Final answer:
A black sweatshirt absorbs most of the light that strikes it, making it an effective absorber and radiator of heat, which is why it feels warmer in sunlight compared to lighter-colored clothing.
Step-by-step explanation:
When you are wearing a black sweatshirt, most of the light that strikes it is absorbed. Black materials are excellent at absorbing electromagnetic radiation, which includes visible light. This is why a black sweatshirt is warmer to wear in sunlight compared to a white one. A blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. Although a black sweatshirt is a good approximation of a blackbody, it is not perfect because it reflects some light, rather than absorbing all of it and re-emitting the energy at different frequencies as a perfect blackbody would.
In hot climates, wearing black clothing can be uncomfortably warm because it absorbs more solar radiation, while white clothing reflects most of the radiation, keeping you cooler. This transfer of heat by radiation is closely related to the color of an object, with black being the most effective absorber and radiator, and white being the least.