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To Absorb or to Scatter?
Grade Level: 5 - 8
Overview
An understanding of both the basic physics of light and the basic physiology
of light perception (vision) is necessary for understanding the more complicated
subject of visual perception. The unique environmental variables of the
space environment (microgravity, vacuum, radiation, etc.) can produce
adaptive responses that may alter both human and animal visual perception.
This is particularly the case when visual cues conflict with normal 1
G (Earth gravity) postural cues, that contributes to our notions of "up"
and "down."
This activity introduces students to the concept of absorption and scattering
of light in "transparent" materials. The lens of the healthy
eye is a transparent biological material that serves to focus the light
we perceive (see Activity 5). The colors of the rainbow demonstrate the
spectrum of visible light. However, light wavelengths both shorter (infrared)
and longer (ultraviolet) than the visible spectrum surround us and affect
us.
Lens cataracts, which are a clouding of the lens of the eye, can ultimately
result in blindness and require surgery for removal and replacement of
the damaged lens. There is evidence that long-term exposure to high levels
of ultraviolet (UV) light can lead to cataracts. On Earth, the ozone layer
in the stratosphere protects us from harmful UV rays from the Sun, but
in space, astronauts must be carefully shielded from UV and other harmful
radiation in order to prevent physiological damage that can lead to such
conditions as cataracts and even cancer. In addition to wave-form radiation
energy (visible, infrared, UV light, and X-rays), space explorers are
also exposed to high energy particles generated by solar flares. NASA's
life scientists are actively studying both wave-form and particulate radiation,
and their biomedical effects.
In this activity, students observe the change in intensity of light when
subjected to absorption.
Key Question
- How do different particles present in a liquid medium affect the transmission
of light through the medium?
Time Frame:
1 class period
Materials
For each pair of students:
- milk
- India ink
- 1 piece of white paper (3 cm x 3 cm)or newspaper with fine black print
- 2 eye droppers
- 1 toothpick
- 2 coasters
- 1 black marker or black ink pen
- 10 ml water
- 1 petri plate lid, glass or plastic
Getting Ready
Before beginning the activity, make sure that you have assembled all
of the necessary materials. This activity is designed to test two liquids
or to contrast the properties of two liquids. If milk is not available,
a starch solution can be substituted.
Classroom activity
1. Set the stage for this activity and motivate the class by explaining
some of the ideas mentioned in the Overview.
In eye physiology, a condition known as cataracts (a clouding of the lens
of the eye) can ultimately result in blindness and require surgery for
removal and replacement of the lens. There is evidence that long-term
exposure to high levels of ultraviolet (UV) light can lead to cataracts.
On Earth, the ozone layer in the stratosphere protects us from harmful
UV rays from the Sun, but in space, astronauts must be carefully shielded
from UV and other harmful radiation in order to prevent physiological
damage that can lead to such conditions as cataracts or skin cancer.
2. Tell the students that they are going to do a little exercise
that will help them understand how light is absorbed or scattered in a
clear or semi-clear substance. They will be adding drops of impurity (milk)
to a dish of water and finding out how many drops are required before
one can no longer see through the water. Challenge the class to guess
how many drops that will be.
3. Hand out materials to pairs of students. Have them draw a 1
cm diameter circle on the small square (3 cm x 3 cm) of white paper. Fill
in the circle with a black marker. Alternatively, students can use a small
piece of newspaper with black printing on it.
4. Have the students:
a. Place two coasters about 5 cm apart on the table.
b. Set one of the empty petri plate lids directly over the two
coasters, over the 5 cm gap.
c. Pour 10 ml of water into the petri plate lid.
d. Place the paper with the dot (facing up) directly under the
petri plate lid, in the small gap under the plate.
5. Explain that when viewed directly from above, the dot (or print)
should be visible through the water. One of the partners should play the
role of observer and position their face directly above the dish looking
straight down, about 30 cm from the dish.
6. The second partner will add one drop of milk at a time and stir
with the toothpick. This process should continue about every 15 seconds.
The students should count the number of drops of milk added. The count
will stop when the observer cannot distinguish the presence of the black
dot. Have the students design a data table for recording their data.
7. Have the class repeat steps 4 - 6 with fresh water. Allow the
original observer to resume the 30 cm position. This time add one drop
of India ink and stir after each addition. Keep track of the drops and
discontinue when the black dot cannot be seen. Record the data.
8. Have students prepare bar graphs of their results and compare
the data.
Wrap-Up Session
1. Allow students to share their observations and findings with
others in their class. You may wish to have students construct posters
or transparencies with their group data and to present their data to others.
