An object is placed at A (OA gt f). Here f is the focal length of the lens. The image is formed at B. A perpendicular is erected at 0 and c is chosen such that angle BCA=90^@. Let OA =a, OB=b and OC=c. Then the value of f is

An object is placed at A (OA gt f). Here f is the focal length of the

1.	An object is placed at A (OA gt f). Here f is the focal length of the lens. The image is formed at B. A perpendicular is erected at 0 and c is chosen such that angle BCA=90^@. Let OA =a, OB=b and OC=c. Then the value of f is
Answer» SOLUTION :From `1/v-1/u=1/F` we have `1/b+1/a=1/f or f=(AB)/(a+b)`…….(1) Further `AC^2+BC^2=AB^2 or (a^2+c^2)+(b^2+c^2)=(a+b)^2` or `a^2+b^2+2c^2=a^2+b^2+2ab, ab=c^2` Substituting this is Eq. (1) we get `f=c^2/(a+b)`