>[!abstract] >The visual line of sight (VLOS) and the radio line of sight (RLOS) do not behave the same for a couple of reasons: >- Light waves have a $10^5$ to $10^6$ times shorter wavelength than radio waves (~500 nm for visible light vs. 12.5 cm for a 2.4 GHz radio signal), so light waves travel strictly in straight lines whereas radio waves can (to some degree) bend around obstacles or follow terrain (or Earth's) curvature. > - The reason visible light does not bend while radio waves do has to do with how diffraction works. Diffraction is the phenomenon where waves bend around corners or spread out after passing through small openings. The amount of diffraction depends on the relationship between the wavelength of the wave and the size of the obstacle or opening it is encountering. > - When a wave hits an obstacle smaller than or similar in size to its wavelength, it diffracts significantly. > - When a wave hits an obstacle much larger than its wavelength, it casts a sharp shadow with minimal diffraction. >- Even if the pilot has unobstructed VLOS to the UAV, part of the [[Fresnel zone]] might still be partially blocked (e.g., by a tree), causing signal degradation or loss. So, a communication link might fail even in VLOS conditions — because radio waves need a wider clear space than just a narrow beam of vision. ## Related - [[Fresnel zone]]