What formula is used to determine the intensity of radiation from a source?

The intensity of radiation from a source is determined by the Inverse Square Law, which states that the intensity of radiation or any point source of energy dissipates as the distance from the source increases. Specifically, the law indicates that the intensity is inversely proportional to the square of the distance from the source. This means that if you double the distance from the source, the intensity becomes one-fourth, and if you triple the distance, the intensity becomes one-ninth.

This principle is fundamental in understanding how radiation spreads in space, such as light from a bulb or radiation emitted from a radioactive source. It helps in calculating exposure levels at various distances and is critical in fields like radiology, radiation safety, and nuclear physics.

The other choices relate to different principles or theories. Fermat's Principle deals with the path that light takes as it travels between two points. Einstein's Theory refers broadly to the theory of relativity and does not specifically address intensity from a radiation source. Planck's Equation pertains to the quantization of energy, specifically dealing with the energy of photons, but not the intensity related to distance from the radiation source.