m = magnification, u = object distance (m), v = image distance (m) Magnification is unchanged with mirror or lens type. QUESTIONS AND ANSWERS
5. An object is placed 3 m from a convex mirror of focal length 1.2 m. Find the image distance and the ratio of the height of the image to the height of the object.
Solution
Use the formula for a convex mirror: 1
__ u – 1
1
__ v = − 1
__ 3 − 1
__ f .
__ v = − 1
_____ 1.2 ⇒ 1
so v = 0.857 m m = v
__ u = 0.857
________ 3
You may now complete Exercise 13B (page 138). Uses of convex mirrors
The type of images produced from convex mirrors means they have many uses.
• Since all convex mirror virtual images are diminished and upright, the mirror can show a greater field of view. This makes them useful for wing mirrors in cars (Fig. 13.17). In practise, this means you may think objects are farther away because they look smaller.
• Again, because of their wide field of view, these mirrors are used at concealed junctions or corners to alert drivers or pedestrians to oncoming traffic.
• In supermarkets, convex mirrors at the end of aisles can alert staff to possible shoplifters.
FIG. 13.17 Wing mirrors use convex mirrors as they show a greater field of view
