According to the inverse-square law, if you move a light source from distance r1 to r2 = 2 r1, how does the intensity change?

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Multiple Choice

According to the inverse-square law, if you move a light source from distance r1 to r2 = 2 r1, how does the intensity change?

Explanation:
The intensity from a point light source falls off with the square of the distance: I ∝ 1/r^2. So if you move from distance r1 to 2r1, the intensity scales by (r1/2r1)^2 = (1/2)^2 = 1/4. Intuitively, the same amount of light spreads over a spherical surface whose area goes up as r^2, and doubling the radius makes that area four times larger. Therefore, the light per unit area becomes one-quarter of what it was originally.

The intensity from a point light source falls off with the square of the distance: I ∝ 1/r^2. So if you move from distance r1 to 2r1, the intensity scales by (r1/2r1)^2 = (1/2)^2 = 1/4. Intuitively, the same amount of light spreads over a spherical surface whose area goes up as r^2, and doubling the radius makes that area four times larger. Therefore, the light per unit area becomes one-quarter of what it was originally.

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