Question

You place a light bulb 8 cm in front of a concave mirror. You then move a sheet of paper back and forth in front of the mirror. The image of the light bulb focuses on the paper when the paper is 12 cm in front of the mirror. What is the focal length of the mirror? Please show work too.

Answer 1

sorry - late reply...just stumbled across tis...hope u can still use it :)

By the mirror equation: 1/di + 1/do = 1/f

where di = distance to image = +12cm (+ for real image)

and do = distance to object = +8cm

Substitute and solve for f, the focal length

1/12 + 1/8 = 1/f

1/f = (8 + 12) / 12 * 8 = 20/96

so f = 96/20 = 4.8 cm