Here's why:

* Diffraction: Diffraction is the bending of waves around obstacles. The amount of diffraction depends on the wavelength of the wave and the size of the obstacle.

* Wavelength: Radio waves have very long wavelengths compared to other electromagnetic waves like visible light.

* Obstacle Size: Hills are large enough to be considered obstacles for radio waves.

How it works: Since radio waves have a long wavelength, they can bend around hills more easily than shorter wavelength waves. This is why you can still receive radio signals even if there is a hill between you and the transmitter.

Other Electromagnetic Waves:

* Visible light: Has a much shorter wavelength than radio waves. It doesn't diffract around hills noticeably.

* Microwaves: Have shorter wavelengths than radio waves, but still diffract somewhat around hills.

* X-rays and Gamma rays: Have extremely short wavelengths and do not diffract around hills.