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

The electromagnetic spectrum is a range of all types of electromagnetic radiation, which includes various forms of waves that travel at the speed of light. These waves are characterized by their wavelength and frequency and encompass a wide array of radiation, from radio waves to gamma rays. Infra-red radiation, X-rays, and gamma rays are all forms of electromagnetic radiation and thus are part of this spectrum.

Sound waves, on the other hand, are mechanical waves that require a medium (such as air, water, or solids) to travel through. They are not electromagnetic in nature, which means they do not belong to the electromagnetic spectrum. Instead, sound waves propagate via vibrations of particles in a medium, involving compressions and rarefactions rather than the oscillating electric and magnetic fields that characterize electromagnetic waves.

Understanding this distinction clarifies why sound waves are not included in the electromagnetic spectrum while other options listed are indeed forms of electromagnetic radiation.