Experimenting within the Third Age: ‘flipped lectures’

 

Four years BP (Before Pandemic) and soon enough after retirement that I could still remember the details of my life in salaried work, I wrote about some of the ‘experiments’ in education I was fortunate enough to be able to pursue (see here for example, final couple of paragraphs). I find that I am still succumbing to this weakness for trying new approaches in science communication. My intended audience is now the membership of the local branch of the University of theThird Age, U3A , rather than undergraduate students, but the passion to share my love of the life scientific remains the same. This post will offer, I hope, a preliminary reflection on a new take on an old theme. The rich vein of serendipity evident to me throughout will also emerge if I can string the right words together.

Cartoon by Jon Butterworth - used with permission.

Although my first 2020 ‘lockdown’ project was actually fairly conventional in many ways – a vaguely straightforward presentation of some basic Physics, albeit tied to objects one might very well find in the home – it did expand my experience of video creation/editing and the use of YouTube. I also learnt how to perch a small whiteboard on the lap and keep it level and in the frame without getting muscle pain and how simultaneously to control a handful of coloured marker pens as they attempted an escape. Such were the skills associated with 2020. A previous blog post provided an overview to the videos and formed, in essence, a Contents Page for the series (you will find it here). Curiously, given the crudity of the setup and my naïvety as a speaker-to-camera (I generally prefer to be behind the lens) this initial blog post has become the most viewed of them all. Having got to the end of my imagination, or perhaps simply my energy – it’s hard to say when in the midst of the stresses and strains of pandemic life – I turned to something more prosaic: making the few surviving recordings of my pre-retirement lectures available via YouTube (see here for details). The potential audience for this particular video series was never going to be large.

In early summer the call came for ideas and proposals for the approaching 2020/21 U3A Autumn & Spring Programme, none of which would take place face-to-face of course. Whilst everyone had ‘made do’, after a fashion, when the pandemic’s grip had first been felt and the second half of 2019/20’s programme had to be cancelled, actually starting a new year without even the remotest possibility of face-to-face meetings seemed worse somehow. My fellow science-based tutors and I ran monthly Q&A/Forum sessions via Zoom on a selection of topics, which went down very well. (I’m glad that I had suggested it earlier in the year, but the truth is that it was my friend and local U3A Science Coordinator, Alan Chadwick, who actually got it up and running successfully; I doubt I could have done so well.) However, this didn’t address the need/demand for the more focused sessions one would normally expect to lead. Some of these could be handled ‘live’ via Zoom, and several tutors took this route, but it didn’t suit the material I had or my presentation style. In particular, it would be difficult to include the demonstrations I try to weave into my sessions. So, without consciously realising it until well after I’d launched myself into my proposed ‘solution’, I turned to the use of an approach I’d tried during the final couple of years of my working life based on flipped lectures. A classic use of this would be to guide students into studying a topic in their own time – I had recordings of lectures I’d delivered in earlier years available for them, together with recommended reading etc. – and to follow this up with face-to-face sessions in which any issues arising from their study could be ironed out. In my ‘locked down’ variant I hoped to translate my pre-existing (face-to-face) sessions into videos that I could upload to YouTube for our U3A members to view as and when convenient. At some point thereafter we’d schedule a Zoom session so that everyone had the opportunity to engage in follow-up discussions and to pose whatever questions might have arisen in their minds.

As with all novel approaches, feedback and proper reflection are important when trying to assess whether the ‘experiment’ has been a success or needs a re-think. Questionnaires and their like do not appeal: too reminiscent of work, and a sure-fire way to dampen enthusiasm. A simpler route would be to assess the overall demand by looking at the numbers registering for the Zoom session and comparing them to those typically associated with a face-to-face session. Then, from the self-selected people who did participate, one might take a look at the questions posed – what folk actually took away as their appreciation of the videos’ content as distinct from my naïve intentions – and any unsolicited feedback. That brings me, at last, to the primary focus of this post.

Although there were decent access statistics for the blog post and evidence that the videos had been viewed, the numbers actually registering and turning up for the follow-on Zoom sessions were relatively small compared to analogous face-to-face sessions. In a ‘normal’ year one might see between 20 and 40 U3A members participating, but there were only a dozen or so at the Zoom sessions. About a quarter of those people sent subsequent feedback by email – all of which was positive I am glad to say. Likewise, the fact that several folk had taken the trouble to formulate and submit questions in advance and/or engaged in the resultant discussion might also be taken as positive. Having said that, a significant number of the questions posed were arguably at a tangent to the actual content of the associated videos; in practice, this doesn’t matter as the topics were fun to discuss: it’s all about science communication after all, and a little diversion can be instructive.

