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Lesson for 16-19
This topic may be either in the first or second year of advanced physics. In some specifications, it appears as an optional topic.
There is some algebra, so you may decide to quote some results, rather than giving full derivations.
Teaching Guidance for 16-19
- Level Advanced
There is little scope for practical work in this topic, so you should look out for paper-based activities, multimedia software etc.
Spectrometers are useful, since spectroscopy plays a central role in astronomy. Hand-held spectroscopes are useful, although you can get away with just looking through diffraction gratings. Check with colleagues from the chemistry department to see what they can make available to you. (Also, ask them what your students may have learned about atomic spectra in their chemistry studies.)
As this is a rapidly-changing area, it is worth making an effort to keep up-to-date by reading magazines such as New Scientist, Scientific American etc. Encourage your students to do the same.
A visit to an observatory can greatly enhance this topic.
Main aims of this topic
- Name different types of spectra
- Describe the information which can be deduced from spectra
- Understand how the Doppler effect can give rise to red and blue spectral shifts
Most students should have heard of the Big Bang and expanding Universe, whose present age ~ 14 billion years. This topic makes it all plausible. Some points which you will need to draw on:
- the wave speed formula v = f × λ
- diffraction by a grating (qualitative)
- the electromagnetic spectrum
Where this leads
This topic acts as an introduction to cosmology and ideas about the history of the Universe.
Lesson for 16-19
- Activity time 100 minutes
- Level Advanced
What we know about the universe comes from observations which rely on the radiation and (to a lesser degree) particles which reach us on Earth.
- Discussion: What is astronomy? (10 minutes)
- Student activity: Reviewing current knowledge (30 minutes)
- Discussion: How do we know? (10 minutes)
- Demonstration: A line spectrum (20 minutes)
- Demonstration: Further spectral types (20 minutes)
- Discussion: A stellar poem (10 minutes)
Discussion: What is astronomy?
Students will have heard of several different terms that you want to disentangle at the start:
- Astronomy is, broadly speaking, the observation of the motion and distribution of celestial objects
- Astrophysics is the application of physics to astronomy
- Cosmology is the study of the Universe as a whole, its origin, development and fate
- Space science is concerned with space exploration, including putting people in space
- Astrology is an attempt to predict the future based on the positions of the stars and planets. Scientists regard this as mumbo-jumbo, but you may have to be sensitive to students’ personal or religious views
Naked eye observation of the night sky can reveal up to 8000 objects, mostly stars, the Moon and various
nebulae. On closer inspection the nebulae are either diffuse dust clouds or galaxies that have a definite shape (spiral, elliptical etc). Repeated observation reveals the
wandering stars now known as the planets (5 are visible to the naked eye). If you are lucky you may observe the odd comet.
Because the speed of light is finite and we see all the light entering our eye at the same time, the further away its source, the father back in time it is being observed.
With the naked eye we can see out to about a distance of 2 × 1018 km corresponding to light that has been travelling for ~ 200,000 years (so the distance ~ 2 × 105 light years). In other words, we are seeing back in time for up to 200 000 years ago (NB even looking in a mirror you see yourself as you were a split second ago!).
Student activity: Reviewing current knowledge
Find out what your students already know (or think they know) by asking them to assess a number of statements about astronomy.
Discussion: How do we know?
All our information about astronomical objects in the Universe beyond our solar system is gleaned by analyzing the electromagnetic radiation they emit or absorb. Visible light is of course a small part of the total spectrum. A useful analogy is a fraction of an octave compared to the range of a concert grand piano.
Temperature, relative speed to Earth, rate of spin, orbital speed (and hence mass), and what they are made from can all be deduced by analysis of their electromagnetic radiation.
Knowing what stars
are, that those twinkling pin points of light have a structure, and that our Sun is made of the same stuff as down here on Earth had a similar intellectual impact as the change from the geocentric to the heliocentric model of the 17th century known Universe.
Some elements were first discovered
out there before being found
down here on Earth. For example, helium was discovered in 1895 in the spectrum of the Sun, and so was named after Greek word for the Sun (Helios).
Demonstration: A line spectrum
It’s worth repeating for emphasis that our knowledge about astronomical bodies is coded in the light that they emit. All we can measure is the colour (i.e. wavelength l) of the
lines in the spectra and the intensity at each wavelength.
There are three categories of spectra: continuous, band and line. Line spectra are subdivided into emission and absorption spectra.
Start with a demonstration of the sodium (Na) spectrum. View a sodium lamp using either an optical spectrometer or hand-held spectroscopes. Students should see a clear set of differently-coloured spectral lines, with (apparently) random spacing.
Now discuss: Why do we observe spectral
line is an image of the slit in the spectrometer (if you have an optical spectrometer indicate it to the students).
(To emphasise this, you could show that a circular laser beam and a grating gives a series of circular diffracted spots . A
laser line parallel to the grating slits gives a series of lines. (A laser line is also the commercial name for a useful and cheap DIY item used to give a line of light for setting tiles horizontal etc.)
