Question

An object is 20 cm away from a concave mirror and it is within the focal length of the mirror. If the mirror is changed to a plane mirror, the image moves 15 cm closer to the mirror.

Focal length of the concave mirror is ______.

Options

• 36.6 cm

• 56.6 cm

• 66.6 cm

• 46.6 cm

Answer 1

Focal length of the concave mirror is 46.6 cm.

Explanation:

`1/"f" = 1/20 - 1/"y"`

`1/"y" = 1/20 - 1/"f"`

y = `(20"f")/("f"-20)`

y – 20 = 15 (given)

`(20  "f")/("f"-20)` = 35

f = `700/15` cm

= 46.6 cm