The absorption of different parts of the electromagnetic spectrum by Earth's atmosphere is primarily due to the specific properties of the molecules present in the atmosphere and their interactions with the incoming radiation.
In the case of the infrared (IR) spectrum, certain molecules in the Earth's atmosphere, such as water vapor (H₂O), carbon dioxide (CO₂), methane (CH₄), and ozone (O₃), have specific vibrational modes that correspond to the energy levels present in the IR range. When infrared radiation interacts with these molecules, it can be absorbed, causing the molecules to vibrate and gain energy.
The absorption of infrared radiation by these molecules is referred to as the greenhouse effect, as it plays a crucial role in regulating Earth's temperature. The absorbed infrared radiation is re-emitted in various directions, including back toward the Earth's surface, leading to warming.
On the other hand, the atmosphere has minimal absorption in the radio and visible parts of the electromagnetic spectrum. In the radio spectrum, the atmosphere is relatively transparent because the molecular properties of the atmospheric components do not strongly interact with radio waves. This transparency allows radio waves to propagate through the atmosphere with little attenuation.
Similarly, in the visible spectrum, the atmospheric constituents have minimal absorption because the energy levels associated with visible light do not match the specific vibrational or electronic transitions of the molecules in the atmosphere. Hence, visible light can freely pass through the atmosphere, enabling us to see objects on Earth's surface.
It's important to note that while the atmosphere allows most of the visible light to pass through, certain atmospheric phenomena like scattering (e.g., Rayleigh scattering) can cause the sky to appear blue during the daytime. However, this is not due to significant absorption of visible light by the atmosphere but rather the scattering of shorter wavelength light by gas molecules and particles in the atmosphere.
In summary, the selective absorption of different parts of the electromagnetic spectrum by Earth's atmosphere is a result of the specific molecular properties and energy levels associated with the atmospheric constituents, leading to absorption in the infrared range but minimal absorption in the radio and visible ranges.