2. Ask the class if their results agreed with any predictions made
before the lab?
3. Did the black color of the India ink and the black color of
the dot have any effect on the results? Have them explain their responses.
4. You may wish to have the class construct a table where each
group's data is recorded then expressed individually and collectively.
Allow the class to calculate the mean number of drops before transparency
was lost. To do this place a large data table on the chalk board and allow
students to enter their individual group data and then to obtain an average
(mean, median, and/or mode for the class).
5. Ask the students, "Was the light absorbed or scattered
as it passed through the water?" This can lead to a discussion of
the meaning of the terms absorb and scatter. In the process of scattering,
light is actually first absorbed by a molecule and then re-emitted in
a different direction. If there is absorption only, i.e. a photon is not
re-emitted, then the light energy is usually converted into thermal energy,
heating up the water.
6. Discuss this example of light absorption/scattering in the context
of other forms of radiation. X-rays can be used to produce an image that
would not otherwise be visible to the naked eye. Many students will be
familiar with X-rays of bones and teeth (try to get an X-ray image to
show the class). How does this type of radiation interact with living
matter? What are the benefits and risks of using radiation?
7. Finally, you can return to the original "motivator,"
by asking, "For astronauts who may have to spend long periods of
time in space suits, how might they be protected from UV radiation that
could cause cataracts?"
More Activity Ideas
1. Compare different brands of milk (regular, low-fat, condensed)
and the differences in their ability to affect the transmission of light
by a water medium. This activity could help introduce the topics of suspensions,
solutions, and colloids. Demonstrate the Tyndall effect-scattering of
light.
2. Explore the manner in which different colored filters affect
the transmission of white light through water; or repeat the activity
with different colors of food coloring.
3. Have students see how much reflection is a factor in seeing
through the water in the petri dishes. With clear water, have them try
looking at the dot (or newsprint) from increasing angles? Is there some
critical angle beyond which the dot (or print) can no longer be seen?
This is due to the phenomenon of reflection of light. Any smooth surface
can reflect light that strikes at a shallow enough angle. Everyday examples
of this include seeing bright reflections of streets and roads, smooth
table tops, and other smooth surfaces. It is also the effect that can
cause "mirages," the illusion of bodies of water in the desert,
when there is no water there.
4. Compare the ability of various soils to diminish the transmission
of light through water. Do a turbidity analysis (measure of sediment levels
in water) of water systems in the area with a spectrophotometer (instrument
to measure the intensity of different wavelengths of light). In images
of the Earth from high altitudes and from space, observations of sediments
and plant life in the waters of oceans and rivers can tell reasearchers
about the water quality and health risks associated with disease spreading.
Many of these images are produced using infrared photography, which indicates
heat by capturing light of the infrared wavelength.
Background for Teachers
It is the degeneration of lens cells or fibers that can lead to opacification
(cloudiness) of the eye lens. This can occur simply as a result of old
age. Also, people with metabolic diseases such as diabetes are at higher
risk of an accelerated process of cataract formation.
Here are some other things you should be aware of about light. When light
moves through a vacuum, it travels relatively undisturbed in its speed,
direction, and intensity. However, when light enters a transparent medium,
whether solid, liquid, or gas, it slows down slightly. If the light enters
at an angle into the new medium, it undergoes refraction (change of direction).
In addition to refraction, the light may be subjected to absorption or
reflection, or both. Absorbtion may be greater if foreign material is
present in the medium. Due to absorption and reflection, the transmitted
light is reduced in its intensity.
Two substances, milk and India ink, are tested in this activity to measure
their ability to block the transmission of light through water. This type
of turbidity testing is routinely done as a measure of water quality in
terrestrial environments.
When light enters a medium it may undergo reflection or scattering. This
is a phenomenon whereby light is redistributed in other directions from
the original path. On the other hand, light may undergo absorption, whereby
some of the light from a beam is irretrievably removed and transformed
into some other energy form. These two phenomena may occur independently,
but tend to occur together.
Vocabulary:
- Absorption - the process of light being "soaked up"
by a substance
- Transmission - the process of light passing through a substance
- Reflection - the process of light bouncing off of an object
- Medium - a substance
- Light beam - a ray of light
- Scattering - the process of light being absorbed and then radiated
from an atom
Skills:
- Data collection
- Data analysis
Concepts:
- Absorption of light
- Transparency
Keywords: Transmission, Reflection, Scattering, Absorption, Light,
Visual Perception
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