The Greenhouse Effect

Introduction

We hear that the Earth is warming because of the “greenhouse effect.” What is the greenhouse effect? How did it get this name? Demonstrate the principles of the greenhouse effect for your students.

Concepts

• Absorption

• Reflection
• Greenhouse effect

Background

In a greenhouse, visible light (medium wavelength) and ultraviolet light (short wavelength) pass through the glass while infrared radiation (long wavelengths) are absorbed or reflected. The visible light and UV light from the Sun that pass through the glass are absorbed by dark-colored surfaces, such as plants and soil, inside the greenhouse. These dark-colored objects absorb the light energy and heat up. These dark objects inside the greenhouse then re-radiate energy from their surfaces. The re-radiated energy, however, is infrared (long wavelength) radiation and not the shorter wavelengths like those that entered the greenhouse. The longer wavelength radiation is absorbed or reflected back into the greenhouse from the glass as it tries to pass back out through the glass. Thus, the original short wave light rays have been transformed and “trapped” inside the greenhouse. The greenhouse thus acts as a one-way valve for heat energy. The entire structure becomes a “heat trap.”

The plastic in the bottles in this demonstration has the same properties as glass. Thus, heat will build up inside the bottle similar to that in a greenhouse. The bottle, with one-half of its surface black, is very efficient at absorbing and converting the short wavelengths to long wavelengths and will very efficiently trap the energy inside the bottle and dramatically increase the internal temperature.

The “greenhouse gases” in the Earth’s atmosphere (CO₂, water vapor, ozone, methane, nitrous oxide and chlorofluorocarbons) act somewhat like the glass panels in a greenhouse. They allow visible and UV radiation from the Sun to pass through the troposphere. The Earth’s surface then absorbs much of this solar radiation and degrades it to longer wavelength energy (heat) which then rises back to the troposphere. Some of this heat does escape into space, but much of the energy is “trapped” by the greenhouse gases and is reflected back toward the Earth. The net result is the trapping of heat and a warming of the atmosphere. Because it is very similar in action to the glass in a greenhouse, it has become known as the “greenhouse effect.”

Materials

Safety Precautions

Be extremely careful when inserting thermometers through rubber stoppers. Be sure to use glycerin or other lubricant before attempting to insert the thermometers. The light bulb used in this demo will be unshielded and will get extremely hot. Follow all other laboratory safety rules.

Disposal

All the materials for this demonstration may be saved for future use.

Prelab Preparation

1. Select three identical thermometers that provide consistent temperature readings at the same location.
2. Carefully fold the aluminum pieces around the end of the thermometers to make flags as shown in Figure 1. Needle-nose pliers might be helpful in shaping the flags. Do not shape the flags with the thermometers in place! Attach the flags after they have been formed.
3. Cut black construction paper to fit over one-half of the outside surface of one of the bottles.

Procedure

1. Set up two bottles as shown in Figure 2.
2. Tape the black construction paper on the outside of one of the bottles.
3. Set up the demonstration as illustrated in Figure 3. Use a ring stand to position the third thermometer at approximately the same distance away from the lamp and the same height from the tabletop.
4. Have students record the temperature for each of the three thermometers before the light is turned on.
5. Turn on the light and record the temperature for the three thermometers every five minutes for 15–30 minutes. Stop when the “effect” is obvious.
6. Discuss the results and relate the results to the “greenhouse effect.”
7. All the materials for this demonstration may be saved for future use.

Teacher Tips

• The temperature in the bottles will vary rather quickly if the bulb is intense, unshielded, and the right distance from the bottles. Try your actual setup before doing the demonstration with students. A 200-watt bulb at 15 cm distance works well.

• The bottle with the black surface will get the warmest and will likely get to 60–70 °C. The clear bottle will likely be between 50–60 °C and the unbottled thermometer will likely stay at room temperature, between 25–30 °C.
• Extreme care must be taken when inserting thermometers through rubber stoppers. Do not force them! Use glycerin to coat the thermometers before inserting them. Use a cork borer to enlarge the holes if your thermometers do not fit in the stoppers provided.
• The aluminum flags provide a greater surface area for absorbing the radiation. The demonstration would work without the flags but not as quickly or as dramatically. What effect would making larger flags have?