Question

focal length of concave mirror is​

Answer 1

The focal length of a concave mirror is negative and that of a convex mirror is positive. You can also prove the same by using the mirror formula (1/f = 1/v +1/u).

Let's see how,

Since we know object is always placed on the left side or direction opposite to the incidence ray of the mirror. Therefore object distance will always be negative.

u = -u

v = -v (Image distance is negative, since image formed by concave mirrors are generally on the left side or direction opposite to the incidence ray)

Using mirror formula,

1/f = 1/v +1/u

or, 1/f = (-u-v)/uv

or, f = -uv/(u+v)

As you can see, The value of f is negative here…

However v is not always negative in concave mirror, when object is placed between focus and pole, you will get a virtual image on the right side of the mirror, so in this case image distance will be positive. But even in this case focal length will be negative. How?

Just use the mirror formula again, and you will know how.

Answer 2

The focal length of a concave mirror is the distance between the pole and the focus of the mirror along the principal axis which is known as the focal length of a concave mirror.

• Concave and convex mirror both are kinds of spherical mirrors. If the internal side of the spherical mirror is reflecting it is known as a concave mirror. Similarly, the principal focus of a spherical mirror is the point existing on the principal axis of the mirror at which the light rays which occur parallel to the principal axis converge in actuality or appear to diverge after reflection.
• Proper care of the indication convention is to be accepted while substituting the values of f,u or v in the mirror equation. All the distances are calculated from the pole of the mirror. All the distances in the direction of the incident light are held as positive, while those contradicted by the direction of the incident light are held as negative.

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