As shown, a narrow beam of light is incident onto a semi-circular glass cylinder of radius R. Light can exit the cylinder when the beam is at the centre. When the beam is moved parallel to a maximum distance d from the central line, no light can exit the cylinder from its lower surface. Find the refractive index of the glass.
A. `(R)/(d)`
B. `(d)/(R)`
C. `(R)/(sqrt(R^(2)-d^(2)))`
D. `(sqrt(R^(2)-d^(2)))/(R)`
