Title: The Greenhouse Effect
Author: Benjamin Masella and Ying Geng
Basic Idea: When the earth’s surface is warmed by the sun, some of this energy is reradiated back in the form of infrared light. Certain gases in the atmosphere absorb this light, keeping the energy from escaping into space.
Jargon: Infrared, greenhouse gas, absorption, blackbody radiation
Deeper Explanation:
All matter above absolute zero temperature will emit blackbody radiation. The spectrum of this radiation is a function of the temperature of the matter. The earth’s average temperature puts the peak of its blackbody radiation curve in the infrared. Certain gases in the atmosphere, such as CO2 and water vapor can absorb light at these wavelengths, and as a result, energy that would have escaped into space is trapped in the atmosphere and reradiated down to the earth. This has the effect of increasing the average temperature of the earth’s surface.
Importance in Science:
Nature: The greenhouse effect keeps more of the energy that the earth receives from the sun within the biosphere. Without this effect, the earth would be too cold to support life.
Extended Discussion:
Heat is energy. Unless it is at absolute zero, matter at any temperature has some amount of energy. Some of this energy can and will be released as electromagnetic radiation. The wavelength spectrum thus emitted is defined by Planck’s Law, which gives the emission of an ideal emitter/absorber (black body) as a function of temperature, adjusted for the emissivity of the real object. Molten steel, for example, appears to glow an orange red because, at roughly 1500 K, it’s radiation spectrum peaks at the red end of the visible spectrum. At lower temperatures, the peak of this radiation will be shifted to longer wavelengths. The average temperature of the earth’s surface is just over 300 K, which puts its blackbody peak in the infrared.
Thus, some of the solar energy that heats the earth’s surface is reradiated in the form of infrared light. If the earth had no atmosphere, this energy would escape into space. But, because the earth is surrounded by a layer of gas, some of this energy is absorbed by certain molecules. These molecules have just the right configuration so that infrared light causes them to vibrate, putting them into what is called an “excited state.” The molecule will then either dissipate its new energy by colliding with another gas molecule (causing a temperature increase) or it will reradiate the infrared light. Some molecules will radiate toward space, others will radiate toward the earth; but, overall, a large fraction of the energy that would have escaped into space is trapped on earth to warm the surface and continue the cycle.
Equipment:
Big stuff: An infrared camera (may need to be cooled by liquid nitrogen). Volunteers from The Institute of Optics could bring this equipment to class for a presentation.
Small around-the-house stuff: Ziploc bags, a balloon, hot water, cold water, clear glasses or cups or plates, a window
Demonstrations:
The infrared camera can be used to demonstrate how some materials interact differently with infrared light than they do with visible light. For example, a Ziploc bag or balloon will be mostly transparent in both the visible and IR regimes. However, a container of water will be transparent to visible light, but absorb infrared light, appearing black on the camera’s monitor. Similarly, a pane of glass will transmit visible light, but reflect infrared light. Thus, it is possible to see the reflection of a warm object in a window using the infrared camera.
These activities can be used to stimulate discussion about visible and invisible light, and also lead into environmental ideas such as the greenhouse effect.