8.3.1 Red-shift and the expanding universe

Light from distant galaxies

1. The Sun is one of more than a hundred billion stars in our own galaxy, the Milky Way, and the universe contains billions of other galaxies.
2. Other galaxies are far too distant to visit, so almost everything astronomers know about them comes from studying the light they send to Earth.
3. As light from a star passes through the cooler gases in its outer layers, each element in those gases absorbs light of certain exact wavelengths.
4. When this light is spread out into a spectrum, it shows a continuous band of colours crossed by narrow dark lines at the absorbed wavelengths.
5. This pattern is called an absorption spectrum, and each dark line is a spectral line.
6. Every element produces its own fixed pattern of spectral lines, so the pattern works like a fingerprint that identifies the element.
7. Scientists measure the wavelengths of these lines for each element in a laboratory on Earth, where the source is not moving away from the measuring equipment.
8. Comparing the laboratory pattern with the pattern in light from a galaxy shows whether the lines have shifted, and by how much.

What red-shift is

1. In the light from almost every distant galaxy, the whole pattern of dark lines is found at longer wavelengths than in the laboratory.
2. The red end of the visible spectrum has the longest wavelengths, so a shift to longer wavelengths is a shift towards the red end, which is why it is called red-shift.
3. Light always travels at the same speed, so when the wavelength $\lambda$ increases in $v=f\lambda$, the frequency $f$ must decrease.
4. Red-shift therefore means that the observed light has a longer wavelength and a lower frequency than the light measured from the same element in the laboratory.
5. The lines keep the same spacing pattern, which is how astronomers know that the same elements are present and that the whole pattern has moved together.
6. Light from a source moving towards the observer is shifted the other way, to shorter wavelengths and higher frequencies, which is called blue-shift.

Why receding galaxies show red-shift

1. Red-shift happens because the galaxy is receding, which means it is moving away from the observer.
2. A source of waves sends out one crest after another at regular intervals.
3. If the source is moving away, it has travelled a little further away by the time it sends out each new crest.
4. Each crest therefore has slightly further to travel than the crest before it, so the crests arrive more spread out.
5. Crests that are further apart give a longer observed wavelength, and fewer crests arriving each second give a lower observed frequency.
6. The light leaving the galaxy is unchanged, and only the wavelength and frequency measured by the observer are different.
7. This change in the observed wavelength and frequency caused by the motion of the source is the Doppler effect.
8. You can hear the same effect with sound when an ambulance siren drops to a lower pitch as the ambulance drives away from you, because the sound waves reaching you have been stretched to a lower frequency.
9. Light travels so fast that the shift only becomes large for sources moving at enormous speeds, such as galaxies moving away at thousands of kilometres per second.

Bigger shifts mean faster recession

1. The size of the red-shift depends on how fast the galaxy is moving away.
2. A galaxy receding slowly shows a small shift, and a galaxy receding quickly shows a large shift.
3. Measuring how far the lines have moved therefore lets astronomers compare the recession speeds of different galaxies.
4. Astronomers also estimate how far away each galaxy is using other methods, such as measuring how bright certain types of star in it appear.
5. Plotting recession speed against distance for many galaxies shows that the more distant a galaxy is, the faster it is moving away.
6. This link between distance and speed was first reported by the astronomer Edwin Hubble in 1929.

Evidence for an expanding universe

1. Galaxies in every direction show red-shift, so they are moving away from us whichever way we look.
2. Astronomers in any other galaxy would see the same pattern, because every distant galaxy is moving away from every other galaxy.
3. This pattern is explained if space itself is stretching, carrying the galaxies further apart from each other.
4. When space stretches evenly, the gap between two nearby galaxies grows slowly, while the gap between two distant galaxies grows much faster because there is more stretching space between them.
5. A useful model is a balloon with dots drawn on it, where blowing up the balloon moves every dot away from every other dot and the furthest dots separate fastest.
6. This matches the observed link between distance and recession speed, so the change in speed with distance is evidence that the universe is expanding.
7. A few very close galaxies, such as the Andromeda Galaxy, show blue-shift instead, because gravity is pulling them towards the Milky Way more strongly than the expansion carries them away.

8.3.2 Evidence for the Big Bang model

The Big Bang model

1. The Big Bang model is the scientific explanation of how the universe began and how it has changed since.
2. According to the model, everything in the universe was once packed into an extremely small, extremely hot and extremely dense region.
3. About 13.8 billion years ago this region began to expand, and the universe has been expanding and cooling ever since.
4. As the universe cooled, the first particles formed, then atoms of hydrogen and helium, and much later the first stars and galaxies.
5. The Big Bang was not an explosion of matter into empty space that already existed, because the model describes space itself expanding.
6. Scientists accept the model because it explains the observations made of the universe today, and each piece of evidence is linked to something the model explains or predicts.

