Which of the following statements is not true with respect to microwaves ?

Answer: The statement that electromagnetic waves have frequencies from 300 GHz to 400 THz is not true.

Why this is incorrect:

• Microwave frequency range is approximately 300 MHz (0.3 GHz) to 300 GHz, not 300 GHz to 400 THz.

• Frequencies from about 300 GHz up to 400 THz lie in the far-infrared through the visible-light region, which is outside the microwave band.

Brief notes on the other statements (explaining why they are essentially true):

• Microwave propagation is primarily line-of-sight because the short wavelengths limit diffraction around large obstacles; clear paths or repeaters are often required for reliable links.

• Very high-frequency microwaves have poor penetration through walls and are strongly attenuated; however, the degree of penetration depends on wall material and frequency, so 'cannot penetrate' is an overstatement.

• Use of parts of the microwave spectrum is regulated; some bands require licensing or permission while certain bands (e.g., ISM bands) permit unlicensed use.

Conclusion: The incorrect statement is the one that gives the frequency range 300 GHz to 400 THz; the correct microwave band is roughly 0.3 GHz to 300 GHz.