Taken together, it is reasonable to conclude that the format of ‘pre-recorded video plus Zoom follow-up’ was not popular amongst the membership as a whole. However, judging by the feedback received, the brave few who did embrace the experiment seem to have got something worthwhile from the experience.

On that basis, and bearing in mind the large investment of time required, would I seek to offer ‘flipped’ sessions again in the future? Frankly, given the pandemic-derived impetus for the experiment, I sincerely hope the question doesn’t arise! The truth of the matter is that I needed a project on this scale to help fill the year and would almost certainly have proceeded even if I’d known no-one would turn up; moreover, I enjoyed doing it. Thus, every one of the lovely U3A members who did engage with my experiment provided a distinct bonus: each and every one of them rendered my investment worthwhile.

There is a postscript: now that the major part of my pre-existing U3A material is available 24/7 on YouTube I can’t see myself ever presenting it ‘live’ hereafter. This might be considered a negative consequence were it not for the fact that my next U3A project is thereby called into being – to put together some brand new material from scratch; watch this space …

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I append below the feedback I’ve received and the questions submitted in advance by email (in italics) and the few notes I pulled together in case my brain ‘froze’ during the live Zoom sessions. There were plenty of follow-up/'live' questions of course, but I’m not sufficiently skilled at multi-tasking that I could jot all those down whilst also answering them, so only a few are listed below. I promised participants that I’d include all this material in the present post but, unless you particularly want to read it, do feel free to stop at this point.

Feedback

Thank you very much for this excellent course.

I wondered if we would be able to have access to these valuable lectures on the internet indefinitely please, or is there a limited timescale (lifespan?)? [There is no time limit currently envisaged or planned.]

Thank you so very much for fascinating videos and this morning’s session on radiation.

This went so much further than what we learnt as radiotherapy students with the much broader aspect of the subject so well presented. I am new to U3A and have found the course content really stimulating with the standard of lectures.

I will be looking up your other u tube videos.

Thank you for another really fascinating set of videos and the question and discussion session today. I missed out on science education when I was at school and have been trying to fill the gaps ever since.

Thanks for a fascinating Zoom.

Really enjoyed your presentation … I've always felt quite comfortable to have my understanding stretched. Your presentation continued the process especially explaining gravity as fundamental, pervasive, measurable but not yet fully explained. There were good further reading hints too ...

Thank you again for your excellent courses, they make a huge difference.

Your lectures open new doors of learning, & hopefully understanding, food for the soul & life enhancing.

I’m enjoying the videos – at least I appreciate now why you always talk about glasses plural rather than glass!

Thank you so much for this talk – fascinating... I hope I didn’t ask too many questions, but I was riveted.

I have just finished watching your last video on glass. Thank you for putting so much effort into the presentations. As a complete novice I found the material fascinating and although some of the chemistry was above my head, I now have an appreciation of what a versatile and valuable material glass is. It must have been difficult to retire from such interesting work when your passion for the subject is still there!

Questions posed

1) Radiation

What happens to nuclear waste, and will it be a problem long term, or will we have mastered it safely?

It’s a big question, and a serious one. One of the ways forward for the high level waste that needs long-term storage is to incorporate it into a glass. Glasses may be designed to be stable for thousands of years; if in turn these are enclosed in outer protective layers and then carefully stored deep underground away from water courses we are likely to be well protected. There are some exceptionally talented people involved in the research behind these vitrification processes (e.g. https://twitter.com/@clairecorkhill  Sheffield) See also ukinventory.nda.gov.uk Remember, there is always risk – it’s a matter of selecting the optimum way forward on the basis of all available reliable evidence.

Polonium 210 - one of my friends stayed at the London hotel where it was believed Litvinenko was poisoned. About a week later, he was contacted by police at his home in Naples, he was asked to submit to a medical, luckily all clear. Given polonium 210 is an alpha emitter, and apparently was transported in a flask, how could radiation leak out to contaminate hotel rooms, aeroplanes?

