Electromagnetic Spectrum:

Don't be confused by the different names; Gamma rays, X rays, Microwaves, etc. All electromagnetic energy is basically the same. The only difference is wavelength: The distance between adjacent wave crests. Wavelengths decrease to the left and get longer to the right in the diagram above.

Short wavelengths = high frequency
Long wavelengths = low frequency

The Sun emits all wavelengths but not all are given off equally. 90% of the Sun's output is visible light! Our eyes have evolved to see only visible light because we evolved in a solar system whose star gives off mostly visible wavelengths.

Because of the Doppler Effect light from distant objects is shifted to the blue (short WL) end of the spectrum if the object is moving towards us. This is called 'blue shift'. Light from objects moving away from us is shifted towards the red (long WL) end of the spectrum. This is called 'red shift'. Since the light from most distant galaxies is red shifted we know they are moving away from us and that the universe is expanding.

Blue shift (wavelengths appear shorter) = moving closer.
Red shift (wavelengths appear longer) = moving away.

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