# Hands On Physics

## Haze Messing Around SCATTERING

In this activity, you simulate a hazy atmosphere with milky water. By shining light through the water and carefully observing what happens, you gain important insights about haze in air.

The "Milky Way": Scattering in Water.
In this activty, you investigate light scattering. You simulate a hazy atmosphere with milky water. By shining light through the water and carefully observing what happens, you gain important insights about haze in air. Milk cosists of tiny droplets of fat that are about the same size as haze particles, so putting milk into water is a good way to model a hazy sky. Start with clear water and then add drops of milk, a few at a time. Predict and then observe changes in the intensity and color of the light, as the amount of milk increases. Keep track of both the light that goes straight through and the light that gets scattered out the side.This is a model for the scattering caused by haze in air. Later on you will investigate whether milky water resembles hazy air.

#### Questions to think about:

• How does milky water influence the color and intensity of light passing through the water?
• How does milky water influence the color and intensity of light scattered by the milk particles?
• How does a clear tint influence the color and intensity of transmitted and scattered ligh

### Materials

You need a clear container for water, milk, water tints, a bright light, and some polarizers. The idea is to shine light through at least 20 cm of water onto a white surface. You will want to be able to add milk in equal amounts. An eye dropper to measure out the milk would be helpful.

The container can be a graduated cylinder, a glass milk bottle, an aquarium, or a clear tube sealed at one end with plastic. Whatever you use, it is important to be able to shine the light down through at least 20 cm of water, to look in the side to see the light beam, and to see the amount of light that gets through and falls on a white card.

Figure M1
Scattering in Water

The light source can be a flashlight, a reading lamp, or a slide projector with a mirror to direct the light down through the water.

Figure M2
Projector as Light Source

We need some way to color water. You can use food coloring, water colors used in painting, or colored chemicals like copper sulfate.

### Activities

These activities are designed to help you see how light is effected when it travels through clear liquid and and then when it travels through murky liquid. Observe the intensity and color of the light that goes straight through the liguid as well as the light that is scattered out to the side

#### Questions to think about:

• Which scatters light most, clear or murky liquid?
• Are all colors of light scattered equally?
• Is most light polarized?
• Where do you observe polarized light in nature?
• Does polarized light stay polarized when it strikes different surfaces?
• What do you think "partially polarized" means?
• Is the light that goes through milk become polarized?
• Is light scattered off milk polarized?
Start by shining light through clear water. Experiment with a clear water colored with a tint such as a food color. Now add a small amount of milk to clear water and observe what happens. Continue adding the same amounts and observing.

Investigate the polarization of scattered light. If you are not familiar with polarized light, start by exploring with the polarized filters. When you look at a source of light through a polarized filter, you see only light polarized in one direction. Look around you, through a polarized filter, and rotate the filter to check for polarization.

If you see no changes as you rotate the filter, then the light you are observing is randomly polarized. Look at the sky, at haze, at light reflected off water (glare). Make polarized light by placing a polarizer in front of a light source. Look at the polarized source through your filter as you rotate it. Look at various surfaces illuminated by polarized light. Finally, shine unpolarized and polarized light through the milky water.

### Reporting

Keep a journal as scientists do, even when messing around. Record all your questions and observations in a bound notebook. Don't erase, just cross out errors. Write enough so someone else could reproduce your experiments. Diagrams can save lots of words and help make your ideas clear. If you make measurements, record your data, with units. Tables are useful for repeated similar measurements. To make the logic of any calcuations clear, first record the formula you used, then show the numbers used to evaluate the formula, and finally your answer.

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