In ultraviolet radiation, the intensity of radiation varies inversely with the square of the distance from the source. Which law applies?

• A. Angulation of Rays law
• B. Inverse square law
• C. The law of Grotthus
• D. DuBois-Reymond's law

Answer: (B)

The main concept is that radiant intensity from a point source falls off with distance due to geometric spreading. Energy emitted by the source must pass through larger and larger spherical surfaces as you move away, and the area of a sphere grows with the square of the radius (area = 4πr^2). Since the total power is the same, the power per unit area, i.e., the intensity, decreases as 1/r^2. This gives the relation I(r) = P / (4πr^2). Ultraviolet radiation behaves this way in free space when the source is effectively point-like and there's little absorption or scattering. If the source isn’t point-like or the medium absorbs strongly, deviations can occur, but the inverse square law is the fundamental rule for how intensity changes with distance in this context. Other named laws describe unrelated phenomena (like conduction in electrolytes or neural activity) and don’t govern how light intensity varies with distance.