A circular disc is placed in front of a narrow source. When the point of observation is 2 m from the disc, then it covers first HPZ. The intensity at this point is I. When the point of observation is 25 cm from the disc then intensity will be

$\left(\frac{R_9}{R_2}\right)^2 I$

$I=\frac{R_2^2}{4}$  given  $n_1 b_1=n_2 b_2 \Rightarrow 1 \times 200=n_2 \times 25$

∴ $n_2=8 HPZ$

∴ $I=\left(\frac{R_9}{2}\right)^2$

$=\left(\frac{R_9}{R_8} \times \frac{R_8}{R_7} \times \frac{R_7}{R_6} \times \frac{R_6}{R_5} \times \frac{R_5}{R_4} \times \frac{R_4}{R_3} \times \frac{R_3}{R_2} \times \frac{R_2}{R_2}\right)^2=\left(\frac{R_9}{R_2}\right)^2 I$