What electromagnetic waves have the longest wavelengths?

Hint: This question is based on the concept of wavelength. Wavelength of electromagnetic waves is defined as the distance between successive crests of a wave. These waves are generally sound waves or the electromagnetic waves.

Complete step by step answer:
An electromagnetic wave is a wave radiated by an accelerated or oscillatory charge in which a varying magnetic field is the source of electric field and varying electric field is the source of magnetic field. In simple words we can say that the electromagnetic waves are composed of oscillating magnetic fields and electric fields. The abbreviation for electromagnetic waves is EM waves.
Charged particles—such as electrons and protons—create electromagnetic fields when they move, and these fields transport the type of energy we call electromagnetic radiation, or light.
There are basically seven types of electromagnetic waves which are a part of the electromagnetic spectrum. These are radio waves, microwaves, infrared waves, X-rays, gamma rays, ultraviolet waves and visible rays. In all these seven electromagnetic waves, radio waves have the longest wavelength and the shortest frequency whereas gamma rays have the shortest wavelength and the shortest frequency.
So, the final answer is that the radio waves have the longest wavelength among all the electromagnetic waves.

Note: There is an inverse relation between wavelength and frequency. This means that the wave with the shortest wavelength has the longest frequency and the wave with the longest wavelength has the shortest frequency. As an example, radio-waves have the longest wavelength but the shortest frequency.