Why can radio-frequency waves not be seen?

Radio-frequency waves cannot be seen because they fall below the sensitivity range of the human eye. The human visual system is designed to detect only a specific range of electromagnetic radiation, which we perceive as visible light, typically between wavelengths of about 400 to 700 nanometers. Radio-frequency waves, on the other hand, have much longer wavelengths, ranging from one millimeter to several kilometers, and these wavelengths are outside the visible spectrum. Therefore, while these waves can carry information and are integral to communication technologies, they do not stimulate the photoreceptors in our eyes, making them invisible to us.

In contrast, factors such as the speed of radio waves or their absorption by the atmosphere do not directly relate to visual perception. Radio waves travel at the speed of light, which is not a reason for invisibility. The absorption characteristics of the atmosphere can affect the propagation of radio waves, but this does not influence their visibility. The notion of radio waves being 'too weak' does not apply here either, as it is their wavelengths that govern their visibility, not their strength.