Question

The refractive index of glass which is a symmetrical convergent lens if it's focal length s equal to radius of curvature of it's surface
A.infinity
B.0
C.3/2
D.1/2

Answer 1

correct Answer is C:  3/2

explanation:
Convex lens focal length=R
Symmetrical lens=Hence=R1=R2=R
By sigh conventions convex lens: R1=+R
R2= -R
Lens makers formula :
1/f=(n-1)[1/R1-1/R2]
1/R=(n-1)[1/R+1/R]
1/R=(n-1)[2/R]
1=2n-2
2n=3
n=3/2 =1.5 

Answer 2

hope you get it............