Name one type of invisible radiation that is re-emitted by the Earth after absorbing sunlight.

• A. Longwave infrared radiation
• B. Visible light
• C. Ultraviolet radiation
• D. Microwave radiation

Answer: (A)

When you absorb sunlight, the Earth warms up and then cools by radiating energy back into space. The kind of radiation a body emits depends on its temperature, and the Earth’s average surface temperature is around 290 Kelvin. At that temperature, the thermal radiation is mainly in the infrared part of the spectrum, specifically the longwave infrared. That’s invisible to the naked eye, which is why it’s called invisible radiation. So longwave infrared radiation is the form the Earth re-emits after absorbing sunlight.

The other options don’t fit the usual re-emission from a body at Earth’s temperature: visible light is the part of the spectrum we can see and is largely reflected or absorbed rather than emitted as the primary re-radiation at Earth's temperature; ultraviolet radiation is higher-energy solar radiation mostly absorbed higher in the atmosphere; microwave radiation is not the typical peak of planetary thermal emission at around 290 K.