Question

What is the focal length of a makeup mirror that produces a magnification of 1.90 when a person’s face is 12.0 cm away? (Make sure your focal length is positive if the mirror is concave, and negative if the mirror is convex.)

Answer 1

1/f = 1/d_o + 1/d_i

m = - d_i /d_o

1.9 = - d_i /12

d_i = -22.8 cm

1/f = 1/12 - 1/22.8

f = (22.8*12)/10.8

f = 25.33cm