Question

the general formula of you used to find focal length of concave mirror

Answer 1

Objective -

To determine the focal length of a concave mirror, by obtaining image of a distant object.

Theory

A concave mirror, like a plane mirror, obeys the laws of reflection of light.

Rays of light from object - The rays of light coming from a distant object e.g. sun or a distant building can be considered to be parallel to each other. When these parallel rays of light fall on a concave mirror along its axis, reflect and meet at a point in front of the mirror, which is called as Principal focus of the mirror.

A real, inverted and very small image size is formed at the focus of the mirror.

Focal Length - The distance between the pole P of the concave mirror and the focus F is the focal length of the concave mirror. Thus, the focal length of a concave mirror can be estimated by obtaining a 'Real image' of a distant object at its focus, as shown in the figure.

Answer 2

Let the focal length be f .

Let the distance of image be v .

Let the distance of object be u .

General formula :

1 / f = 1 / v - 1 / u

Note :

In concave lens :

f = negative .

v = negative .

u = negative .

Solved example :

f = 8

v = 24

u = ?

1 / f = 1 / v - 1 / u

= > 1 / ( - 8 ) = 1 / ( - 24 ) - 1 / ( -u )

= > - 1 / 8 + 1 / 24 = 1 / u

= >  1 / u = 1 / 24 - 1 / 8

= >  1 / u = ( 1 - 3 ) / 24

= > 1 / u = - 2  / 24

= > 1 / u = - 1 / 12

= > u = - 12

Hope it helps !

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