Refraction
This follows the pages about understanding
light and introducing a ray-diagram of image
formation within the eye.
What is Refraction and why study it
?
Refraction
is an important concept describing how electromagnetic
waves (including
visible light) change direction
when passing from one medium
to another. Examples of different media
include air, water,
glass and transparent plastics.
Biological examples include the transparent parts
of the eye, such
as the cornea,
aqueous
humour, lens
and vitreous
humour.
As mentioned
on the previous page:
Refraction is an important means
by which the eye controls
and re-directs in
such a way as to produce
the good-quality clear
images on the retina that
are essential for good
vision / eyesight,
the light it receives
from objects in the person's
field of view. |
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Summary from
previous page:
The previous
page stated that ray-diagrams can be used used to show how light
from an object outside the body passes into the eye
in such a way as to form an image at the back of the eye. The ray-diagram showed light leaving the object in all directions, so not all of the
light travelling (in straight lines)
away from the object reached the eye.
Considering a simple object defined
by just two (2) points, it is possible
to see an example of image formation
on the retina by tracing just four
(4) rays through the eye.
Rays representing
light travelling in straight lines
from the object to the eye change
direction on entering the eye
through the cornea. |
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More detailed consideration of the
optical path through the eye would
include further re-direction of light
passing through the lens of the eye.
In general, the changes of direction
of light passing through the eye to
produce an image on the retina are largely due to the effect of refraction. |
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The Principle of Refraction:
Some people remember how refraction works using this simple statement (which
is explained below):
Light bends towards the normal1 when moving into a denser2 medium. |
e.g. When travelling from air into water,
or from air into glass.
- The normal is a theoretical
line (i.e. it is not present in real objects
but is often drawn on diagrams) at right-angles
to the tangent to a surface at the position
at which a ray of light crosses that surface
between media. This means that the normal to a plane surface is at right-angles
to the surface - as shown opposite.
The normal is usually drawn as a dashed
line.
- Refraction is concerned with the refractive
index
(a form of "optical density")
of materials, rather than with actual density (density
= mass/volume).
However, use of the idea of density
may be helpful to convey and remember
the basic concept of refraction - and
loosely applies for the simple examples
of light passing from air into water,
or water into glass etc..
The opposite is also true, i.e. light
bends away from the normal when moving into a less dense medium.
e.g. When travelling from water into air,
or from glass into air.
Another way to remember this is to note
that:
- The angle of incidence is larger than the angle of refraction when
light travels into a denser medium.
and, conversely,
.
- The angle of incidence is less than the angle of refraction when
light travels into a less
dense medium.
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A more detailed explanation
including the significance of refractive
index is included below and is important
for numerical examples (calculations) and
when considering different types of glasses,
as necessary for opticians and optical physicists.
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The Law of Refraction:
The Law of Refraction
is also known as Snell's
Law, as Descartes' Law,
and (less usually) as The Snell–Descartes
Law.
Knowledge of the mathematics of the Law
of Refraction is not necessary
for an introductory-level understanding of how
the eye works but it is interesting and essential
for more advanced study. The Law
of Refraction is more useful than
the simple "Principle of Refraction"
(above) because the Law of Refaction is quantitative,
hence enables calculations to be made.
The change in direction of a wave
(e.g. green light) that is known as refraction is due to the change in speed at which that wave (e.g. green light) travels
through one medium compared with another
medium - or sometimes even through different
areas of the same medium such as
areas of different temperatues.
An equation is a simple way to summarise
the relationship between the angles (which, together, describe the change in direction of the wave) and the speeds at which the wave (e.g. green light) travels
through the different media.
In the following equation:
i
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-
represents the angle of incidence
meaasured in degrees (o),
as labelled in the diagram opposite,
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r
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- represents
the angle of refraction measured in
degrees (o), as labelled
in the diagram opposite, |
v1
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- represents
the speed of the wave (in the example
shown, a wave of green light) in Material
(1) which could be air in this case, |
v2
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- represents
the speed of the wave (in the example
shown, a wave of green light) in Material
(2) which could be glass in this case.
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There are also two other terms in the equation
form of the Law of Refraction.
They are n1 and n2,
which represent the refractive
indices of the two materials:
n1 represents the refractive
index of Material (1), and n2 represents the refractive
index of Material (2).
Strictly, one would
specify the refractive
index of a material for
a particular wavelength (or frequency, as the wavelength and frequency
of electromagnetic radiation are related).
However, the variation of the refractive
index of most common materials/substances
with wavelength (within the visible spectrum)
is sufficiently small that in non-specialist
situations a value of refractive index for
a single (specified) wavelength is usually
given and used for the visible range of
wavelengths generally. Exceptions to this
simplification apply in cases of advanced
high-specification optical design, and when
considering wavelengths from different parts
of the electromagnetic spectrum e.g. IR,
UV, radio waves, etc..
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Refractive Index
The refractive
index of a material
is a property of that material that is related to the extent
to which a specific type of electromagnetic
energy (e.g. a wavelength of light)
travels through that material more slowly than that
same type of electromagnetic energy
(e.g. that wavelength of light) would
travel through a vacuum. |
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There are equations defining the refractive
index of materials in terms of
other properties of the material
(including e.g. the phase velocity, relative
permittivity, and relative permeability
of the material) but those equations and their
explanations are beyond the knowledge necessary
to understand how the structures of the human
eye operate to form good quality inverted
images on the retina.
Lenses and Refraction
The above descriptions of refraction
indicate that there are important parameters
controlling how light is refracted
(i.e. re-directed) at surfaces between different
media. These parameters are:
- The refractive
indices of the materials on
either side of the surface (e.g. the refractive
indices of air and glass).
The mathematics in the equation form of the
Law of Refraction
means that, generally, the bigger the difference
in refractive index between the two materials,
the larger the change in direction (i.e. the
bigger the difference between the angle of incidence
and the angle of refraction).
and
- The angle
of incidence at the interface
between the two media.
The extent to which light bends (changes direction)
at a surface also depends on the
angle at which the light
reaches that surface (i.e. the angle
of incidence).
One way to control the direction of travel of
light using the effect of refraction
is by the use of lenses.
Choice of the material from
which the lens is made determines its refractive
index and
specification of the curvature
of the surfaces of the lens determines
the range of angles
of incidence of light arriving
at the lens from any particular point in space.
Lenses
are very important components in optical physics.
A basic understanding of lenses is also necessary
to explain how the human eye works.
The next page is about convex lenses
(because the lens
in the eye is convex).
Note: Understanding of parts of
the nervous system (such as the eye / visual system)
are required for many exams - e.g. GCSE Physics, GCSE
Biology, and AS and "A"-Level Biology and
Human Biology.
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