Many people have seen a greenhouse for growing crops, and this is how it works. When sunlight hits the glass or transparent film of a greenhouse, most of the light will penetrate into the interior of the greenhouse. This light is then absorbed and re-radiated as infrared light with longer wavelengths. We can't see this infrared light, but we can feel it because it makes us feel warm. However, glass traps infrared, preventing the energy from scattering outward and thus warming the greenhouse.
Our Earth is also like a greenhouse. The atmosphere is like glass and is transparent to most of the light from the sun: some light is absorbed by the upper atmosphere, some is reflected, but most of the light can reach the Earth's surface. This causes the surface to begin to heat up, which in turn emits re-radiated infrared light. But infrared light doesn't travel easily through the atmosphere because some molecules in the air absorb it. These are called greenhouse gases, and carbon dioxide is one of them. More carbon dioxide causes more infrared radiation to be absorbed, warming the planet.
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In fact, the greenhouse effect is not that simple. This time we add some physical knowledge to make our understanding of the greenhouse effect more complete. Sunlight contains many different frequencies, and the frequency is directly proportional to the energy of the photon, so higher frequency means higher energy. Frequency is inversely proportional to the wavelength of light, with longer wavelengths meaning lower energy.
If you plot energy as a function of its frequency, you get what's called a spectrum, and the picture below is what's called the solar spectrum. But the shape of this spectrum is not specific to the Sun; the spectrum of any object at constant temperature will have a shape like this. The higher the temperature, the more the spectrum moves to higher frequencies, and the higher the energy emitted; the lower the temperature, the less energy emitted, this is Planck's law. If you know the temperature, then Planck's law tells you what frequency of light is emitted, and how much energy is emitted in total.
If sunlight hits the Earth's surface, it is absorbed and re-emitted. As we just said, the spectrum of radiation depends on temperature. But what is the temperature of the earth? As the Earth receives a certain amount of energy from the Sun, the Earth's temperature increases until the energy emitted is the same as the energy from the Sun. When the energy input is the same as the energy output, the system is in equilibrium and does not change further, meaning one can calculate the temperature of the planet's surface based on the amount of sunlight.
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If the Earth had no atmosphere, its surface temperature would be -18 degrees Celsius. Luckily, the Earth does have an atmosphere that keeps us warm, and here's how it works. When light of certain frequencies hits a molecule, the molecule resonates, which converts the light energy into motion of the molecule, which is heat energy. But most air molecules do not vibrate due to infrared rays. Nitrogen and oxygen make up 99% of the earth's atmosphere, but neither of them resonate due to infrared rays.
On the other hand, when light hits greenhouse gases, they vibrate, and these vibrating molecules then bump into other air molecules, which distributes energy throughout the air. These molecules in turn emit infrared light again, spreading it throughout the atmosphere. The most relevant greenhouse gases on Earth are water vapor, carbon dioxide and methane, which warm the planet.
The Earth's atmosphere contains greenhouse gases, so infrared radiation emitted by the surface is absorbed by them, which heats the air and emits some radiation again, so infrared radiation slowly passes from the surface through the atmosphere. As you go higher and higher, the air becomes thinner and thinner. Even if the concentration of greenhouse gases remains constant, the total amount of greenhouse gases per unit volume will still decrease. Eventually the greenhouse gases become so small that infrared radiation can escape into outer space. This means that most of the infrared radiation leaving our planet does not come from its surface, it comes from a few kilometers above the surface.
This is how the greenhouse effect really works. Greenhouse gases in the atmosphere prevent infrared radiation from the surface from reaching directly into space. In contrast, infrared radiation entering space comes from the upper atmosphere. Radiation from the upper atmosphere must balance the influx of energy from the sun. Because the temperature of the atmosphere decreases with altitude, the Earth's surface must be much warmer than it would be in the absence of greenhouse gases.
In addition, we need to realize that these greenhouse gases do not absorb infrared light equally at all wavelengths. They only absorb light in certain parts of the infrared spectrum. By studying the absorption spectra of these gases, scientists found that there are bands with weak infrared absorption in the wavelength range of 0.8 to 15 μm. These bands are called atmospheric windows.
The discovery and research of infrared atmospheric windows have a profound impact on infrared detection technology. In these bands, the atmosphere has better transmission characteristics of infrared radiation, allowing the infrared radiation of objects to be identified by detection equipment. This is crucial for fields such as weather forecasting, environmental monitoring, astronomical observation, and military reconnaissance. In addition, some scientists are studying some special materials that can radiate heat directly to outer space through the atmospheric window, thereby enhancing the cooling of the material.