Activity #6
To Absorb or to Scatter?

Grade Level: 5 - 8

Module: Visual Perception

Overview

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:

Materials

• milk
• India ink 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

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

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.

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:

• Light refracts (bends) through various surfaces and liquids.
• Absorption of light
• Transparency