Why is the radio horizon for VHF and UHF signals more distant than the visual horizon?
Why The lower atmosphere's density, temperature and humidity decrease with height, so radio waves passing through it bend slightly downward, following the curve of the Earth a little way past the geometric line of sight. Engineers model this by pretending the Earth's radius is about 4/3 its actual value, which puts the VHF/UHF radio horizon roughly 15 percent farther than the visual horizon. Light is refracted too, but much less at these wavelengths, so radio reaches farther.
Watch out Nothing travels faster than light in vacuum, and radio in air is very slightly slower, not faster, so the speed explanation is out. Dust is irrelevant at VHF and UHF wavelengths, which are thousands of times larger than dust particles.
Radio bends around the bulge: assume a 4/3 Earth, about 15% beyond the visual horizon.