Po-210 appears to have been introduced via a cup of tea drunk by Alexander Litvinenko during a meeting with former associates. Po-210 was finally confirmed as he was nearing death precisely because it is an alpha emitter and therefore almost ‘invisible’ to a standard Geiger counter: it took the involvement of specialist scientists with more sophisticated equipment. Once they were involved the trail of contamination could be followed back in time. The two poisoners had contaminated themselves and shared that with items and people around them; obviously, the table setting in the place he’d drunk the tea was also contaminated – especially the dregs in the tea cup. Po-210 is only dangerous as a source of radiation once inside the body, where the alpha radiation kills living cells as the metal is carried around the body (including to the bladder – causing more contamination).

If you keep a glass vase coloured with uranium salts long enough (700 million years!) will it lose some of the green colour.

On the face of it the answer is “yes”, but bear in mind the fact that the decay chain for U-238 takes us, via intermediaries, to U-235 – an isotope which is of course chemically identical to its parent isotope. All radioactive isotopes of U have an associated decay chain but U-238 forms 99.3% of the element, which is why I focused on that in the slides. Thus, given that the half-life of U-238 is about the age of the Earth, the colour will fade – but maybe not at a rate one would notice ;-)

How is a GM tube made sensitive to alpha radiation?

The key is to ensure that the end window is thin and made from something of low density (i.e. relatively few atoms to get in the way). Whilst gamma rays and the higher energy beta particles will have no problems entering the gas within the thin-walled tube, alpha particles will. End windows of mica or occasionally even beryllium (element 3 in the periodic table, a metal – extremely toxic) are commonly used. Thus, whilst it’s harder to detect alpha radiation with a G-M tube it’s far from impossible with the right set-up.

In your demonstration with the various radioactive minerals and aluminium and lead absorbers you used lead wrapped in plastic.  Is there a significant hazard in handling lead?

Lead is toxic; it affects the nervous system and has a tendency to stick around in the body for a long time; it’s an example of heavy-metal poisoning. Lead water pipes began to be phased out decades ago, and lead additives in fuel were removed during the ‘90s and outlawed in the EU in 2000. Lead may be absorbed through the skin – hence the plastic bag.

At a few places in the videos there is a bit of blurring between X-rays which are electromagnetic radiation but are not produced by radioactive decay and gamma radiation also em radiation but is. In everyday language the word radiation is applied to many situations in which the radiation referred to is not nuclear.  I think I understand this but wonder if all your audience do.

This is a good point. ‘Radiation’ may be used of more than one phenomenon; however, at the core of our present topic is the word as applied to radiation having its origin within the nucleus of unstable atoms. (The cross-over in the video was, I think, associated with making the point that gamma rays are themselves electromagnetic in nature – so, a higher energy analogue to x-rays, which in their turn are higher energy electromagnetic radiation than the colours we perceive in a rainbow and so on.)

Comment: The nucleus changes during radioactive decay: it’s not that a part of the nucleus is ‘thrown out’ but rather that there’s a change to the nucleus which results in a newly-created entity leaving with the excess energy of the change. (e.g. a nucleus splitting into two, with one being an alpha; a neutron decaying into a proton and an electron, with the electron leaving; a whole nucleus shuffling down to a less excited state by emitting a gamma ray photon)   

2) What’s so special about the Earth?

How come we’ve landed up in a highly desirable area (estate agents) as a third area out from the Sun?

We are able to ask questions like this because we live on this particular planet – we wouldn’t be around to pose such questions from Venus or Neptune. There is a major philosophical theme here.

What pulled the plug out to get rotation going. Why does everything have to rotate and not stay still.

Early universe: once cool enough for atoms to form (H, He) they were at high T and therefore moving fast; more cooling meant that they could clump together (gravity), but all it takes is a tiny instability in one place to begin to affect all other places around. Locally, there are also the effects of collisions.

Gravity is a complete mystery to me. Everyone takes it for granted. I can’t. Want to know more. What is this prime force and how did it start?

The classic description by Newton tells us that it is a fundamental property of anything that has mass: it’s intrinsic to the universe. It’s not a strong force and may be dominated by other effects at short distances (e.g. magnetic, electrostatic) but for large masses and over long distances it becomes the boss. Einstein’s theories of Special and of General Relativity offered us a model of space-time which is curved/distorted and in which we ‘fall downhill’. We feel our weight because the Earth’s surface is preventing out fall down the slope towards its centre. A key observation was that the path of photons from distant stars is bent as it comes close to the Sun. Listen to ‘The Curious Cases of Rutherford and Fry’. (It was at this point in our Zoom session that we engaged in an extended discussion on the myth of the ‘lone genius’.)

