Example from Physics, 16.1 Reflection

Calculating Focal Length

A person standing 6.0 m from a convex security mirror forms a virtual image that appears to be 1.0 m behind the mirror. What is the focal length of the mirror?

Work it out on paper first. Then open the solution one step at a time, and stop as soon as you can finish on your own.

Strategy
The person is the object, so d_o = 6.0 m. We know that, for this situation, d_o is positive. The image is virtual, so the value for the image distance is negative, so d_i = –1.0 m.

Now, use the appropriate version of the lens/mirror equation to solve for focal length by substituting the known values.
Solution
$f = \frac{d_{i} d_{o}}{d_{o} + d_{i}} = \frac{(- 1.0) (6.0)}{6.0 + (- 1.0)} = \frac{- 6.0}{5.0} = - 1.2 m$

0 of 2 steps open

How did it go?

Strategy

Solution

Strategy

Solution