Which of the following statements about the electromagnetic waves is correct?

Analyze the question: We need the one true statement about electromagnetic (EM) waves.

Concept: All EM waves (radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays) travel at the same speed in a vacuum, c = 3.0 × 10⁸ m/s. They differ in frequency and wavelength, linked by c = fλ.

Order of the spectrum, from lowest to highest frequency: Radio → Microwave → Infrared → Visible → Ultraviolet → X-rays → Gamma rays

Wavelength is the reverse: radio waves have the longest wavelength and gamma rays the shortest.

Check each option:

• A. Radio waves have longer frequencies than X-rays. False. Radio waves have the lowest frequencies, and X-rays have much higher frequencies. (The word "longer" is also wrongly used for frequency, since wavelength is what is "long".)
• B. X-rays have the longest wavelength. False. X-rays have very short wavelengths. Radio waves have the longest.
• C. Some of the waves travel at 3.0 × 10⁸ m/s. True. In fact, all EM waves travel at this speed in a vacuum, so "some of the waves" is certainly correct, even though it understates the fact.
• D. White light occupies a region beyond ultraviolet rays. False. Visible light lies below ultraviolet in frequency, not beyond it. Beyond ultraviolet come X-rays and gamma rays.

Answer: Option C.

Exam tip: Remember the spectrum order with "Rich Men In Very Ugly X-Garments" (Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma). Moving to the right, frequency and energy increase while wavelength decreases. Also remember that all EM waves share the same speed in a vacuum.