Category: Physics

Rainbow over the town

Sunlight makes two internal reflections in a raindrop in mid air and these reflections send the light back at a fixed angle from the sun, but as a ray of sunlight enters a raindrop it is not only bent at an angle, each wavelength (or colour) is bent by a slightly different amount.

Double rainbow over Cirencester Market Place

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Image of the day – 204

What’s in an image? Sometimes quite a lot, more than meets the eye. I’m posting an image whenever I can.

Click thumbnails to enlarge

On 27th October I walked to town for some shopping, coming out of Boots in Cricklade Street I found it had started raining, so went into Bishop’s Walk to find shelter. As the rain eased I moved toward the entrance and looking out saw a bright rainbow so made my way to the Market Place for a better look.

Rainbows are interesting apparitions. Sunlight makes two internal reflections in a raindrop in mid air and these reflections send the light back at a fixed angle from the sun, but as a ray of sunlight enters a raindrop it is not only bent at an angle, each wavelength (or colour) is bent by a slightly different amount. The same happens again as the light leaves the raindrop. So these rays of separated, coloured light now come back at similar but not identical angles from the sun so raindrops in exactly the right place send back a single colour to the viewer eyes. The rainbow is part of a circle, cut off where it meets the ground as there are no raindrops below that point.

Enhanced image

Furthermore, a small amount of the light is reflected three times internally in each raindrop and comes out at a wider angle. If there are enough raindrops in the air, you can then see another rainbow outside the circle of the main bow. This outer rainbow is faint and not always visible though you can just make it out in the photo. The colours in the outer rainbow are reversed because of the additional reflection inside the raindrops. In the second photo (click it to enlarge it), I’ve increased the intensity of the colours for you. You can just see the outer rainbow in both images (look above the chimneys over the yellow shop on the right (Joules). You may notice the colours are reversed in the outer bow, red on the inside and blue outside.

A closer look

Finally, took a look at the final photo where I’ve pushed things to the limit. I’ve enlarged a small part of the photo and enhanced the colours even more. Again, click the image for a closer look. Traditionally a rainbow was said to have seven colours – red, orange, yellow, green, blue (a pale blue), indigo (a deeper blue) and violet. In actual fact it’s a steady slide through the range of colours.

Do you notice anything else about this rainbow? If so, leave a comment below.

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A glorious colour contrast
Dark sky, bright light
Wall art in a cafe garden

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If you enjoyed this or found it useful, please like, comment, and share below. (If you don’t see those links, click the article’s title above the main photo and they will appear.) Send a link to friends who might enjoy the article or benefit from it – Thanks! My material is free to reuse (see conditions), but a coffee is always welcome and encourages me to write more often!

First generation Star formation

Our Sun is a typical, smallish star, it has been around for some five billion years so far and probably has about another five billion years to go. No need to panic, the Sun is middle aged! Steady as you go.

Artist’s impression of early stars (Wikimedia)

Part 4 of a series – Emergence

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RequiresExtensive cold gas clouds
Results inStars producing elements up to iron, gas giant planets
EnablesNovae, Supernovae

Two features of the birth of a star system are important here, matter and energy. The first stars formed from the gradual collapse of clouds of cold gas consisting mainly of hydrogen with some helium and a trace of lithium. Gravity slowly pulls a gas cloud into an ever-shrinking volume, and slow, drifting motions lead to increasing rates of rotation as this shrinkage proceeds. Compression of gases always results in heating, so over a long period of time, a diffuse cloud of cold gas becomes a rotating mass of increasingly hot gas.

Sufficient collapse eventually causes the internal pressure and temperature to reach a critical point at which nuclear fusion becomes not just possible, but inevitable, and conditions then settle to a point where the fusion energy dramatically increases the core temperature and pressure, pushing outwards more and more strongly until the the gravitational collapse is stopped. The rotating, hot mass is a young star, converting hydrogen to helium.

Over time it settles down more and more to a stable state, though this lasts for a limited time, basically until no further hydrogen fusion is possible because there is insufficient hydrogen remaining. The length of time of that stable state is related to the mass of the star. Small, light stars process their hydrogen slowly. Large, very massive stars burn through their supply much faster. Although they have a great deal more to begin with, the temperatures and pressures at the centre are much higher so there is a faster reaction in a larger volume of core. That’s why large stars run out of fuel faster than small ones. These earliest stars are called Population III stars by astronomers, it seems they were usually very large and therefore short-lived.

