The term 'Radiation Source' can be summarized as the SUN, which is a three letter word. The SUN is the largest and most important source of energy for life on Earth, providing light and heat. It is a star at the center of our solar system, composed mostly of hydrogen and helium. The SUN produces energy through nuclear fusion, releasing radiation in the form of light and heat. This radiation is essential for life on Earth, and is the primary source of energy for plants, animals, and humans.