No, visible light is not the only electromagnetic radiation emitted when hydrogen atoms are heated. When hydrogen atoms are heated or energized, they can emit electromagnetic radiation across a broad range of wavelengths, not just visible light. This phenomenon is known as atomic emission.
Hydrogen has a unique spectrum of emission lines known as the Balmer series, which corresponds to the transitions of electrons between different energy levels in the hydrogen atom. The Balmer series includes visible light wavelengths, such as the well-known Balmer-alpha line at 656.3 nm (red light), but it also includes other lines in the ultraviolet (UV) and infrared (IR) regions of the electromagnetic spectrum.
In addition to the Balmer series, there are other series of spectral lines associated with hydrogen, such as the Lyman series in the UV region and the Paschen series in the IR region. Each series corresponds to different electron transitions within the hydrogen atom and results in the emission of specific wavelengths of light.
Therefore, when hydrogen atoms are heated, they can emit electromagnetic radiation across a wide range of wavelengths, including visible light, ultraviolet, and infrared, depending on the specific transitions of electrons within the atom.