Our Sun is much more recent, a typical, smallish star, it has been around for some five billion years so far and probably has about another five billion years to go. No need to panic, the Sun is middle aged! Steady as you go.

Eventually, as the hydrogen is used up, energy production falls and gravity can no longer be resisted, so the star shrinks and heats up further. As the internal temperatures and pressures increase, the star shrinks until the temperature at the core is sufficient to fuse helium. Once again, further gravitational collapse is halted by increasing core temperatures and this lasts until the helium supply is exhausted. Through a whole series of similar steps the star creates heavier and heavier elements all the way up to iron, but fusing atoms of iron absorbs energy so gravity wins out in the end. Small stars slowly cool and eventually become inactive and unchanging. Particularly large stars have a different fate.

We’ll consider those details in a future article.

See also:

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Image of the day – 27

In the natural way of things, each Aloe will produce an average of one new plant, and the population will remain in balance.

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What’s in an image? Sometimes quite a lot, more than meets the eye.

I’m posting an image every day (or as often as I can). A photo, an image from the internet, a diagram or a map. Whatever takes my fancy.

Aloe aristata

Today’s picture is a close up of an Aloe aristata plant with a developing flower bud. All plants, animals, fungi, bacteria and even viruses have ways of reproducing themselves. That’s one of the defining characters of life of any kind. We can be absolutely confident that the same will be true of any life forms anywhere in the universe.

The Aloe flower bud will develop on a tall stalk and if the flowers that form are pollinated they will produce and release seeds that stand a chance of germinating and growing into new, similar, Aloe plants. In the natural way of things, each Aloe will produce an average of one new plant, and the population will remain in balance.

The only choices available to life are to survive for ever with no reproduction, or to live for a limited time and leave behind new versions to carry on the process. What life cannot do is live forever and reproduce: that would lead to overpopulation and catastrophic failure of resources. Even with reduced family sizes, the planet is no longer capable of supporting the billions of people on our planet. We face catastrophic population collapse due to lack of resources at some point unless we can reduce our population size in some other way first. That’s a matter of simple arithmetic, not a political statement or some kind of guesswork. If we don’t face and fix the issue, something else will sooner or later.

Themed image collections

The links below will take you to the first post in each collection

Cirencester, Favourites, Irish holiday 2024, Roman villa

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Useful? Interesting?

If you enjoyed this or found it useful, please like, comment, and share below. (If you don’t see those links, click the article’s title above the main photo and they will appear.) Send a link to friends who might enjoy the article or benefit from it – Thanks! My material is free to reuse (see conditions), but a coffee is always welcome and encourages me to write more often!

Image of the day – 21

The plant, on the other hand, is a living organism. Nobody designed or manufactured it – life is much more wonderful than that!

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What’s in an image? Sometimes quite a lot, more than meets the eye.

I’m posting an image every day (or as often as I can). A photo, an image from the internet, a diagram or a map. Whatever takes my fancy.

It’s quite amazing how life clings on, even in the most adverse circumstances. This plant was growing in my front drive, somehow finding a way to get its roots into a narrow gap in the block paving. The blocks were designed by a garden landscaping company and manufactured to particular standards of hardness and resistance to my car rolling over them. They were designed to last.

The plant, on the other hand, is a living organism. Nobody designed or manufactured it – life is much more wonderful than that! The universe we live in is tailored to build ever more complex things from very simple beginnings. A handful of quantum fields is all it takes, and these are exquisitely able to give rise to fundamental subatomic particles. These group together, eventually settling into simple atomic nuclei. As the universe expanded and cooled, atoms of simple elements appeared, almost entirely hydrogen and helium. Stars condensed and formed heavier elements up to iron. I could go on, but it’s a long story! Maybe some other time?

For now, just consider the battle between order (my paving blocks and the urge I have to remove weeds that neither I nor my wife want to see growing there) and disorder (weeds thriving wherever they can, despite my best efforts). Life always wins in the end, it seems!

Themed image collections

The links below will take you to the first post in each collection

Cirencester, Favourites, Irish holiday 2024, Roman villa

< Previous | Index | Next >

Useful? Interesting?

