Question

here is my problem:
a concave mirror forms a real image at 17.0 cm from the mirror surface along the principal axis. if the corresponding object is at a distance of 36.0 cm, what is the mirror's focal length?

here's my work: 1/f = 1/v + 1/u
where f=focal length=?
u= -36 cm (object distance)
v= -17 cm (image distance)
1/f = -1/17 - 1/36
f= -11/54 cm

is this the correct way to do this? is there any other way that is better? is the answer correct? thanks

Answer 1

Both u and v should be positive. The object is in front of the reflecting surface (so u is positive) and the image is on the same side (so v is positive also). The computed f will be positive, as it should be for a concave mirror.