The radiation force experienced by body exposed to radiation of intensity $$I$$ , assuming surface of body to be perfectly absorbing is :

For a perfectly absorbing surface, the radiation pressure is  $$P=\frac{I}{c}$$

Since the radiation falls on the curved surface, its projected area in the direction perpendicular to the radiation is the triangular area shown in the figure.

$$A_{\text{proj}}=\frac{1}{2}(2R)(H)=RH$$

Therefore, the radiation force is  $$F=PA_{\text{proj}}$$

$$F=\frac{I}{c}(RH)$$

$${F=\frac{IRH}{c}}$$

Hence, the correct option is D.