Question

An object is 20 cm away from a concave mirror with focal length 15 cm.if object moves with a speed of 5 m/s along the axis ,then the speed of image will be

Answer 1

Answer:

Vi = 45m/s

Explanation:

Given object is at a distance (u) = -20cm

from a concave mirror of focal length f = -15cm.

➞ Using Mirror's Formula

1/f = 1/v + 1/u

⟹ - 1/15 = 1/v + (-1/20)

⟹ -1/15 = 1/v - 1/20

⟹ 1/v = -1/15 + 1/20

⟹ 1/v = 20 + (-15)/-15 × 20

⟹ 1/v = 5/-300

⟹ v = -300/5

⟹ v = -60cm

now consider object and image velocities be

Vi & Vo

Vo = 5m/s

We know that —

★ m = -v/u

⟹ m = -(-60)/20

⟹ m = 3

We have to find speed of image

•°• Vi = m²Vo

⟹ Vi = (3)²×5

⟹ Vi = 45 m/s