A rod of length 10cm lies along the principal axis of a concave mirror of focal length 10cm in such a way that its end closer to the pole is 20cm away from the mirror. The length of the image is?

Rod AB of length 10 cm lies on the axis in front of a concave mirror such that end A lies at a distance of 20 cm from the mirror.
Using mirror formula : 1/v + 1/u = 1/f
Case A : u = -20 cm, f = -10 cm
∴ 1/vA + 1/-20 = 1/-10 ⇒ vA = -20 cm
Case B : u = -30 cm, f = -10 cm
∴ 1/vB + 1/-30 = 1/-10 ⇒ vB = -15 cm
Thus length of image L = 20 - 15 = 5 cm