Home > Biology > Options > Communication > Communication: 3. Refraction of light in the eye
9.5 Option – Communication: 3. Refraction of
light in the eye
Extract from Biology Stage 6 Syllabus (Amended October
2002). © Board of Studies, NSW.
Prior learning: Stage 4-5 Syllabus,
5.8.4(b) and 5.12 (c)
Background: Light travels in straight lines, but may be bent
when it passes from one substance to another. This bending or refraction can
be used to increase the clarity of the visual information sent to the brain.
conditions under which refraction of light occurs
- Light rays travel in straight lines. However, if a light
ray passes from one medium to another at an angle other than
90 º, the ray is bent. This bending, as light passes
from one medium to another, is called refraction.
- Refraction of light occurs when light passes from one
substance to another.
cornea, aqueous humor, lens and vitreous humor as refractive
- In the eye, refraction occurs when light passes from the
air to the cornea, from the cornea to the aqueous humor, from
the aqueous humor to the lens and from the lens to the
vitreous humor. Light spreading out from one point on an
object can therefore be focused on a particular point on the
plan, choose equipment or
resources and perform a first-hand
investigation to model the process of accommodation by passing
rays of light through convex lenses of different focal
A biconvex lens is shaped as shown in the diagram below.
Convex lenses magnify images by causing rays of light to
The lens in the human eye is a convex lens. When parallel
rays of light are passed through a convex lens, they will
converge at a point, known as the focal point.
The focal length of a lens is the distance of the focal
point from the centre of the lens.
- This activity requires you to select glass or perspex
lenses of different focal lengths and compare how they
refract light. One possible method is to use a ray box and
observe how the lenses refract light. Make sure that you
compare how lenses of different focal lengths can focus
objects at different distances away from the lens. As the
curvature of the lens increases the focal length deceases.
Remember that the same lens in the eye can vary its focal
length by accommodation.
accommodation as the focusing of objects at different
distances, describe its
achievement through the change in strength of the lens and
- Light rays reflected from an object six metres or more
away are almost parallel to each other. The lens refracts
these rays so that they fall on the fovea, the part of the
retina where vision is sharpest. If an object is closer than
six metres the light rays reflected from it will be diverging
rather than parallel. To bend these light rays so that they
fall on the fovea the lens must become more rounded.
- Accommodation is the ability of the lens to change shape
and focus light from objects at a range of distances. If the
lens becomes more rounded (greater curvature) it refracts
light to a greater extent and close objects can be focused.
If the lens becomes less rounded (less curvature) it refracts
light less and distant objects can be focused.
- The ciliary muscles are responsible for adjusting the
shape of the lens. When they relax, the lens is less rounded.
When they contract, the lens becomes more rounded.
- Accommodation is important because it allows the eye to
form focused images on the retina from objects at a range of
distances from the eye.
- As a person ages the lens gradually loses its elasticity
and can no longer accommodate for viewing close objects.
Corrective spectacles are required.
from secondary sources to describe changes
in the shape of the eye’s lens when focusing on near and
- Try to gather information from a range of resources,
including popular scientific journals, digital technologies and the Internet.
- Process the information by preparing a diagram to show accommodation by
the eye at a short distance and one at long distance. Write a brief description
of what occurs at each distance to accompany the diagram. A useful starting
point is the web site Accommodation
(HyperPhysics, Georgia State University). Then go to Refraction
(York University, Canada)
- Analyse by looking for the relationship
between the shape of the eye and the distance to the object
change in the refractive power of the lens from rest to
- When the eye is accommodated it focuses on close objects.
- In the eye at rest (unaccommodated) the lens is flattened
because it is subjected to tension by the suspensory
ligaments. The focal length of the lens is long and so distant objects are in focus.
- When accommodation occurs the ring of ciliary muscles
contract, the tension in the suspensory ligaments is reduced
and the lens bulges due to its natural elasticity. The refractive power of
the lens increases therefore shortening the focal length.
between myopia and hyperopia and outline how
technologies can be used to correct these
- Myopia is short-sightedness. A person with myopia sees
objects that are close clearly but objects in the distance
are out of focus. Rays from distant objects are focused in
front of the retina rather than on the retina. The usual
cause of myopia is that the eyeball is too long. Some forms
of myopia improve with age.
- Hyperopia is long-sightedness. A person with hyperopia
sees objects that are in the distance clearly but close
objects are out of focus. Rays from distant objects are
focused behind the retina rather than on the retina. The
usual cause of hyperopia is that the eyeball is too short or
that the lens gradually hardens with age, reducing its power
- Technologies used to correct these conditions include
eyeglasses or spectacles, contact lenses or surgery. Myopia
can be corrected with concave lenses worn for distance
viewing. These lenses cause parallel rays to diverge slightly
before they enter the eye so that the lens can focus them on
- Hyperopia can be corrected with convex lenses worn for
viewing close objects. These lenses cause parallel light rays
to converge slightly before entering the eye so that the lens
can then converge the rays to a point on the retina.
process and analyse information
from secondary sources to describe
cataracts and the technology that can be used to prevent
blindness from cataracts and use available
evidence to discuss the
implications of this technology for society
- Cataracts are a clouding of the lens. Focus on preparing
a brief description of what a cataract is and how it is
caused. Also describe examples of technologies used to treat
Cataract factsheet Vision Australia
- Use the discussion text type to highlight how this
technology impacts on society. A useful starting point is the
Allabout vision, USA.
- Use the available evidence to predict what the effect
will be on society in the light of the work of organisations
such as the Fred Hollows Foundation in restoring eyesight to
the 3.5 million people who have cataracts in Africa.
Hollows Foundation The Fred Hollows Foundation,
explain how the
production of two different images of a view can result in
- Some animals have forward facing eyes. This means that
there is considerable overlap between the views on the left
and the right. Because the two eyes are a few centimetres
apart, each eye sees a slightly different view of an
- The images formed by each eye are superimposed by the
brain, and because each view is slightly different, objects
appear to have depth as well as height and breadth, that is
we see in three dimensions. This is known as stereoscopic or
- This type of vision also makes it possible to judge
distances of near objects.
- Climbing animals such as monkeys and predators such as
cats have forward facing eyes, but grazing animals such as
horses have eyes on the side of the head so they have a wider
field of view.