Who names and accepts planet names?

Many different cultures named them independently. Earth from 8th century Anglo Saxon word ‘Erda’ meaning ground/soil; Sun – from middle English sunne (Chaucer’s Canterbury Tales). Planets names from Roman myth. Stars often named from Arabic Polaris has also been known by the names Alruccabah, Angel Stern, Cynosura, the Lodestar, Mismar, Navigatoria, Phoenice, the Pole Star, the Star of Arcady, Tramontana and Yilduz at various times and places by different cultures in human history. Some things are named after their discoverer: Kuiper belt, Oort cloud. All objects, bodies and surface features now overseen by the International Astronomical Union.

Termination shock? Heliosphere: You mentioned it is like a 'shock wave'. Could you explain a bit please? Is it just the name of a region that has certain properties or is there something physical there? How much of a barrier is it? Does it behave as a partial two-way barrier? As I understand it, we do get some cosmic radiation incident on Earth, as well as particles from the solar wind.

The term came from a diagram I had inserted into a slide. Heliopause – when the influence of the Sun (magnetic field, solar wind) no longer dominates over galactic forces. This might be thought of as the extent of the Solar System. (The Oort cloud is further out – held there in orbit by the Sun’s longer-ranged gravitational attraction.)

Can you say more about the creation of the molten core and solid centre of the Earth?

Early stage of solar system the planets formed from the gravity-driven) aggregation of gas, and small particles. As ever-larger clumps collided and coalesced the energy of the collisions heated everything up – everything from Mercury to Mars began life as a ball of molten material which then slowly cooled and began to solidify. (The same is true of the outer planets, but they became large enough that their gravity could hold on to a lot of gas as well.) In fact proto-Earth was re-melted in a hugely violent collision with another proto-planet – estimated to be about the size that Mars is today) – out of this collision came the debris which formed our relatively huge Moon.

Is there anything similar on Venus, Mars or the Moon?

Mars is relatively small compared to the Earth, so it has cooled faster but it does still have a molten core – however, perhaps due to the size, there isn’t the motion required to generate a magnetic field. Venus is more similar to Earth in size, but it rotates very slowly (one day on Venus is almost 4 Earth months – in fact its day is slightly longer than it year) and so we don’t get the motions required for either tectonic plate movement or a magnetic field.

I was particularly interested in the equation relating to the likelihood of developed life on a planet in other galaxies. Mostly obvious and difficult to get your head around the infinity of infinities of possible galaxies but what I had never thought of before, and should have, is the fact you have to take time into account too.  That is that life could have existed and finished or maybe not started yet.  So not only the number of galaxies and planets to consider but also the coincidence of life occurring on one at the same time as a life form such as humans on earth is capable of making contact.  Not to mention that contact must be recognisable by the other.

Yes indeed. It sounds as though you know of the Drake equation (see https://exoplanets.nasa.gov/news/1350/are-we-alone-in-the-universe-revisiting-the-drake-equation/ for example, noting the ‘guesstimates’ required) and the Fermi Paradox (e.g. https://www.forbes.com/sites/startswithabang/2018/06/26/no-we-cannot-know-whether-humans-are-alone-in-the-universe/); I am content to await evidence.

Age of the Universe: You mentioned about 14 billion years. Has this been calculated by working backward to the Big Bang and based on current observations of the expansion rate? Or, is the calculation much more sophisticated than that? Detail of the maths not required!

We can get a ‘guesstimate’ of the age simply by reversing the clock on what we currently observe, but there is a need to be a little more subtle than that. We now know, on the basis of our expanding body of observations (which have, in effect, taken us back to within a few million years of the Big Bang), that the rate of expansion was different in earlier epochs. Consider, for example, the fact that when everything was closer together the gravitational attraction between masses was greater – so more tendency to slow the rate of expansion.   (We engaged at this point in a discussion of what the ‘observable universe’ means, introducing Dark Matter/Energy etc.)

Earth's Magnetic field: I can see why relative motion between a solid ferrous inner core and molten (ferrous?) outer core will produce a magnetic field, but how do they think these relative motions might have come about?

