Question

A concave mirror has a focal length f=20 meter. An object is at a distance 100 meters. What is the image distance?

a) -16 b)16 c)20 d)infinity e)25

Answer 1

We will use the mirror formula

1/u + 1/v = 1/f

Where f = focal length = -20 m (Negative sign is used as the focus lies on the same side as the mirror)

u = object distance = -100 m (Negative sign is used as the object lies on the same side as the mirror)

v = image distance

Now, 1/v = -1/20 - (-1/100) m-1 = -1/25 m-1

Hence, v = -25 m  (Negative sign shows that the image lies on the same side as the mirror)

Removing the negative sign we see that the image is formed at a distance of 25 m (Option- E)