Sunsets and Light Scattering

Introduction

Who has not paused at twilight to gaze at a dazzling sunset with its brilliant oranges and reds? Use this demonstration to help explain the beautiful hues of a sunset.

Concepts

• Light transmission versus scattering
• Rayleigh scattering
• Tyndall effect

Experiment Overview

The purpose of this activity is to simulate the effect of light scattering as it travels through the Earth’s atmosphere. A light beam will be transmitted through water in an aquarium and then scattered as Pine-Sol^® is added to the water.

Materials

Safety Precautions

Pine-Sol^® is a mild eye irritant; wear chemical splash goggles when performing this demonstration. Clean up spills immediately. Follow all laboratory safety guidelines. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The Pine-Sol^® solution may be poured down the drain in accordance with Flinn Suggested Disposal Method #26b.

Prelab Preparation

The day before the demonstration, fill the two aquarium tanks with 980 mL of tap water, leaving room to add another 60 mL of liquid. Let sit overnight so the water is room temperature.

Set up the two aquarium tanks, one behind the other, with the longer side of each tank facing the students. The tank farther from the students will serve as a control. Position the flashlight at the short end of the front tank and the poster board at the opposite end at an optimal viewing angle for the students (see Figure 1). The poster board should be next to the aquarium tanks at a 45° angle, allowing the students to see the board while still providing a surface for the light to strike. The light should shine on the poster board as it is transmitted through either tank. Place the flashlight on a book or box if necessary so the beam will travel through the center of each tank as needed.

Procedure

1. Darken the classroom and turn on the flashlight, shining it onto the poster board through the water in the experimental tank. Instruct students to make observations of the light on the poster board and in the tank. The students will easily see the light beam projected onto the poster board, but should not easily see the light beam as it passes through the tank of water.
2. Measure and add 20 mL of Pine-Sol^® to the water in the experimental tank and stir the resulting solution. Allow students to record all observations. Note: The light beam traveling through the tank should become more visible.
3. Move the flashlight beam from the experimental tank to the control tank and back again to show the contrast of the transmitted light on the poster board.
4. Repeat steps 2 and 3 until the experimental tank contains 60 mL of Pine-Sol. Students should observe the path of light in the experimental tank becoming more and more visible, and showing a faint bluish-white color (similar to a very cloudy sky). Meanwhile, the transmitted light projected onto the poster board should change from white to yellow, then become more orange and eventually turn red.
5. Over time, as the dispersion of the Pine-Sol continues (approximately 4–6 minutes), the transmitted light on the poster board from the experimental aquarium will become deep red. Eventually the water will become cloudy enough to block the transmission of light and the red color disappears—the sun has set!

Student Worksheet PDF

12507_Student1.pdf

Teacher Tips

• This kit contains enough material to perform the demonstration at least ten times: 28 oz of Pine-Sol^®, two reusable plastic aquarium tanks, and one flashlight.
• Darkening the classroom will enhance the visibility and color of the transmitted light.
• If your water source (tap water) is hard, the light beam may be visible without the addition of Pine-Sol, and the solution may appear too opaque once the Pine-Sol is added. If this occurs, use distilled or deionized water for a portion or all of the water needed to fill the experimental tank.
• The rate of dissolution or dispersion of the Pine-Sol is temperature-dependent. Room temperature water works best for a gradual color change of the transmitted light. For a slower change, use less Pine-Sol rather than colder water. For a color change to red in about 10 minutes, use 50 mL instead of 60 mL of Pine-Sol. For a faster color change, use warm tap water, but do not exceed 35° C or the change will happen too fast for good observations.
• For further demonstration of the Tyndall effect (the scattering of light off of colloids), consider the Aloha Chemical Sunset Demonstration Kit, Flinn Catalog No. AP8988.

Sample Data

Observations
Draw and label the setup for this demonstration, including the control tank and the experimental tank after Pine-Sol^® was added. Describe the visibility and color or appearance of both the light beam and the transmitted light.

Answers to Questions

1. What effect did adding Pine-Sol^® to the water have on the beam of light in the tank? Explain why.
   

   The light beam was not visible before the Pine-Sol was added, but became more and more visible and eventually a pale bluish-white in color as more Pine-Sol was added. The particles of Pine-Sol scatter light, especially blue light, causing some of the light to be diverted from its path in all directions through the tank.

2. Describe how the transmitted light projected on the poster board changed as Pine-Sol was added to the water. Explain why.
   

   The light on the poster board appeared yellow-white before the Pine-Sol was added and slowly turned yellow, orange and eventually red. The violet and blue wavelengths of light were largely scattered, leaving mostly reds and oranges being transmitted to the poster board.

3. From this demonstration, what can you conclude about the source of colorful sunsets?
   

   During sunset, light travels a further distance through the atmosphere, resulting in more of the blues, violets and greens being scattered away, leaving mostly reds and oranges to be transmitted.

Discussion

The scattering of light is best understood as light deviating from its intended path due to some obstacle. A laser pointer, for example, has a beam that is typically invisible and only appears as a narrow dot on the surface onto which it is projected. To view the path of the laser beam, some particles (e.g., chalk dust or smoke) must be dispersed in the air in order to scatter the light, thus making it visible to the viewer.

The colors of the sky during the day and at sunrise or sunset are best explained by Rayleigh scattering. This occurs when light scatters off of particles, such as nitrogen and oxygen molecules in the atmosphere that are much smaller than the wavelength of the light. The intensity of the scattered light is proportional to the intensity of the incoming light, as seen in Equation 1.

where

I₀ is the intensity of incoming light
I is intensity of scattered light
λ⁴ is the wavelength of light raised to the 4th power

As shown by the Rayleigh scattering relationship in Equation 1, wavelength has a significant effect on the intensity of scattered light—it is an inverse fourth-power dependency. The shorter the wavelength of light, the higher the scattering intensity will be. Thus the shorter violet and blue wavelengths are scattered preferentially. Although the shorter-wavelength violet scatters more easily than blue light, the sky appears blue. This is true predominately because all wavelengths are scattered to some degree, bringing the average wavelength—what our eyes end up perceiving—to reside in the blue region. Other factors also play a role, such as our eyes being less sensitive to violet light and the sunlight being naturally depleted in shorter wavelengths as it travels through the atmosphere. At sunrise or sunset the light must travel a greater distance through the atmosphere to reach our eyes. This long-distance travel scatters a greater amount of the blue and violet light, resulting in the average intensity of transmitted wavelengths centering more on orange or red colors (see Figure 2).

Conversely, clouds, which are composed of larger particles such as water droplets, tend to scatter all wavelengths of visible light, as the particles are roughly comparable in size to the wavelength of light. This results in their white appearance, as white is the color of all wavelengths of visible light combined. As the clouds fill with more and more large water droplets, light is scattered and reflected off an increasing amount of surfaces, decreasing the light transmission and resulting in the gray appearance of rain clouds.

This demonstration mimics the effects of light scattering in the atmosphere. When the beam of light is first transmitted through the water, it is hardly visible. Not enough light is scattered during its short path to be detected. Adding Pine-Sol^® to the water produces the Tyndall effect. This is similar to Rayleigh scattering, but with a much more intense scattering of light off larger particles in solution. These particles scatter the light, resulting in a cloudy white beam with a very pale blue tint as the blue wavelengths are more easily scattered. As more Pine-Sol is added, more blue light is scattered, with the average transmitted wavelength becoming longer and longer, resulting in an orange and eventually red projection on the poster board.

References

Bilash, B. & Maiullo, D. A Demo a Day™—A Year of Physics Demonstrations; Flinn Scientific: Batavia, IL, 2009; pp 324–5.