Question

The focus length of a plane mirror is?
(a) 0
(b) Infinite
(c) 25 cm
(d) -25 cm

Answer 1

Answer:

Its infinite..................

Answer 2

Answer:

f = ∞ ( Infinite )

Explanation:

We know that

$\frac{1}{f}\ =\frac{1}{v} +\frac{1}{u}$

Now ,

f = focal length

v = image distance

u = object distance

In a Plane mirror a object and image  at always equal now the object and plane image  is lies on the opposite  sides  of the mirror

so

Sign convention will be opposite

v = -u

$\frac{1}{f} =\frac{1}{v} +\frac{1}{-v} \\\frac{1}{f } = 0$

f = ∞ ( Infinite )

focus length of a plane mirror is f = ∞ ( Infinite )