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When we say that an object is "transparent" because we can see through it, we do not necessarily mean that all types of light can pass through it. Through a red crystal, for example, it can be seen, being, therefore, transparent. But instead, the blue light does not pass through it. Ordinary glass is transparent for all colors of light, but very little for ultraviolet and infrared radiation.
Think now of an open-air glass house in full sun. The visible light of the Sun passes through the glass and is absorbed by the objects that are inside the house. As a result, these objects are heated, just as those outside are heated, exposed to direct sunlight.
Objects heated by sunlight again give up that heat in the form of radiation. But since they are not at the temperature of the Sun, they do not emit visible light, but infrared radiation, which is much less energetic. After a while, they give up the same amount of energy in the form of infrared that they absorb in the form of sunlight, so their temperature remains constant (although, naturally, they are hotter than if they were not exposed to the direct action of the Sun).
Outdoor objects have no difficulty getting rid of infrared radiation, but the case is very different for objects located in the sun inside the glass house. Only a small part of the infrared radiation they emit manages to penetrate the crystal. The rest is reflected in the walls and accumulates inside.
The temperature of the interior objects rises much more than that of the exterior. And the temperature inside the house is rising, until the infrared radiation that filters through the glass is enough to establish the balance. It has become a greenhouse.
That is the reason why plants can be grown inside a greenhouse, although the outside temperature would be enough to freeze them. The additional heat that accumulates inside the greenhouse - thanks to the fact that the glass is quite transparent to visible light but very little to infrared - is what is called the "greenhouse effect".
The Earth's atmosphere consists almost entirely of oxygen, nitrogen and argon. These gases are quite transparent both for visible light and for the kind of infrared radiation emitted by the earth's surface when it is hot. But the atmosphere also contains 0.03 percent carbon dioxide, which is transparent to visible light, but somewhat more opaque to infrared rays. The carbon dioxide in the atmosphere retains heat and acts as the glass in the greenhouse.
Since the amount of carbon dioxide in our atmosphere is very small, the effect is relatively secondary. Even so, the Earth is a little hotter than in the absence of carbon dioxide. Moreover, if the carbon dioxide content of the atmosphere were double, the greenhouse effect, now greater, would heat the Earth a couple more degrees, enough to cause the gradual defrosting of the polar caps.
An example of a large-scale greenhouse effect is on Venus, whose dense atmosphere seems to consist almost all of it in carbon dioxide. Given their greater proximity to the Sun, astronomers expected Venus to be hotter than Earth. But, ignorant of the exact composition of their atmosphere, they had not counted on the additional warming of the greenhouse effect. Their surprise was great when they found that the surface temperature of Venus was well above the boiling point of the water, hundreds of degrees more than expected.
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