If distance increases from r to 3r, how does the intensity change?

If distance increases from r to 3r, how does the intensity change?

• A. I
• B. I/3
• C. I/9
• D. 9I

Answer: (C)

Intensities from a point source follow the inverse square law: intensity is proportional to 1 over the distance squared. If the distance grows from r to 3r, the distance multiplies by 3, so the intensity scales by 1/(3)^2 = 1/9. Therefore the new intensity is I/9. This happens because the emitted energy spreads over a spherical surface whose area grows as r^2, reducing the energy per unit area with distance. The other options don’t fit this 1/r^2 relationship.

Intensities from a point source follow the inverse square law: intensity is proportional to 1 over the distance squared. If the distance grows from r to 3r, the distance multiplies by 3, so the intensity scales by 1/(3)^2 = 1/9. Therefore the new intensity is I/9. This happens because the emitted energy spreads over a spherical surface whose area grows as r^2, reducing the energy per unit area with distance. The other options don’t fit this 1/r^2 relationship.