Lesson: Bending Light
Field Museum Extensions
a. Related Exhibitions.
1. Rocks and Minerals Exhibit: Muscovite Sample. Some minerals that you
see in this exhibit are transparent while others are opaque. Before the
technology to make glass cheaply became widespread, muscovite was
used for windows. Take a look at the muscovite sample in this exhibit and
identify at least two properties of this mineral that made it ideal for
windows. Knowing that rays of light travel in straight lines, can you
explain why light might pass through one mineral and not another? Look
for other minerals in this exhibit that are transparent. Find other miner als
in this display that show unique optical properties and then research how
light interacts with these minerals. Categorize your minerals as ones that
reflect, scatter, absorb, or refract the light rays that interact with them.
Use light ray diagrams to show how the light interacts with these
materials.
Muscovite has perfect cleavage and splits into thin sheets. Cleavage is a
property of a mineral breaking along a plane. Perfect cleavage is when the
break leaves no rough edges. As thin sheets, muscovite exhibits various
degrees of transparency. A window-sized crystal of muscovite would thus
be a reasonable substitute for a pane of glass.
The ability of a mineral to transmit light, rather than reflect it or absorb it,
depends on the spacing of the particles (atoms or ions) in the mineral and
on the arrangement of these particles. Minerals where the particles are, on
average, relatively widely spaced and also disordered tend to be
transparent.
For minerals with widely spaced particles, light is able to pass through
with little or no interaction. The light moves through these like raindrops
falling through a roof made of chicken wire—rarely meeting any
resistance. In most minerals, particles are closely spaced, and light moves
through these like raindrops falling through a fine mesh, onto a solid roof,
or into a sponge. In other words, the light is scattered, reflected, or
absorbed.
A mineral absorbs light when the light interacts with outer energy level
electrons. Visible light does not have the energy required to change the
behavior of electrons in minerals with disordered particles and so passes
through without being absorbed.
Even though light might pass through a mineral for these reasons, it still
can be affected by the electrical charge in the mineral, changing speed and
direction as it passes through. In this case, the path of the light will be
bent on its way through, a process known as refraction.
In some minerals, certain colors of visible light are absorbed when the
light is able to change the behavior of electrons in the mineral. The
energized atoms then release the energy as a combination of light forms
outside of the visible spectrum. The light that is not absorbed is reflected
and this is the color seen.
Some minerals absorb light in the ultraviolet range and emit a combination
of colors in the visible spectrum. This is called fluorescence.
There are many examples of minerals with interesting optical properties in
this exhibit. Some of these are the transparent minerals like quartz,
calcite, and muscovite. Others include colored transparent minerals like
amethyst and citrine. Still others include the fluorescent minerals such as
fluorite.
Student drawings of ray diagrams might appear as follows:
These show reflection, scattering, and refraction, from left to right.
2. Rocks and Minerals Exhibit: Double Refraction display. In this display
you will see the following information: “Crystalline minerals refract or
bend light as it enters them. Many crystalline minerals cause a beam of
light entering them to divide. The path of light in the stone becomes
double. This property is called ‘double refraction’.” Find the example of
double refraction in the display and describe what you see. Using a
drawing of the rays of light, can you explain what is happening to the light
as it passes through the mineral? How does this happen?
When light passes through this type of crystal, electric fields within the
crystal cause the light ray to break up into two. The part of the light beam
with its electric field pointing in a certain direction passes through the
crystal faster than the part of the light beam with an electric field in a
perpendicular direction. The difference in speeds causes a difference in
paths with one part of the beam refracting at a greater angle. Thus, when
the light passes through on the other side of the crystal, there are two
images. Students might draw a ray diagram to show double refraction as
follows:
b. Harris Educational Loan Center.
1. Mineral Match Experience Box. Look at seven common minerals and
identify as many properties as possible that would help to identify the
mineral. Is color a property that could be used to identify an unknown
mineral that you found on the ground? Research the reasons that some
minerals are colored and others are not. How does light interact with
minerals and how do you explain these interactions?
Some of the mineral samples in this kit are colored, translucent,
transparent, and/or opaque. These samples can be used to discuss how
light interacts with matter.
Translucent minerals that let scattered light pass through (like frosted glass
doors, these are not transparent) may contain contaminants, or
microcrystalline forms of the mineral, or a multitude of small fractures,
which would scatter the light beams enough to keep the mineral from
being transparent.
Color is not a good identifying characteristic, since so many different
minerals have the same color and since shades of color can change from
one specimen to another.
The interaction of light with minerals is explained in “a.1.” above.
2. Rocks and Minerals Experience Box. Look over the samples in this
experience box. What is the difference between a rock and a mineral?
Look at the mineral samples in this box. Do you see some ways in which
light is interacting with the mineral samples? Research the ways in which
light interacts with minerals and use your findings to explain what you
see.
This could be used in place of the Mineral Match Experience Box. This
set of minerals will also show light being absorbed, reflected, scattered,
and/or transmitted.
The difference between a mineral (a naturally occurring pure substance,
also termed an ionic compound, or salt) and a rock (a mixture of minerals)
can also be clarified using this Experience Box.
c. Field Museum Science/Website Resources.
1. Underground Adventure: Rock Crystal Cycle Webpage
(http://www.fieldmuseum.org/undergroundadventure/families/rock_crystal
.shtml) Create your own crystals with different optical properties! Make
rock candy crystals using table sugar and boiling water—leave out the
food coloring. Then try making large crystals of normal table salt. Do
you see any difference in the crystals?
The sucrose crystals should show transparency if made carefully, allowing
the crystals to form on a stick or a string as the water cools very slowly.
At the least, large sucrose crystals with some translucent character can be
created. The table salt crystals will be opaque.
2. Evolving Planet Website: http://www.fieldmuseum.org/evolvingplanet/
Consider how a complex organ such as the human eye might have
evolved. How did the first organisms that inhabited the earth see? The
trilobite eye has been a focus of study and an interesting example of a
prehistoric visual organ. Visit http://www.trilobites.info/eyes.htm to
supplement the museum information and displays in Evolving Planet.
What are the differences between the human eye and the trilobite eye?
How did light pass through the trilobite eye lens? Can you draw a light
ray diagram to explain?
The trilobite eye, unlike a human eye, is a hard and fixed lens, not unlike
the lenses of eyeglasses or a microscope. Light rays bend through the lens
and are focused on a point beyond. Many such “eyes” were a part of the
trilobite visual organ, with different eyes used to focus on objects at
different distances. Examples of the refraction of light beams through the
curved lens can be found on the trilobite.info site.