Exemplar for internal assessment resource Physics 2.2A for Achievement Standard 91169
Exemplar for Internal Achievement Standard
91169 Level 2
This exemplar supports assessment against:
Achievement Standard 91169
Demonstrate understanding of physics relevant to a selected context
An annotated exemplar is an extract of student evidence, with a commentary, to explain
key aspects of the standard. It assists teachers to make assessment judgements at the
grade boundaries.
New Zealand Qualifications Authority
To support internal assessment
© NZQA 2014
Exemplar for internal assessment resource Physics 2.2A for Achievement Standard 91169
Grade Boundary: Low Excellence
1.
For Excellence, the student needs to demonstrate comprehensive understanding
of physics relevant to a selected context.
This involves linking ideas to integrate physics relevant to the selected context. It
may involve elaborating, justifying, relating, evaluating, comparing and contrasting,
or analysing the physics underpinning the context.
The student has demonstrated comprehensive understanding of how principles of
physics are relevant to the correction of impaired vision.
The student has linked the focal length of a convex lens to its shape (1), and has
justified the use of a concave lens to correct short sightedness (2).
The student had made an attempt to elaborate on the use of a concave lens to
correct short sightedness by attempting to link the focal length of the lens with the
severity of the deficiency (3).
The use of silicone instead of water has been justified in terms of the wear-ability
of the glasses, and the safety of the substance for the wearer (4).
For a more secure Excellence, the student could:
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© NZQA 2014
explain long sightedness and how it can be corrected
give a more comprehensive explanation for how a concave lens can
correct short sightedness.
Student 1: Low Excellence
When we look at something the light rays from the object first enter our cornea, which is the
transparent dome on the outside of the eye. The iris, the colour muscle of the eye which
controls the size of the pupil, controls the amount of light according to light conditions so it
controls the amount of light that enters the pupil after it travels through the cornea. After light
rays reach the pupil, they reach the lens whose shape is controlled by ciliary muscles and
the lens focuses the light rays on the retina. Once light rays reach the retina, the light
sensitive receptors; cones and rods, in the retina change the light rays into electrical
impulses which are then sent down through the optic nerve to the brain, which then tells us
what we are looking at. (Image of the eye given – not included here)
Since most of our vision is not 100% effective we don’t all have 20/20 vision. Different types
of lenses can be used to correct inaccuracy. There are two types of lens, concave and
convex. A concave lens diverges light rays to a virtual point and is fatter on the edges than it
is in the centre whereas convex lenses converge light rays to a point and are fatter in the
middle than they are at the edges. Lenses can either produce a real image or a virtual
image.
Having knowledge of both concave and convex lenses, helps us to solve eye problems such
as short and long sightedness. Short sightedness, is when people find it hard to see objects
far away because the light rays are focused in front of the retina. To make the rays focus
further back (on the retina) the focal length of the lens has to be made longer by making the
lens thinner but the ciliary muscles aren’t strong enough to do this. Short sighted people can
see things clearly close up because the light enters the eye at a slightly diverging angle
which means the rays have to be bent more to get them on to the retina. Fatter lenses have
a shorter focal length so can bend light more and so the ciliary muscles just relax till the lens
is fat enough. [1]. Short sightedness can be corrected by concave lenses which make the
light rays diverge before they enter the lens, just like they are when they are when they are
coming from close things, so the light rays can be bent enough to meet at the retina.
[2]
John Silver used this knowledge of how concave lenses are used to correct short
sightedness to create his fluid filled lenses. The silicone oil used in the flexible membrane
controls the shape of the lens. As fluid is pumped out, the centre of the lens membranes will
pinch in together to create a concave lens suitable for short sighted people. The more short
sighted a person is the more fluid they pump out the lens to create a thinner concave lens
which has a shorter focal length and so can bend the light more. [3]
In order to understand or justify the materials John Silver used to create his fluid filled lenses
it is essential to understand the terms refraction and refractive index. Refraction is when light
is bent when it travels through two mediums with different optical densities.
