Which layer of the atmosphere acts as a shield against harmful ultraviolet radiation?

The stratosphere is the layer of the atmosphere that plays a crucial role in protecting life on Earth by absorbing harmful ultraviolet (UV) radiation from the sun. This layer contains the ozone layer, which is composed of ozone (O3) molecules. These ozone molecules are effective at absorbing UV radiation, preventing most of it from reaching the Earth's surface. This absorption is vital because excessive UV radiation can lead to harmful effects, such as skin cancer, cataracts, and other environmental impacts.

In contrast, the other layers do not have the same protective capability. The troposphere, being the lowest layer, is where most weather phenomena occur but does not have a significant concentration of ozone to protect against UV radiation. The mesosphere sits above the stratosphere and also lacks the ozone layer's protective quality. The thermosphere is farthest from the Earth's surface and contains very sparse gas molecules; it interacts with solar and cosmic radiation, but it does not provide meaningful UV protection for life at the surface.

Thus, the stratosphere’s composition and its role in hosting the ozone layer make it the primary shield against ultraviolet radiation, ensuring a safer habitat for all living organisms on Earth.