Focal length of the lens remains same. Intensity of image formed by lens is proportional to area exposed to incident light from object. i.e., intensity ∝ area i.e.,
I2
I1
=
A2
A1
Initial area. A1=(
d
2
)2=
πd2
4
After blocking, exposed area, A2=
πd2
4
−
π(d/2)2
4
=
πd2
4
−
πd2
16
=
3πd2
16
∴
I2
I1
=
A2
A1
=
3πd2
16
πd2
16
=
3
4
or I2=
3
4
I1=
3
4
I(∵I2=1) Hence focal length of a lens =f1 intensity of the image =