Red-shift and an expanding universe

1. Light from distant galaxies is red-shifted, which shows that they are moving away from us.
2. More distant galaxies have larger red-shifts, which shows that the universe is expanding.
3. If the universe is getting bigger now, then at earlier times the galaxies must have been closer together.
4. Running the expansion backwards in time leads to a moment when everything was squeezed into a tiny region.
5. Squeezing matter into a smaller space raises its density and its temperature, so the early universe must have been very hot and dense.
6. This is the starting state that the Big Bang model describes, so red-shift is evidence that supports the model.
7. Measuring how fast galaxies are moving apart, and how far away they are, also lets scientists work out how long the expansion has been going on, which gives the age of the universe.

Cosmic microwave background radiation

1. The Big Bang model predicts that the young universe was so hot that it was filled with intense electromagnetic radiation.
2. For the first few hundred thousand years, the universe was a hot, glowing gas that the radiation could not pass through.
3. When the universe was about 380 000 years old, it had cooled enough for atoms to form, and the radiation was set free to travel through space.
4. At that time the radiation had short wavelengths in the visible and infrared parts of the spectrum, matching a temperature of about $3000K$.
5. Ever since, the expansion of space has stretched these waves to much longer wavelengths.
6. The radiation now has wavelengths of around a millimetre, which places it in the microwave region of the electromagnetic spectrum.
7. This leftover radiation is the cosmic microwave background radiation, usually shortened to CMBR.
8. The CMBR was discovered by accident in 1965 by Arno Penzias and Robert Wilson, who picked up a faint microwave signal from every direction they pointed their antenna.
9. The CMBR is almost perfectly uniform, arriving with nearly the same intensity from every part of the sky, which shows that it fills the whole universe rather than coming from any single object.
10. Its spread of wavelengths matches the radiation emitted by an object at only about $2.7K$, which is just above absolute zero.
11. The Big Bang model predicted this radiation before it was found, and the measured radiation matches the prediction, so the CMBR is strong evidence for the model.

Big Bang versus Steady State

1. In the middle of the twentieth century, many scientists supported a rival idea called the Steady State model.
2. The Steady State model accepted that the universe is expanding, but it claimed that the universe had no beginning and that new matter was continually created to fill the gaps.
3. Both models could explain the red-shift of distant galaxies, so red-shift alone could not decide between them.
4. Only the Big Bang model includes an early hot, dense stage that would leave behind radiation filling the whole of space.
5. The Steady State model has no early hot stage, so it cannot explain why the CMBR exists.
6. After the CMBR was discovered, most scientists rejected the Steady State model and accepted the Big Bang model.
7. This shows how scientific models develop, because a model is kept while it explains the evidence and is replaced when new evidence cannot be explained by it.

8.3.3 Formation of the Sun and stellar equilibrium

From nebula to protostar

1. The Sun formed about 4.6 billion years ago from a nebula, a vast cloud of dust and gas made mainly of hydrogen.
2. Every particle in the nebula has mass, so every particle attracts every other particle by gravity.
3. In a slightly denser region of the cloud, gravity pulled the dust and gas inwards, so that part of the cloud began to collapse.
4. As the material fell inwards, energy was transferred from the gravitational store of the particles to their kinetic store, so they moved faster.
5. The fast-moving particles collided more often and more violently, so energy was transferred to the thermal store of the gas and its temperature rose.
6. The collapse also squeezed the particles closer together, so the density and pressure at the centre increased.
7. The hot, dense, glowing ball of gas at the centre of the collapsing cloud is a protostar.
8. Leftover dust and gas spinning in a flat disc around the young Sun later clumped together to form the planets.

Fusion begins

1. Hydrogen nuclei are positively charged, so they repel each other strongly.
2. Two nuclei can only get close enough to fuse if they collide at extremely high speeds, which needs an extremely high temperature.
3. The collisions must also happen very often, which needs a very high density and pressure.
4. Gravity kept squeezing the protostar until its core reached a temperature of around 10 million ${}^{\circ }C$, which was hot and dense enough for hydrogen nuclei to fuse.
5. In the core, hydrogen nuclei fuse to form helium nuclei, and each fusion releases energy from the nuclear store.
6. This energy heats the core further and travels outwards, finally leaving the surface as light and other electromagnetic radiation.
7. Once fusion is under way, the protostar has become a star.

