3 Glass transmits the visible light radiated by the whitehot

3. Glass transmits the visible light radiated by the white-hot sun but it absorbs the infrared light radiated by objects near room temperature. This selective absorption and transmission gives rise to the “greenhouse effect.”

a. A small patch of garden is bathed in sunlight. The sun is transferring heat to this garden by radiation but the garden’s temperature remains essentially constant. Why doesn’t the garden get hotter and hotter?

b. If you cover the garden with a glass dome, what happens to the garden’s ability to get rid of heat via radiation?

c. How does the glass dome affect the garden’s temperature?

d. Like glass, molecules such as water vapor, carbon dioxide, and methane transmit visible light while absorbing some infrared light. How does the presence of these “greenhouse gases” in the earth’s atmosphere affect the earth’s ability to get rid of heat via radiation?

e. How do these greenhouse gases affect the earth’s temperature?

f. What is a cold frame and why do gardeners use them?  

Solution

(a) The garden transfers heat elsewhere exactly as fast as heat arrives because the garden warms up until its rate of heat loss is exactly equal to its rate of heat gain. With as much heat leaving as arriving, there is no further change in the thermal energy content of the garden and its temperature remains constant.

(b) The garden has more difficulty getting rid of heat beacuse some of the thermal radiation emitted by the garden is absorbed by the glass. The glass gets hotter and it also emits thermal radiation. Some of this thermal radiation is sent back toward the garden.

(c) The garden\'s temperature rises,since the garden is now receiving more thermal radiation than before, its temperature rises until it\'s able to eliminate heat as fast as that heat is arriving.

(d) The earth has more difficulty getting rid of heat beacuse some of the thermal radiation emitted by the earth is absorbed by the gases. The gases get hotter and they also emits thermal radiation. Some of this thermal radiation is sent back toward the earth.

(e) The earth\'s temperature rises,since the earth is now receiving more thermal radiation than before, its temperature rises until it\'s able to eliminate heat as fast as that heat is arriving.

(f) In agriculture and gardening, a cold frame is a transparent-roofed enclosure, built low to the ground, used to protect plants from adverse weather, primarily excessive cold or wet. The transparent top admits sunlight and prevents heat escape via convection that would otherwise occur, particularly at night. Essentially, a cold frame functions as a miniature greenhouse to extend the growing season