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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

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.

 Gamma-ray X-ray Ultra Violet Visible InfraRed Microwave Radio Energy (eV) 106-105 105-102 102-3 3.0-1.7 1.7-10-3 10-3-10-6 10-6-10-14 Frequency (Hz) 3x1020-3x1019 3x1019-3x1016 3X1016-8x1014 8x1014-4x1014 4x1014-3x1011 3x1011-3x108 3x108-1 Wavelength (m) 10-12-10-11 10-11-10-8 10-8-4x10-7 4x10-7-7x10-7 7x10-7-10-3 10-3-100 100-108

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.

 Often you will see that in Astronomy the unit for wavelength is the non-SI unit Ångström, named after the Swedish scientist Anders Jonas Ångström . Ten Ångström is one nm (nanometre) when we use the SI unit for length and the unit prefixes for powers of ten. Hence 1 Ångström = 10-10 metre. The Ångström is generally not pronounced properly because the Swedish Å is pronounced as “O” not as “A”.