Actually, this is a complex problem: a fuller picture is that the solid inner core (about the mass of the Moon, but almost all Fe) rotates eastward a little faster than the Earth overall (it laps the rest of the planet once every 400 years or so) – it gets this ‘push’ from the action on the Fe of the Earth’s geo magnetic field. This means that the fluid outer core is travelling. In effect, in a westward direction compared to the inner core. The magnetic field in created by complex convection currents in the fluid outer core, and its relative motion compared to the inner core is what leads to the tendency for the poles to drift over time. The Fe sank to the centre early on in our history; the inner core is gradually growing as the Earth slowly cools

Does the vortex in the bath go the same way in the southern hemisphere as here? Do Runner Beans twine the same way in Australia as they do in UK ?  (ref Flanders and Swann, The Honeysuckle and the Bindweed)

Yes, no difference – urban myth. They follow the Sun, wherever they are.

And a bit about Sun Dogs which we see occasionally during our sundowners on Whitstable beach, and the Northern Lights which we were amazed to watch from a beach on Sheppey one night a few years ago.

Refraction through hexagonal ice crystals in the upper atmosphere; 22º to either side of the Sun, sometimes with an arc (type of halo).

3) Glass: a look inside – science, technology and art

Please discuss de-colouring agents a little more, specifically Manganese dioxide added to make greenish glass clear.  I did research several years ago on 19th Century lavender window glass.  Manganese was added to glass to remove the green tinge to glass.  But with oxidation/ exposure to sunlight, the glass windows turned lavender.  You can still see some of these lavender windows in vintage buildings…. Back Bay homes in Boston, Walmer Castle in Kent, Sanssouci Palace in Potsdam, Germany.   Occasionally I’ve seen a window or two of the same purple glass in old buildings which haven’t been “restored”.  I would love to hear your opinion about this effect.  

We can purposefully add a metal in order to induce a colour, but the raw ingredients may contain metal impurities; a common one is Fe. Contaminants need only be present in tiny amounts to introduce a colour. The green tinge is often associated with Fe and it’s possible to mask this by using a metal offering complimentary colours (e.g. Se – pink; Co – blue). Thus, a better term would be neutralising or counter-dyeing. These will reduce the overall light transmission, but can lead to a greying of the glass.

High purity raw materials will help, and it’s often important to control the oxidation state of the metal (controlling the proportions of O present in the furnace) and the heat-treatment of the melt.

Mn was popular because it has a range of oxidation states and the highest, Mn(VII), absorbs green light but transmits to either side … generating shades of purple; Mn also helps ensure the iron present is in the Fe(III) state (Fe3+), which imparts only a pale yellow to the glass and is therefore easier to neutralise. The action of UV light imparts sufficient energy to alter the oxidation state of the metals present – including Mn – and so initial colours may alter over time. Indeed, the use of artificial UV lighting in so-called ‘purpling boxes’ will accelerate such effects.

Mn ceased to be used in this way from WW1 as the major source at that time was Germany.

I do stained glass and glass mosaics as a hobby.  Some glass breaks evenly, and some breaks irregularly. Please discuss this effect a little further.         

From Léonie Seliger, head of the Cathedral’s Glass Studio:  “Some glasses are simply harder than others due to their composition. American opalescent glasses for instant are really difficult to cut; the feel under the glass cutter is almost as if you are trying to get through very hard plastic. You don’t hear the nice musical sound a glass cutter’s wheel makes on normal glass (the sound the French call ‘le chant du diamant’), and the cut is almost invisible on American opalescent glass.  It’s hard to break, and I find the break runs away from the cut more often than on normal glass. A major suspect for glass breaking irregularly is poor annealing. Glass that retains internal stresses (because they were not given enough time to dissipate as the glass cools down) will jump unexpectedly, sometimes even while you are still running the glass cutter over it. That can give you quite a start, wastes glass, and makes me very uneasy about using glass from a poorly annealed sheet. It’s alright if you cut it into small pieces, but I would steer away from using large pieces from a stressed sheet in a window. They could fracture when experiencing a measure of strain that well-annealed glass would withstand without any problems.”

My perspective:   I think there are two principal length scales: atomic and mesoscopic.

On the atomic scale it's the chemical bonding that dominates of course; composition is the key factor here. Some of the hardest, most brittle glasses we ever made were phosphates containing multivalent rare earth metals; that was all down to their atomic-scale structure and the bonding that went with it. Move to silicates in which the 3D [SiO4] tetrahedral network is heavily disrupted by alkali metals and these physical properties alter considerably. At longer length scales, the effects of inhomogeneities come into play: variations in local density and/or composition, which are highly likely to have their origins in processing (- batch mixing, heating profile and annealing). I suppose one might tack on a third scale associated with microscopic/macroscopic factors like inclusions in the glass, including bubbles, and variations in thickness - but that's beyond my understanding.

