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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.
identify the 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.
identify the cornea, aqueous humor, lens and vitreous humor as refractive media
- 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 retina.
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
lengths
Background
A biconvex lens is shaped as shown in the diagram below.
Convex lenses magnify images by causing rays of light to
converge.
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.
identify accommodation as the focusing of objects at different distances, describe its achievement through the change in strength of the lens and explain its importance
- 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.
analyse information from secondary sources to describe changes in the shape of the eye’s lens when focusing on near and far objects
- 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 being focussed.
compare the change in the refractive power of the lens from rest to maximum accommodation
- 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.
distinguish between myopia and hyperopia and outline how technologies can be used to correct these conditions
- 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 of accommodation.
- 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 the retina.
- 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.
- Refractive surgery may also be used to treat both myopia and hyperopia.
A thin flap of the cornea is cut and folded back. A laser is used to reshape
the cornea to a more suitable shape. The fold of skin is then folded back
into place.
A good website that explains refractive errors
Wikipedia.
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
cataracts.
Cataract factsheet Vision Australia
- Use the discussion text type to highlight how this
technology impacts on society. A useful starting point is the
web site
Cataracts
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.
Fred Hollows Foundation The Fred Hollows Foundation,
Australia.
explain how the production of two different images of a view can result in depth perception
- 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 object.
- 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 binocular vision.
- 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.
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