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what is called focal length and how can we find focal length?

 The distance from the surface of a lens or mirror to its focal point is called focal distance. The focal length of a lens determines how much magnification it provides. The focal length of an optical system is a measure of how strongly the system converges or diverges light.
A system with a shorter focal length has a greater optical power than one with a long focal length.
A practical method of finding focal length is explained below.

Set up the camera on the tripod about 4 to 5 meters from a brick wall. The vertical  focus the camera and stick a small piece of tape on the wall in the centre of the field of view and take a picture.
View the picture on  monitor and  see that the field of view will cover six courses of bricks. Measure the distance from the lens to the wall , this is the object distance. 

 Measure the size of object. this is the distance measured on the wall corresponding to the field of view in the image. 
First  calculate the magnification, m.

m = (size of image)÷(size of object) 

The “size of image” is the size of the sensor in the camera. You should be able to find this in your camera manual. 
Knowing the object distance, u,  we can calculate the focal length, f, from the expression

                         f = u÷(1+1/m)  
                                 OR
                          1/f = 1/u + 1/v

where,
 f is focal length,
 u  is object distance and
 v is image distance. 


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