Question

In a movie hall, the distance between the projector and the screen is increased by 1% illumination on the screen is

• A. Increased by 1%
• B. Decreased by 1%
• C. Increased by 2%
• D. Decreased by 2%

Answer 1

Answer: (D)

Decreased by 2%

Answer 2

$I = \frac{L}{r^{2}}$ $\Rightarrow \mspace{6mu}\mspace{6mu}\frac{dI}{I}\mspace{6mu} = - \frac{2dr}{r}$ ($\because$L = constant)

⇒ $\frac{dI}{I} \times 100\mspace{6mu} = - \frac{2 \times dr}{r} \times 100$ $= - 2 \times 1 = - 2\%$