Remember that a class 2 laser can be used with the simple warning
Do not stare down the beam. Other lasers require precautions.
Each atom has a unique line spectrum, a sort of
bar code that is used to identify each atomic constituent.
For each line the spectrometer determines the wavelength λ , hence c = f × λ , hence the energy difference between two energy levels in the atom from Δ E = h × f. (Refer to the relevant lessons, or flag forward to that section of your specification.)
A quite common misapprehension is that the line spectrum is the energy level diagram
turned on its side.
Because of the high stellar temperatures many atoms become ionized, (at the centre of a star atoms are totally ionized). It is common to see a spectrum due to partially ionized atoms rather than that of just a neutral atom. This is particularly true of sodium and calcium. Only in the coolest stars will the spectrum of only neutral atoms be present.
Demonstration: Further spectral types
Demonstrate an emission spectrum using a spectrometer. Simply look at a Bunsen flame into which you sprinkle salt.
Demonstrate an absorption spectrum using a spectrometer – position a white light source so that it shines through a cloud of sodium atoms – arrange the light beam above a Bunsen flame into which you sprinkle salt.
Now show the band spectrum from the hydrogen molecules in a hydrogen discharge tube (or similar). It’s also worth looking at the historically important Balmer series from the atomic hydrogen.
Finally, look at the continuous spectrum from a white light source.
Your students may wish to see examples of spectra used to classify star types. These are available on:
Discussion: A stellar poem
The poem by G M Minchin sums up nicely the story so far, and provides an opportunity to mention how some of the scientific assumptions have changed over the last hundred years or so.
It was written before the source of stellar energy (nuclear fusion) was known, so a star’s death was thought to be due to collision. Stars were just assumed to have planetary systems (extra-solar planets were not actually discovered until 1995), and light although known to travel at a finite speed was assumed to need an ether in which to propagate.
To finish this episode, discuss the sort of evidence that can be gleaned from spectra. It should be evident that we can deduce the chemical compositions of stars, as well as interstellar dust and gas. You can add that spectra can also tell us about the following:
- Relative quantities of different elements
- Movement (see episode 702: red shift)
Download this episode
Lesson for 16-19
- Activity time 90 minutes
- Level Advanced
Changes in wavelength of spectral lines allow us to determine the motion of astronomical objects relative to ourselves.
- Discussion: Red shift (10 minutes)
- Discussion: The Doppler effect (10 minutes)
- Demonstration: Doppler effect for sound (20 minutes)
- Demonstration: Doppler effect with microwaves (20 minutes)
- Discussion: Speed and frequency (10 minutes)
- Student questions: Binary stars (20 minutes)
Discussion: Red shift
The wavelengths of spectral lines emitted by atoms in an astronomical object are often increased compared to a similar source in the laboratory. We see the same pattern of lines (so we can recognise the elements from which they arise), but the whole pattern is shifted to longer wavelengths. The colour is not necessarily actually red, or even visible. Red shift simply means an increase or shift to a longer wavelength. (For visible light, this means towards the red end of the spectrum.)
There are two distinct explanations: the Doppler effect (due to relative motion of source and observer) and the Cosmological Red Shift (due to the expansion of space).
Both effects result in the same formula for calculating speed v of the object emitting the light.
Discussion: The Doppler effect
The Doppler effect is common to all types of wave motion. It is characteristic of a wave that:
- its frequency depends on its source
- its velocity depends on the medium through which it moves. (Its velocity is not affected by the motion of the source.)
- it is the wave’s wavelength that is affected by relative motion
Demonstration: Doppler effect for sound
Show the Doppler effect for sound using a whirling loudspeaker.
When approaching you (the detector), the crests bunch up as the source is catching up with the wave; l gets smaller, so f gets larger because fl is a constant c, i.e. pitch rises.
Travelling away from the detector, source gets away from the wave, crests stretched out and the pitch drops.
Demonstration: Doppler effect with microwaves
You can show the Doppler effect for microwaves reflected by a moving barrier.
Discussion: Speed and frequency
Derive the relationship Δ λ λ = Δ ff or Δ λ λ = vc
This says that the fractional change in wavelength or frequency is equal to the ratio of the speed of the source to the speed of light.
The consequence is that the frequencies of spectral lines change in proportion. NB many text books have diagrams that do not really show the increasing separation of spectral lines with wavelength, as though all the lines in a spectrum were shifted by an equal amount rather than by an equal fraction. The diagram above shows the correct version.
An interesting example: the Sun rotates, and its light is therefore Doppler shifted. The radiation from the side approaching the Earth is blue shifted; the radiation from the side moving away from the Earth is red shifted. The speed of rotation can be determined from these frequency shifts.
Student questions: Binary stars
When two stars orbit about one another, one may be moving towards us (blue shift), and the other away (red shift).