Question 13 Marks
A point source emitting light uniformly in all directions is placed 60cm above a table-top. The illuminance at a point on the table-top, directly below the source, is 15 lux. Find the illuminance at a point on the table-top 80cm away from the first point.
Answer
given that $\text{E}_\text{a}=15\text{ lux}=\frac{\text{l}_0}{60^2}$
$\Rightarrow\text{l}_0=15\times(0.6)^2=5.4\text{ candela}$
So, $\text{E}_\text{B}=\frac{\text{l}_0\cos\theta}{(\text{OB})^2}=\frac{5.4\times\Big(\frac{3}{5}\Big)}{1^2}=3.24\text{ lux}$
View full question & answer→given that $\text{E}_\text{a}=15\text{ lux}=\frac{\text{l}_0}{60^2}$$\Rightarrow\text{l}_0=15\times(0.6)^2=5.4\text{ candela}$
So, $\text{E}_\text{B}=\frac{\text{l}_0\cos\theta}{(\text{OB})^2}=\frac{5.4\times\Big(\frac{3}{5}\Big)}{1^2}=3.24\text{ lux}$