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The Universe - Big Bang - Red-Shift - Line Spectra.
When we say
that light from other galaxies is red-shifted
we mean that light in line spectra appears at longer wavelengths.
We are
familiar with the idea that different
elements
show different colours in a flame test.
Sodium is yellow/orange,
potassium is lilac, lithium is
red.
The
particular colour which an
element shows depends
on the difference in
energy between its electron shells.
Light is emitted at a particular wavelength
(colour) when
electrons move
from a high energy (outer)
shell to a
lower
energy (inner) shell.
The colour that we see is a
combination
of a number of
different wavelengths
as electrons move between different shells
in
the same atom.
We can see these different wavelengths
as lines on a spectrum for a particular element.
The
spectrum below shows three important lines for sodium.
If the above sodium spectrum is
compared
with spectra for sodium from other
galaxies, you can
see that the positions of the
lines have changed.
The lines have moved
towards the red end of the
spectrum
changing colour as they go. This is called the red-shift.
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