Describe what the greenhouse effect is and how it is caused on a molecular level.

Sun emits a broad band of visible light spanning from red to blue colour. In addition to this, it emits UV and IR radiation. Infrared radiations have wavelength longer than IR, while UV ray have wavelength shorter than visible light. Each of these radiations are received on the earth's surface. On receiving radiation, part of it is absorbed by earth and remaining is reflected back. Generally, light of longer wavelength (IR) is reflected to larger extend as compared to shorter wavelength.  Due to this, air present in atmosphere gets heated up. It must be noted that,  molecule like CO2, which is present in air absorbs light from both direct and reflected  fraction of light. This phenomenon is referred as  green house effect. Molecule which absorb long wavelength light (IR) are present in atmosphere due to human activities. Hence, green house effect is often considered to arise because of human activities of altering the composition of air.