1. A concave mirror forms a real image at 25cm from he mirror surface along the principal axis. If the corresponding object is at a 10cm distance, what is the mirror's focal length?

Question

2. A woman looking in a makeup mirror sees her face at twice its actual size and right-side up. If she is 28cm from the mirror, what is its focal length?

Answer 1

1. Learn the formula
1/Do + 1/Di = 1/f
Then use it.

In this case Do = object distance = 10 cm
Di = image distance = 25 cm.
Solve for f

2. Do = 28
Di = -56
The magnification is 2 = |Di/Do|
Di is negative for an upright image.
Solve for f
Use the same equation as part 1 to solve for f.

Answer 2

1. 14.2