A Level Chemistry OCR Practice Exam 2025 - Free Chemistry Practice Questions and Study Guide

The greenhouse effect is best defined as the absorption and subsequent emission of infrared radiation by atmospheric gases, which leads to the warming of the lower atmosphere and the planet's surface. This process is critical in understanding how energy from the sun interacts with the Earth.

When sunlight reaches the Earth, some of that energy is absorbed by the surface, warming it. The Earth then emits this energy back towards space in the form of infrared radiation. Greenhouse gases, such as carbon dioxide, methane, and water vapor, absorb a portion of this infrared radiation and re-radiate it in all directions, including back towards the surface of the planet. This re-emission of energy effectively traps heat in the atmosphere, raising the temperature of the lower atmosphere and impacting climate systems.

This explanation highlights the fundamental role of greenhouse gases in regulating Earth's temperature, making option B the most accurate definition of the greenhouse effect. Other options either describe unrelated phenomena or mischaracterize aspects of atmospheric processes, emphasizing why this option stands out as the correct one.