The index of refraction is the ratio of the speed light rays travel in a vacuum compared to the
speed they travel in the new substance. For example water has a refractive index of 1.33 so
light travels 1.33 times faster in a vacuum than it does in water. Therefore this justifies why
John Silver used silicone oil as a material for filling up the lenses because silicone oil has a
refractive index of 1.406 which is higher than the refractive index of water 1.33 so silicone oil
refracts light more than water, for the same amount of liquid. Therefore a thin silicone oil
lens, with a higher refractive index will have the same effect as a thicker water filled lens with
a lower refractive index. It is more desirable to use a thinner lens than a thicker lens because
it is less bulky and lighter therefore silicone oil is a better option than water. [4]
John Silver having known the basic principles of refraction, refractive index, concave lens,
convex lens, short sightedness, he created fluid filled lenses using the appropriate materials.
He was a very clever man and his invention could change the world for the better.
Exemplar for internal assessment resource Physics 2.2A for Achievement Standard 91169
Grade Boundary: High Merit
2.
For Merit, the student needs to demonstrate in-depth understanding of physics
relevant to a selected context.
This involves providing reasons as to how or why the physics applies to the
selected context.
This student has demonstrated in-depth understanding of how principles of
physics are relevant to the correction of impaired vision.
The student has used diagrams to show how the thickness of a lens affects the
amount of bending and has attempted to link this to the focal length of the lens (1).
The student has also used diagrams to show how long sightedness (2) and short
sightedness (3) can be corrected.
Reasons are given for why silicon oil is used in the adjustable glasses (4).
To reach Excellence, the student could demonstrate comprehensive
understanding by:
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© NZQA 2014
showing, on the diagram, how the focal length of each type of lens can be
determined, and then explain the relevance of focal length with respect to
the shape of the eyeball
explaining why the bending of the light by the corrective lens enables the
eye lens to focus on the retina, and why different degrees of sight defect
require lenses of different power
explaining the need for the adjustable lenses to be able to change the
shape of the lens (relating the explanation to differing degrees of sight),
and hence explaining the relevance of why a higher refractive index fluid is
desirable.
Student 2: High Merit
Lenses are transparent objects made normally from glass or plastic. As light travels through
the lens, it bends or refracts. Different mediums refract light differently depending on their
optical density. This is measured using a refractive index. The higher the RI the more the
light bends as it passes into the medium.
For lenses, another factor that changes how much light bends, is the focal length of the lens.
Focal length depends on how thick the lens is. For example the thicker a lens is, the more
the light will bend and so the shorter the focal length.
[1]
We use lenses all the time in everyday life, in microscopes, telescopes, cameras but most
importantly in glasses. Eye glasses are used to correct problems that we can get with our
eyes. Long sightedness is when the eye cannot focus on objects close up, so the person can
only see long distances. This can be caused by the eye ball being too small, so the image
that the eye sees does not fall upon the back of the eye (on the retina) but behind it.
Long sightedness can be corrected by a convex lens, as the convex lens causes the light
rays to converge further forward on the retina.
Normal sight
Long sight
Corrected sight
[2]
Short sightedness is when the eye cannot focus on objects far away so the person can only
see short distances. This can be caused by the front of the eye being more curved than
round or the eye ball being too long, so the image falls before the retina, rather than on it.
Short sightedness can be corrected by a concave lens, as the concave lens causes the light
rays to diverge further before the lens in the eye converges the light rays on the back of the
eye.
Normal sight
Short sight
Corrected sight
[3]
New fluid filled adjustable lenses are now available they are much like normal glasses only
the glass lens is replaced with flexible plastic lens with a compartment of silicon oil, and
there is an adjustable pump on the side of the glasses.
The pump can pump the silicon oil in and out of the compartment changing the shape of the
lens to suit each person’s individual eye needs. Silicon oil is used instead of water, as
silicone oil has a refractive index (light bending value) of 1.406. This is higher than the light
bending value of some other liquid, such as water (RI of 1.33) Therefore, less silicone oil
would be necessary to bend light compared to a larger amount of water. [4]
Exemplar for internal assessment resource Physics 2.2A for Achievement Standard 91169
Grade Boundary: Low Merit
3.
For Merit, the student needs to demonstrate in-depth understanding of physics
relevant to a selected context.
This involves providing reasons as to how or why the physics applies to the
selected context.
This student has demonstrated in-depth understanding of how principles of
physics are relevant to the correction of impaired vision.
The student has given reasons why a concave lens can correct short sightedness
(1).
The student has also attempted to give reasons why a convex lens can correct
long sightedness (2).
The student has attempted to give reasons why silicone oil is used in the fluid filled
adjustable glasses (3).
