Let D1 be the diameter of the beam at the base of hemisphere
For objects at infinity Image will be formed at :
Applying the formula for the spherical curved surfaces:
n2/v−n1/u=(n2−n1)/R
(3/2)/V=(1/2)/R
v=3R
for similar triangles ABC and ADE
D1/2r=D/3R
D1=(2/3)D