Does this have possibilities ?

“in which they took balsa wood and removed its lignin – a component of wood that gives it [compressive] strength and colour [cell walls and related tissue; cellulose provides tensile strength]. Acrylic, which is non-biodegradable and water-repellent, was introduced into the remaining tissues where it filled both the tiny pores left by the removal of lignin and the hollow vessels that carried water in the tree. That, said Montanari, not only helped maintain the wood’s structure but also restored its strength and improved its optical properties. The upshot was a frosted-looking wood-based material. In the latest work the acrylic was mixed with another substance called polyethylene glycol, which permeates wood well.” https://www.theguardian.com/environment/2019/apr/03/scientists-invent-transparent-wood-in-search-for-eco-friendly-building-material

How do they make these?

Millefiori Glass Paperweights; it’s a bit like the glass version of ‘Brighton rock’, but perhaps easiest to watch:

https://www.youtube.com/watch?v=CYUqvLAeyg4&ab_channel=KSMQPublicTelevision

https://www.youtube.com/watch?v=dQw_yUsTVS0&ab_channel=PanosEgglezos

 

I was quite surprised to find how recent plate glass windows are.

Yes, the really smooth, highly transparent and toughened variant arrived during our lifetime. There were large sheets before that, but of relatively poor/non-uniform quality. For details see: ‘Float: Pilkington’s Glass Revolution’ by David J. Bricknell (ISBN 978-1-905472-11-6, 2009.

Fascinated by the idea of neutron diffraction.  I'm OK with wave particle duality, de Broglie wavelength, electron diffraction etc so am fine with the principles, but how do you separate the neutrons, how can they be accelerated to required energies, and how are they detected once diffracted?   

You’ve done the hard part; coming to terms with the fact that sub-atomic particles can behave like waves and thus be used to perform diffraction experiments is the key step; thereafter it’s fairly conventional.

Neutrons streaming from a nuclear reactor, for instance (e.g. https://www.ill.eu/about-the-ill) emerge with a range of speeds – i.e. energies, or wavelengths – so we can use a suitable crystal as a monochromator via Bragg’s Equation: hey presto we have a monochromatic beam of neutrons heading towards our sample. Of course, we first have to define a ‘beam’, but this can be done using standard shielding material to make a collimator. Post-sample detection as a function of angle is also relatively straightforward since neutrons, though uncharged, will interact with matter; a common form of detector uses scintillation: the neutron is absorbed by an atom’s nucleus – for example, Li, doped into a glass, is highly efficient at absorbing neutrons and when it does so a gamma ray is released which may then be detected relatively straightforwardly.

One can separate the neutrons generated by an accelerator source into their energies, or wavelengths, simply by timing their flight between two fixed points (using small detectors that sample the passing beam). The detectors will not now sort what emerges from the sample as a function of angle but rather on the basis of their time of flight (e.g. where I worked for a few years in the early ‘80s https://www.isis.stfc.ac.uk/Pages/About.aspx - I’ve posted on this a few years ago on my blog: https://bobreflected.blogspot.com/2016/03/large-scale-facilities-for-small-scale.html ).

How long does it take to grow a ‘Venus Flower Basket’ skeleton?

Because euplectella aspergillum is found at such great depths the information about its active life is limited. This is an animal that protrudes from the rocky ocean bottoms, also making it a benthic (bottom dweller) animal. Details of reproduction of E. aspergillum are not known, therefore we can only assume it’s similar to the normal forms of reproduction in related sponges.

How much stronger is glass in compression than in tension?

The compressive strength of glass is extremely high: 1000 N/mm² = 1000 MPa (for comparison, atmospheric pressure is 101 KPa – i.e. about 10,000 times smaller). This means that to shatter a 1 cm cube of glass, it requires a load of some 10 tonnes. Its resistance to tensile stress is significantly lower:  40 MPa (N/mm²) for annealed glass and 120 to 200 MPa for toughened glass (depending on thickness, edgework, holes, notches etc). (by the way, glass is a perfectly elastic material: it does not exhibit permanent deformation, until breakage. However it is fragile, and will break without warning if subjected to excessive stress.)