Video Transcript
What type of spectrum is shown
below? An emission spectrum or an
absorption spectrum.
The spectrum we are trying to
identify is this one here. The first thing we notice is that
this spectrum consists of discrete dark lines separated by regions that show a
continuous spectrum of light, much like a rainbow.
Now, we can recall that both
emission and absorption spectra result from electrons transitioning between energy
levels in an atom or molecule. Now, we can recall that the energy
levels in an atom or molecule are discretely spaced. So transitions between them only
occur with particular values of energy. Since each wavelength of light has
a unique energy, this explains why there are discrete lines in our spectrum. Each of these discrete lines
corresponds to one of the possible transitions.
Okay, so we understand where our
spectrum is coming from, but which of the spectra is it, emission or absorption? As its name suggests, an emission
spectrum is the light that is emitted by an atom or molecule as the electrons change
energy levels. Since each of these energy levels
corresponds to a discrete wavelength, in an emission spectrum, we will see the light
from the transitions. That is, we will see discrete
bright lines. These discrete bright lines will
also be on a dark background because the atom or molecule can only emit light at
these particular wavelengths, not at any wavelengths in between.
Now, our spectrum consists of
discrete dark lines on a bright background. So it is not an emission
spectrum. We could conclude at this point
that our spectrum is an absorption spectrum. But let’s recall the
characteristics of an absorption spectrum just to be sure. In an absorption spectrum, all
wavelengths of light are shined at our atoms or molecules. Because energy levels are discrete
and the differences between them are discrete, only those wavelengths of light that
correspond to those energy differences can be absorbed by electrons during the
transitions. This means that all of the light we
shine on our sample will pass through it except for those wavelengths of light that
correspond to the discrete transitions.
So we expect to see all of the
wavelengths of light, that is, a dark background, except for those particular
wavelengths that are filtered out by our sample, which will appear as discrete dark
lines. And again these lines appear dark
because these are the wavelengths that were absorbed by our sample and therefore did
not make it to our final detector. So, because our spectrum is
discrete dark lines on a bright background, we conclude that we’re looking at an
absorption spectrum.
It’s interesting to note that both
emission and absorption spectra have lines in exactly the same places. This is because the wavelengths of
light that an atom or molecule can absorb when electrons transition from a less
energetic to a more energetic state are exactly those wavelengths that the atom or
molecule can emit when the electron transitions from a more energetic state to a
less energetic state.
