The Spectrum

If we go back to visible light, we see that different wavelengths (or frequencies or energies) are identified as colour of light, and we use the term spectrum to indicate the range of the different colours from red to violet.

The table below shows the visible spectrum with its various colours and values.

Colour
Wavelength	380–450 nm	450–495 nm	495–570 nm	570–590 nm	590–620 nm	620–750 nm
Frequency	789-668 THz	668-606 THz	606-526 THz	526-508 THz	508-484 THz	484-400 THz

The complete spectrum of EM-Radiation

Credit: Wikipedia

This visible spectrum is only a very small fraction of what we know as the entire EM-spectrum, from Radio waves with a wavelength of up to hundreds or thousands of kilometres, to Gamma waves with a wavelength of typically 0.01 nm or less.

All this radiation behaves fundamentally in the same way and travels through the vacuum of space at the constant speed of light.

The boundaries are not always sharply defined and are rounded off here.
Theoretically the spectrum extends until infinity on both sides.

Units used are:

• eV (Electron Volt) for Energy
• Hz (Herz) for Frequency
• m (metre) for Wavelength

For the notation of Powers of Ten and prefixes used, see the document Prefixes and Symbols in our Physics Study Notes

Only within the last hundred years astronomers can use this entire EM-spectrum of radiation that is received from celestial objects, because we now have the technology to detect and measure these different parts of the spectrum. Before that, only the visible part of the spectrum could be used.