why radio horizon is greater then optical horizon with respect to ground wave?

Adam Momaya: Radio waves have longer wavelengths than optical waves of visible light. Radio waves diffract more, and thus have much more "fuzzy" shadows. I.e. the radio waves are better at bending in to their shadows than are optical waves.

Blythe Noreiga: Simply that because of the frequencies used (HF), the ionosphere "reflects" the radio waves back down to the surface of the earth, allowing radio range well beyond the horizon (and around the world, should the surface and ionosphere both reflect the signal multiple times).