For a more secure Merit, the student could demonstrate further depth by:
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© NZQA 2014
showing understanding of the nature of long sightedness by showing the
incident rays diverging from a close object, rather than arriving parallel
from a far object
giving a more comprehensive reason why silicone oil is used in the lenses
of the adjustable glasses.
Student 3: Low Merit
There are two types of lenses convex and concave. Convex lenses are lenses that curve
outwards, they converge parallel light rays to a focal point. Concave lenses are thinner in the
middle and thicker at their edges. Concave lenses diverge parallel light rays away from each
other.
Lenses are able to correct short sightedness because short sightedness is when the light
that comes to the convex lens in your eyes converges (meets) at a focal point before your
retina. (You can only see an image when the light ray’s focal point falls on the retina).
So when you wear glasses they put a concave lens in them which diverges the light rays
before they meet the lens in your eye so that when they do converge after going through
your lens they converge to a point on your retina and the image is no longer blurry and you
can see. [1]
Long sightedness is when the lens converges the light rays but they don’t meet on your
retina. Instead, if they could. they would come to a point after the retina. And so they use a
convex lens to converge the light rays before they reach your eye so then when your eye’s
lens converges them again the focal point is reached on the retina.
[2]
It is necessary to pump fluid in and out of the lens to change their shape between concave
and convex to become the type of lens that the person needs to be able to see. So for
example if they were short sighted the liquid would be pumped out to create a concave lens
to help them see.
Silicone was used instead of other substances like water because it is more dense and so it
has a higher refractive index. Water has a refractive index of 1.33 whereas silicone is 1.406.
This means that the lens doesn’t have to be so thick to converge/diverge the light rays to the
right extent needed for the person to see [3]. If they used water to fill the lenses they would
become thicker and the glasses and flexible membrane would have to be bigger to cater for
the larger lens.
Exemplar for internal assessment resource Physics 2.2A for Achievement Standard 91169
Grade Boundary: High Achieved
4.
For Achieved, the student needs to demonstrate understanding of physics
relevant to a selected context.
This involves providing characteristics of, or an account of the physics related to
the selected context.
The student has demonstrated understanding of how principles of physics are
relevant to the correction of impaired vision.
The student has described how convex and concave lenses bend parallel light (2).
The student has described long and short-sightedness, and has attempted to give
reasons for why they happen (3) (4)
A description is provided for how both long and short sight can be corrected (5).
To reach Merit, the student could demonstrate in-depth understanding by:
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© NZQA 2014
giving clearer reasons for why refraction occurs
stating what is meant by the term ‘power’, and given reasons why it is
relevant in the situations of long-sightedness and short-sightedness
explaining correctly why the image is produced in front of the retina for
short-sightedness.
Student 4: High Achieved
Refraction is the bending of light, it occurs as the speed of light changes as it moves from
one medium to another. The denser a substance is, the more difficult it is for the light to
travel [1].
Lenses are transparent material; there are two types of lenses, concave and convex.
concave
convex
Refraction occurs when light travels through these lenses. In a concave lens the parallel light
rays are diverged and in a convex lens the light rays are converged [2].
If someone is long sighted it means that close objects are blurry or fuzzy and distant objects
are not blurred. This is because light rays don’t need as much power to focus, so they can
focus on your retina properly. If you are long sighted the light rays focus behind your retina,
this can be because your eyeball is too short [3].
If you are short sighted it means that you can see close objects well, but distant objects
appear to be blurry and fuzzy. As parallel light rays enter your eye, the lens causes them to
focus behind the retina, so the image is produced in front of the retina. This can be due to
your eyeball being too long from the front [4].
To correct short sight a concave lens is used. The concave lens will cause the parallel light
rays to diverge then converge, this causes the image to be produced on the retina, so a
clear image can be seen.
To correct long sight from the image being produced past the retina to on the retina, a
convex lens is used. A convex lens makes the light rays converge, then converge again to
form an image on the retina, after this the person can see a clear image, as it is formed on
the retina.
[5]
The fluid filled lens is necessary to have fluid pumped in and out of it as you have to make it
adjustable to your eye sight. Silicon oil has a refractive index of 1.406. Silicon oil is used
instead of water because water has a refractive index of 1.33. Silicon oil is more suitable to
use as it has a slight higher refractive index, so less oil has to be used, to correct the vision.
