2. Introduction
From the time our ancestors first gazed at rainbows,
spectra have been a source of both wonder and new
understanding.
From gamma rays to radio waves, spectra have given us
new understanding, ranging from the structure of atoms
to the nature of distant quasars.
In this series of lessons you will learn:
1. About the electromagnetic spectrum,
2. About absorption and emission spectra,
3. How spectra depend upon energy jumps.
2
3. 3
The diagram shows the entire electromagnetic spectrum
All three scales are logarithmic – do not go up in equal
steps, but in equal ratios
Every number is ten times bigger than the adjacent smaller
one
The diagram shows the regions of the electromagnetic
spectrum described by wavelength, frequency and by
photon energy (in eV)
4. Regions of the EM spectrum
4
The diagram on slide 3 shows the usual way in which the
spectrum is divided – but there are no sudden
boundaries between the different regions
Gamma radiation and X-rays
Radioactive nuclei emit gamma rays – range in energy
from 104 eV to 5 x 106 eV
X-rays produced when high speed electrons decelerate
quickly.
High energy X-rays have shorter wavelengths than low
energy gamma rays.
Impossible to tell them apart by observation – given
different names only because of the way they are
produced.
5. Regions of the EM spectrum
5
10 keV (104 eV) is usually chosen as the boundary.
Chandra X-ray observatory satellite is sensitive to
photons up to 10 keV. The Compton Gamma Ray
Observatory satellite detects photons from 10 keV to
beyond 10 MeV
Calculate the wavelength of a high-energy ‘cosmic’ photon
with energy of 1 GeV (= 1 x 109 eV)
Where e = 1.6 x 10-19 C, speed of light , c = 3 x 108 ms-1
and Plank’s constant, h = 6.6 x 10-34 Js)
E = eV = hf = hc/λ hence λ = hc / eV
So λ = (6.6 x 10-34 Js x 3 x 108 ms-1) = 1.2 x 10-15 m
(1.6 x 10-19 C x 1 x 109 V)
6. Regions of the EM spectrum
6
Ultra-violet (UV) – this energetic, ionising radiation is
given off by electrical discharges (sparks and lightening)
Also given off by stars
The ozone layer in our atmosphere absorbs UV with a
wavelength of less than 300 nm (3 x 10-7 m), but its
recent thinning increases the risk of skin cancers.
Visible light – you can either see electromagnetic
radiation or not.
Human eyes are sensitive to the range between 400 nm
to 700 nm.
Other animals have eyes sensitive to different ranges.
Eg bees can see ultraviolet light.
7. Regions of the EM spectrum
7
Infra-red (IR) – produced by all hot bodies.
First invisible part of the EM to be discovered (William
Herschel in 1800)
Common application of IR, produced by LEDs, is found in
remote controls.
Radio wavelengths range from mm, as used in radar and in
microwave ovens, up to tens of kilometres used for
submarine communications.
Their photon energy is small.
8. Line spectra
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The spectrum is from a sodium
street lamp – it is a line spectrum.
It has a few wavelengths only.
There are about 90 different lines
in this spectrum, only the 7
brightest lines are shown.
Why does the light appear yellow when the spectral lines
are evenly spread across the visible range?
Because most of the lines are relatively faint, over 98%
of the energy is given out by 2 spectral lines of
wavelength 589.0 nm and 589.6 nm. (too close to
separate – appear as a slightly thicker yellow line)
9. What causes line spectra?
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Line spectra always observed when atoms have been
excited (heated or by electrical discharge)
Energy is given to electrons, which then is released as light
Line spectra are caused by changes in energy of the
electrons. Large complicated atoms like Neon give very
complex line spectra (many electrons).
Line spectrum of the simplest possible atom, hydrogen, was
investigated first (only one electron) – see below
10. Hydrogen spectrum
10
Observation of a hydrogen discharge tube through a
diffraction grating produces just 4 sharp lines.
4 wavelengths are:
656 nm, 486 nm, 434 nm and 410 nm (in the visible region)
More spectral lines were discovered in the invisible UV
and IR regions that are also similarly grouped.
Each is named after the physicist who investigated it,
Lyman series in the UV, Balmer series in the visible
region, Paschen and Brackett in the IR region.
11. Photon energies
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Need to look at photon energies instead of wavelengths
to see the cause.
Calculate the photon energy of the 656 nm line in the
hydrogen spectrum. Speed of light, c = 3 x 108 ms-1, and
the Plank constant h = 6.6 x 10-34 Js.
E = hc/λ = 3 x 108 ms-1 x 6.6 x 10-34 Js / 656 x 10-9 m
= 3.03 x 10-19 J
As 1.6 x 10-19 J = 1 eV then 3.03 x 10-19 J = 1.9 eV
Repeating this calculation for the other spectral lines,
gives following photon energies (in eV) for the first few
lines in Lyman and Balmer series.
12. Photon energies
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Lyman energies/eV 10.2 12.1 12.8
Balmer energies/eV 1.9 2.6
Can a pattern be spotted?
Each of the Balmer energies is the difference between
two of the Lyman energies.
This is true for all the Balmer lines, not just the two
given above.
The explanation of the spectrum of hydrogen is a story
of energy differences which will be discussed in the
next lesson.
