Final answer:
Internal radiation comes from natural radioisotopes in rocks and soil, cosmic rays from space, and radioactive atoms in the atmosphere.
Step-by-step explanation:
Internal radiation comes from various sources. One source is natural radioisotopes present in the Earth's crust, such as uranium and thorium, which can emit radiation. These isotopes can be found in rocks and soil, giving rise to internal radiation exposure. Another source is cosmic rays from space, which can also contribute to internal radiation exposure. Additionally, radioactive atoms like carbon-14, which are produced in the atmosphere by high-energy cosmic rays, and radioactive radon gas released from the Earth's surface can also lead to internal radiation exposure.