Exemplar for internal assessment resource Physics 2.2A for Achievement Standard 91169
Grade Boundary: Low Achieved
5.
For Achieved, the student needs to demonstrate understanding of physics
relevant to a selected context.
This involves providing characteristics of, or an account of the physics related to
the selected context.
This student has demonstrated some understanding of how principles of physics
are relevant to the correction of impaired vision.
The student has described the process of refraction (1), attempted to describe the
physics of how lenses correct eye defects (2), and attempted to account for the
choice of silicon oil for the fluid in the adjustable lenses (3).
For a more secure Achieved, the student could:
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© NZQA 2014
include appropriate ray diagrams to describe the physics of how lenses correct
eye defects
describe the significance of refractive index in the choice of adjustable lens
fluid.
Student 5: Low Achieved
A lens is a transparent material with a curved surface. Refraction is the bending of light as it
changes from one medium to another. [1]
There are two main types of lenses, converging lens (convex) or a diverging lens (concave).
A converging lens has an outward curving surface, with the lens being thicker at the centre
as opposed to the edges. A diverging lens has inward facing curving surfaces, with the lens
being thicker at the edges as opposed to the centre.
Short-sightedness is when a person has difficulty focusing on objects far away due to the
light rays being focused in front of the retina. They can only see close objects clearly. In
order to correct this vision, a concave lens would be used so that the light is bent outwards
before it enters the eye.
Long-sightedness is when a person can see things far away clearly, but their eye lens is too
weak to focus on close objects, this is because the light rays are focused behind the retina
(converge). This sight problem can be corrected by using convex lenses so that the light is
bent inwards before it enters the eye. [2]
It is necessary to be able to pump the fluid in and out of the adjustable lenses because it is
necessary to make sure the person’s vision is clear. This way you do not have to change the
physical shape of the lens as that is what the liquid does. It changes the glasses to create
either a convex or concave lens to aid the person depending on whether they are long or
short sighted.
The choice of silicone oil in this design is good as they used it because it has a higher
refractive index then water, but lower than glass and plastic. Because of this higher
refractive index it would not be as thick as putting in water. This makes the glasses more
suitable, not a chunky shape which looks funny on. It can refract more light for the same
amount of fluid with its refractive index of 1.406 as opposed to water’s 1.33. [3]
Exemplar for internal assessment resource Physics 2.2A for Achievement Standard 91169
Grade Boundary: High Not Achieved
6.
For Achieved, the student needs to demonstrate understanding of physics
relevant to a selected context.
This involves providing characteristics of, or an account of the physics related to
the selected context.
This student has discussed the relevance of principles of physics that are relevant
to the correction of impaired vision.
The student has attempted to describe the physics of how different shaped lenses
refract light to different degrees (1) (4).
The student has attempted to describe the physics of how lenses can correct sight
defects (2) (3).
To reach Achieved, the student could:
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© NZQA 2014
include appropriate diagrams to describe more clearly how lenses can be
used to correct eye defects
define relevant terms such as refractive power
give more detailed descriptions.
Student 6: High Not Achieved
Our eyes have our own flexible lens. This enables us to change our focus to near or far
objects. The lens is surrounded by a ring muscle called the ciliary muscle. This relaxes to
allow the lens to contract changing its refractive power. Light is refracted to focus as it
passes through the lens [1].
Short sightedness results in blurred distance vision. You will have difficulty looking at
distance objects but close objects won’t be a problem. People with short-sightedness can
see well because when they look into near objects the light gets bent slightly outward.
Concave prescription lens are used when you are short sighted because these are used to
bend light rays slightly outward. The light ray will be at a greater angle to bend back to focus
when it travels through the eye lens [2].
If you are long sighted you will have difficulty seeing objects that are close up. This happens
when the length from the front to the back of the eye is too short. Some people have this
because the cornea lens does not have enough curvature. Convex lenses are used to treat
long sightedness, because they bend light so the retina can focus [3], and these kinds of
lenses are thinner at the edge than at the centre.
Adjustable lenses can change the refractive power. The power of the lens is changed
whether you pump in or pump out fluid because it changes the curvature [4]. When fluid is
pumped out it reduces the power of the lens and this is suitable for short sightedness and
when fluid is pumped in, this increases the power of the lens and this is good for longsightedness.