Question

An object is placed at a distance of 10 cm from a concave mirror of focal length 20 cm.

1. Draw a ray diagram for the formation of image.
2. Calculate the image distance.
3. State two characteristics of the image formed.

Answer 1

a.

 

b. According to the cartesian sign convention:

Focal length (f) = −20 cm

object distance (u) = −10 cm

v = ?

By using the mirror formula:

`1/f = 1/v + 1/u`

⇒ `1/v = 1/f - 1/u`

⇒ `1/v = 1/-20 * 1/-10`

⇒ `1/v = -1/20 + 1/10`

⇒ `1/v = (-1 + 2)/20`

⇒ `1/v = 1/20`

⇒ v = ±20 cm

The positive sign indicates that the image is formed 20 cm behind the mirror.

c. The characteristics of the image formed are:

1. The image is virtual.
2. The image is erect.