Physics - Wave Optics Question with Solution | TestHub

For a point source, illuminance at a distance $r$ is given by Illuminance $I=\frac{\text{ luminous flux }\times \varphi }{4\pi r^2}$ $\begin{array}{rl}& \therefore \frac{I}{I^{\prime }}={\left(\frac{r^{\prime }}{r}\right)}^2\\ & \frac{I}{16I}=\frac{r^{\prime 2}}{(1000\mathrm{m}{)}^2}\\ & \text{ or }{r}^{\prime 2}=\frac{(1000\mathrm{m}{)}^2}{16}\text{ or }{r}^{\prime }=\frac{(1000\mathrm{m})}{4}\\ & r^{\prime }=250\mathrm{m}\\ & \end{array}$