly
elongated state.
Accommodation
 If light rays travel from a close source, the light rays tend to
diverge. These rays need to be refracted to a greater degree, so
that they converge and can be directed onto the retina. For this
increased refraction of rays, it is necessary for the lens to
become rounder, resulting in a focused image forming the fovea
of the retina.
Accommodation
For distant vision the curvature of the lens must be relatively flat.
When the ciliary muscles are relaxed they hold the suspensory
ligaments taut. These ligaments pull on the lens, keeping it
relatively flat and allowing the image of distant objects to be
focused on the retina.
Accommodation
For near vision the curvature of the lens must be increased. The
ciliary muscles contract, pulling the sclera forward, causing the
suspensory ligaments to slacken. As a result, the lens becomes
rounder, refracting the light to a greater degree and allowing a
focused image to fall on the retina for viewing near objects.
Accommodation
 The refractive power of the
lens changes from low when
at rest, to high at maximum
accommodation.
When a person grows older the
lens loses some of its elasticity
and cannot alter its shape as
easily. There is a tendency for it
to remain in its elongated shape
(muscles at rest), resulting in
some people becoming longsighted as they get older.
healthy-ojas.com
Accommodation
It is very important for the visual signal to reach the photoreceptor
cells at the back of the eye in a clear and precise form. If this
doesn’t happen, the visual signal will not be interpreted accurately.
www.thedoctorwillseeyounow.com
Activity/Homework
-Students to complete DOT Point 3.5 Changes in the shape of
the eye’s lens when focusing on near and far objects