Which of the following is not part of the electromagnetic spectrum?

• A. Visible light
• B. Ultraviolet
• C. Infrared
• D. Sound

Answer: (D)

The difference to focus on is between electromagnetic waves and mechanical waves. The electromagnetic spectrum comprises waves like visible light, ultraviolet, infrared, radio waves, microwaves, X-rays, and gamma rays—these are oscillations of electric and magnetic fields that can travel through empty space and move at the speed of light. Sound, in contrast, is a mechanical wave; it travels by vibrating particles in a medium such as air, water, or solids and cannot propagate in a vacuum. Because sound is not an electromagnetic wave, it is not part of the electromagnetic spectrum.