because
positive uses could be communication,
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Radio waves aren't harmful at its usual pattern of operation. They are harmful only when a strong signal is close to a person. It can cause cancer and other problems. Radio waves may also cure. All waves have different results.
Light waves do diffract, but the effect is very very small. In general diffraction effects are important when the object interacting with the wave has dimensions that are comparable to the wavelength of the wave. Light waves have much shorter wavelength compared to the radio waves. Hence the effect. (It should also be noted that radio waves penetrate buildings much better than light waves, but that leads to a more nuanced discussion of diffraction effects.)
no, mechanical waves are not radio waves
Radio waves ARE electromagnetic waves.
Radio waves are electromagnetic waves. The radio waves have the longest wavelengths in the electromagnetic spectrum. A radio wave has a much longer wavelength than does visible light. We use radio waves extensively for communications.