If you enjoyed this or found it useful, please like, comment, and share below. (If you don’t see those links, click the article’s title above the main photo and they will appear.) Send a link to friends who might enjoy the article or benefit from it – Thanks! My material is free to reuse (see conditions), but a coffee is always welcome and encourages me to write more often!

Combining atoms

Atoms began emerging very early in the formation of the universe, perhaps 18 000 years after the origin.

Part 3 of a series – Emergence

< In the beginning – A field | Index | From gas and gravity to galaxies >

A hydrogen atom

Here’s a simple diagram of a hydrogen atom. The little black ball is the nucleus, a proton, 10 000 times smaller than the atom as a whole, the white part represents an electron, spread out like a cloud around the nucleus. The proton and the electron were once thought of as fundamental particles that had no underlying structure. For the electron that remains true. The proton on the other hand consists of three quarks, but for the purposes of chemistry we can still think of it as ‘fundamental’.

A hydrogen atom can react with other atoms in quite specific ways. New and more complex behaviour emerges as atoms combine. Here are some of those emergent properties:

A molecule of methane, four hydrogens attached to a carbon atom
  • Two atoms of hydrogen can combine as a molecule of hydrogen, a gas that can become explosive when mixed with air.
  • Two hydrogen atoms and an oxygen atom can combine as a water molecule. Everyone knows that pure water is safe to drink.
  • Four hydrogens and a carbon atom can combine as a molecule of methane gas. This is the domestic gas that we use for cooking and for heating our homes. Methane is also a powerful greenhouse gas, contributing to global heating.
  • Three hydrogens and a nitrogen atom can combine as a molecule of ammonia, a poisonous gas that dissolves readily in water.
  • Two hydrogens and a suphur atom can combine as a molecule of hydrogen suphide, a gas that smells like rotten eggs.

There are many other molecules that include hydrogen.

Protons, and similar particles called neutrons can combine in larger numbers to make heavier and larger nuclei surrounded by much larger clouds of electrons (we’re leaving out a great deal of detail here). Together, these are the various chemical elements; there are more than 100 different kinds. Sodium, oxygen, phosphorus, chlorine, nitrogen, lead, iron, gold, sulphur, copper, tin and so on.

Chemistry

So – Take 100 different atoms and combine them together in various ways and you can clearly see that many, many different molecules are possible. Imagine 100 different kinds of Lego bricks and you begin to see the range of possibilities. There are rules of chemistry that restrict the combinations that can form, but even allowing for those rules, the number of possible molecules is huge . Here are some examples.

  • Sulphuric acid – two hydrogens, a sulphur, and four oxygen atoms
  • Table salt – one sodium and one chlorine atom
  • Bleach – two chlorine atoms
  • Laughing gas – two nitrogens and two oxygen atoms

We see chemistry appearing as soon as we have atoms. Chemistry just isn’t there in the world of subatomic particles like protons, neutrons and electrons. Like every object you can think of, we are made of atoms in complex chemical combinations so it’s quite hard for us to imagine a universe without chemistry. And atoms began emerging very early in the formation of the universe, perhaps 18 000 years after the origin. Chemistry started around 370 000 years as the universe continued to cool and atoms were able to begin combining ever more freely. At first hydrogen, helium and a small amount of lithium were the only elements available, all the others up to iron formed inside stars, while exploding stars (supernovae) generated the heavier elements and scattered these and the lighter elements far and wide. Once that had happened, perhaps 500 million years ago, the full range of atoms were available and chemistry took off in earnest.

Atoms are emergent, beginning to form once the universe became cool enough. And chemistry emerges given the presence of atoms and even lower temperatures. Could atoms and chemistry have been predicted given the presence and behaviour of subatomic particles? Perhaps. But it would have taken a real genius, a physicist with great foresight and imagination. But physicists are made of atoms and complex chemistry – so the real answer must be ‘no’!

That’s the thing about emergence – new kinds of objects and new processes ’emerge’ when the materials and conditions to do so exist. Sometimes emergence is rapid, even sudden. But as we shall see in a future post, sometimes it’s very slow indeed, or long delayed even after the possibility of emergence has existed for a very long time. Chemistry emerged quickly once atoms and low enough temperatures became available. So the opportunity was ‘slow’ to occur, but the emergence was immediate thereafter. We can think of these things separately – emergence opportunity, emergence delay, and emergence rate.

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