Transcript Document

The Refraction of Light
Light moves at different speeds through
different media. When it travels from one
medium into another, the change in speed
causes the ray to bend.
An analogy for refraction
As a marching
band moves from
an area where the
ground is solid to
one where it is
soft and muddy,
the direction of
motion changes.
Huygens’ Principle and the Law of
Refraction
Huygens’ principle can also explain the law of
refraction.
As the wavelets propagate from each point,
they propagate more slowly in the medium of
higher index of refraction.
This leads to a bend in the wavefront and
therefore in the ray.
Huygens’ Principle and the Law of
Refraction
The Refraction of Light
The angle of refraction is related to the different
speeds:
The speed of light in a medium is given by the
index of refraction of that medium:
The Refraction of Light
We can now write the angle of refraction in
terms of the index of refraction:
Index of Refraction
A beam of light in air enters
(a) water (n = 1.33) or (b) diamond (n = 2.42)
at an angle of 60.0° relative to the normal
The Refraction of Light
Basic properties of refraction:
The Refraction of Light
Here are some typical
indices of refraction:
The Refraction of Light
If light enters a medium of lower index of
refraction, it will be bent away from the normal.
If the angle of incidence is large enough, the
angle of refraction is 90°; at larger incident
angles the light will be totally reflected.
The Refraction of Light
Refraction can make objects immersed in water
appear broken, and can create mirages.
Huygens’ Principle and the Law of
Refraction
Highway mirages are due to a gradually
changing index of refraction in heated air.
The Visible Spectrum and Dispersion
The index of refraction of a material varies
somewhat with the wavelength of the light.
Huygens’ Principle and the Law of
Refraction
The frequency of the light does not change, but
the wavelength does as it travels into a new
medium.
The Visible Spectrum and Dispersion
This variation in refractive index is why a prism
will split visible light into a rainbow of colors.
The Visible Spectrum and Dispersion
Actual rainbows are created by dispersion in tiny
drops of water.
How rainbows are produced
As a single drop
of rainfalls toward
the ground, it
sends all the
colors of the
rainbow to an
observer. The top
of the rainbow is
red, and the
bottom is violet.
Light propagating through a glass slab
When a ray of light passes through a glass
slab, it first refracts toward the normal, then
away from the normal. The net result is that
the ray continues in its original direction but
is displaced sideways by a finite distance.
Disappearing Glass Rods
Disappearing Glass Rods
You can make glass objects disappear!
Glass objects are visible because they reflect some of the light
that shines on them and bend or refract the light that shines
through them. If you eliminate reflection from and refraction by a
glass object, you can make that object disappear.
Materials:
*Wesson oil. (Regular, not lite.)
*One or more Pyrex stirring rods or other small, clear glass
objects.
*A beaker.
Procedure:
Pour some Wesson™ oil into the beaker.
Immerse a glass object in the oil. Notice that the object becomes
more difficult to see. Only a ghostly image of the object remains. If
you do this as a demonstration, keep your audience at a distance
to make it harder for them to see the ghost object.
Experiment with a variety of glass objects, such as clear marbles,
lenses, and odd glassware. Some will disappear in the oil more
completely than others.
You can make an eyedropper vanish before your eyes by
immersing it and then sucking oil up into the dropper.
Immerse the magnifying lens in the oil. Notice that it does not
magnify images when it is submerged.
Explanation:
When light traveling through air encounters a glass surface at an
angle, some of the light reflects. The rest of the light keeps going,
but it bends or refracts as it moves from the air to the glass. You
see a glass object because it both reflects and refracts light.
When light passes from air into glass, it slows down. It's this
change in speed that causes the light to reflect and refract as it
moves from one clear material (air) to another (glass). Every
material has an index of refraction that is linked to the speed of
light in the material. The higher a material's index of refraction, the
slower light travels in that material.
The smaller the difference in speed between two clear materials,
the less reflection will occur at the boundary and the less
refraction will occur for the transmitted light. If a transparent object
is surrounded by another material that has the same index of
refraction, then the speed of light will not change as it enters the
Wesson oil has nearly the same index of refraction (n) as Pyrex
glass (n = 1.474). Different glasses have different indices of
refraction. In Wesson oil, Pyrex disappears, but other types of
glass &emdash; such as crown glass or flint glass &emdash;
remain visible. Fortunately for us, a great deal of laboratory
glassware and home kitchen glassware is made from Pyrex glass.
For most Pyrex glass, the index matching with Wesson oil is not
perfect. This is because the Pyrex glass has internal strains that
make its index of refraction vary at different places in the object.
Even if you can match the index of refraction for one part of a
Pyrex stirring rod, the match will not be perfect for other parts of
the rod. That's why a ghostly image of the rod remains even with
the best index matching.
The index of refraction of the oil (and of the glass, too) is a
function of temperature. This demonstration will work better on
some days than others.
Index of refraction is sometimes called optical density, but optical
density is not the same as mass density. Two materials can have
different mass densities even when they have the same index of
refraction.
Though Pyrex glass and Wesson oil have similar indices of
refraction, Pyrex sinks in Wesson oil because it has a higher mass
density than the oil. Wesson oil has a higher index of refraction
than water (n = 1.33), but it has a lower mass density and floats
on water. The index of refraction depends not only on density, but
also on the chemical composition of a material.
You can also make Pyrex glass disappear by immersing it in
mineral oil, which is available from pharmacies or chemical supply
houses. However, mineral oil comes in light, medium, and heavy
weights, and each variety has a different index of refraction. To
match the index of refraction of Pyrex glass, you'll need a mixture
of mineral oils of different weights. To create the proper mixture,
place a Pyrex glass object into a large glass beaker and pour in
enough heavy mineral oil to submerge it partially. Slowly add light
mineral oil and stir. Watch the glass object as you pour. Most
Pyrex glass will disappear when the mixture is about 2 parts
heavy mineral oil to 1 part light mineral oil. Notice the swirling
refraction patterns as you mix the two oils.
Karo syrup is another material that has an index of refraction
close to that of glass. Karo can be diluted with water to match
some varieties of glass.