Chapter 12.8, Problem 5E

To determine

To calculate: The what illumination is provided on the surface of a desk $2.5$ m directly below a lamp $125$ cd.

Answer to Problem 5E

The illumination is: $20$lx.

Explanation of Solution

Given information:

The what illumination is provided on the surface of a desk $2.5$ m directly below a lamp $125$ cd.

Formula used:

For find the illumination,

  $E=\frac{l}{r^2}$

Calculation:

Consider the illumination is provided on the surface of a desk $2.5$ m directly below a lamp $125$ cd.

Recall that for find the illumination,

  $E=\frac{l}{r^2}$

Here the values are:

  $r=2.5\text{,l=125}$

Therefore, the illumination given by,

  $\begin{array}{c}E=\frac{l}{r^2}\\ E=\frac{125}{{2.5}^2}\\ E=\frac{125}{6.25}\\ E=20\end{array}$

Thus, the illumination is: $20$lx.