The visible part of the EM spectrum
We described the different types of EM radiation which makes up the whole spectrum in 'Waves and the EM spectrum'. Visible light (the type of EM radiation which we detect or see with our eyes) is only a small part of this, and is shown in the diagram below.
As you can see from the spectrum above, red wavelengths of light fall between 620-750 x 10-9m in length (1x10-9m = 1nm or 1 nanometre) whereas bluer wavelengths of light have shorter wavelengths.
Now, imagine that a light bulb is travelling through space......this light bulb emits most strongly at wavelengths of around 550nm (in the green part of the visible spectrum). However, here on Earth, we measure its wavelength to be nearer 620nm - in this case it is clear that the light has shifted toward the red part of the visible spectrum - this is known as redshift.
This type of shifting of light also happens the other way - if the wavelength of light from the light bulb that we imagined above had shifted towards the blue part of the visible spectrum, we would call this blueshift.
So what causes light froim objects in space that we measure here on Earth, to shift towards either the red or blue part of the spectrum? The reason is simple - it depends on whether the object you are measuring the light from is moving away from you or towards you.
Remember, red light has a slightly longer wavelength than blue light.
If an object is moving away from us on Earth, its light gets stretched out, and as a consequence, the object's light, to us, appears redder.
If an object is moving towards us on Earth, the wavelength of the light it emits gets shorter, and so light from the object, to us, appears bluer.
Thinking of light bulbs travelling around space is useful to help us think about light waves in space, but it's not very practical - you don't often see a light bulb floating in the sky above you!
However, anything in space that emits light can be used to determine whether the object is moving away or towards you, from galaxies to stars....
Galaxies in space...
More on how the redshift of galaxies are measured by astronomers, coming soon..