Solution
= Solution
Set $E=0$ in part (a), substitute the two conserved quantities, and multiply the normalization by $f$:
$$
c^2k^2-\dot r^2-f\frac{h^2}{r^2}=0.
$$
Consequently the <null effective potential in Schwarzschild spacetime> is
$$
\boxed{\frac12\dot r^2+V_{\rm eff}(r)=\frac12c^2k^2,\qquad
V_{\rm eff}(r)=\frac{h^2}{2r^2}\left(1-\frac{2\mu}{r}\right).}
$$
Only radii where the total radial energy is at least this <effective potential> are accessible.