Stellar equilibrium

1. Two effects act on the material of a star at the same time.
2. Gravity pulls all of the star's material inwards, trying to make the star collapse further.
3. The energy released by fusion heats the gas, and the pressure of the hot gas pushes outwards, trying to make the star expand.
4. When the inward pull of gravity exactly balances the outward push from the energy released by fusion, the star stops changing size.
5. This balance is stellar equilibrium, and a star held in this stable state is a main sequence star.
6. The balance corrects itself, which is why a main sequence star stays stable for so long.
7. If fusion slows slightly, the outward push weakens, so gravity squeezes the core a little.
8. Squeezing heats and compresses the core, which speeds fusion back up until the outward push again balances gravity.
9. The Sun has been a stable main sequence star for about 4.6 billion years and has enough hydrogen in its core to stay this way for around another 5 billion years.

The life cycle of a star like the Sun

1. A star stays on the main sequence until the hydrogen in its core starts to run out.
2. With less fusion in the core, gravity squeezes the core, which heats it up.
3. The extra heat starts fusion in a shell of hydrogen around the core and makes the outer layers swell enormously.
4. The outer layers cool as they expand, so they glow red, and the star becomes a red giant.
5. In the very hot core of the red giant, helium nuclei fuse to form heavier elements such as carbon and oxygen.
6. Eventually the red giant throws off its outer layers into space as a glowing shell of gas.
7. The hot, dense core left behind is a white dwarf, which is roughly the size of the Earth.
8. No fusion happens in a white dwarf, so it slowly cools and fades over billions of years.

The life cycle of a massive star

1. A star with at least eight times the mass of the Sun follows a different path.
2. Its core is hotter, so it uses up its hydrogen much faster and then swells into a red supergiant, which is even larger than a red giant.
3. Its core is hot enough for fusion to build up heavier and heavier elements, as far as iron.
4. Fusing iron does not release energy, so once the core is mainly iron, fusion can no longer support the star against gravity.
5. The core collapses in less than a second, and the outer layers are blasted into space in a supernova explosion.
6. Some elements heavier than iron, such as gold and uranium, are made in supernovae and are scattered through space.
7. The dust and gas thrown out can later collapse to form new stars and planets, which is how the Earth came to contain these heavy elements.
8. If the collapsed core is up to about three times the mass of the Sun, it becomes a neutron star.
9. If the core is even more massive, gravity crushes it further into a black hole.

8.3.4 Radiation emitted by hot bodies

All bodies emit radiation

1. Every object, whatever its temperature, emits electromagnetic radiation.
2. The particles in every object are constantly moving, and this motion causes the object to give out electromagnetic radiation.
3. Radiation emitted by an object because of its temperature is called thermal radiation.
4. Objects at everyday temperatures, such as you, a cup of tea or a brick wall, emit mainly infrared radiation.
5. Infrared is invisible to the human eye, which is why most objects around you do not appear to glow even though they are emitting radiation all the time.
6. Thermal imaging cameras detect this infrared, which is how they can show people and animals in complete darkness.
7. Even very cold objects, such as an ice cube or a comet far from the Sun, still emit some radiation.

A continuous range of wavelengths

1. A hot object does not emit radiation of just one wavelength.
2. Instead it emits a continuous range of wavelengths and frequencies, with no gaps between them.
3. The different wavelengths carry different amounts of energy, with shorter-wavelength, higher-frequency radiation carrying more energy.
4. The radiation is not spread evenly across this range, because some wavelengths are emitted much more strongly than others.
5. A graph of intensity against wavelength shows a smooth curve that rises to a peak and then falls away.
6. The wavelength at the top of the curve is the peak wavelength, where the object emits most strongly.
7. The curve falls steeply on the short-wavelength side of the peak and more gradually on the long-wavelength side.

Hotter bodies emit more intensely

1. As the temperature of an object rises, it emits more radiation at every wavelength.
2. The whole intensity curve therefore becomes taller, and the area under it, which represents the total energy emitted each second, becomes much larger.
3. A small rise in temperature produces a large rise in the total radiation emitted, so hot objects transfer energy by radiation far faster than cool ones.
4. You can feel this near an electric heater, where the radiation reaching your skin becomes much stronger as the heating element warms up.

Hotter bodies emit shorter wavelengths

1. As the temperature rises, the peak of the curve moves to shorter wavelengths and higher frequencies.
2. A greater share of the radiation is therefore emitted in the visible part of the spectrum, and at very high temperatures in the ultraviolet.
3. This explains the changing colour of a steel bar heated in a forge.
4. At first the bar emits only infrared, so you can feel its warmth but it does not glow.
5. At around $500{}^{\circ }C$ the bar emits enough red light to glow a dull red.
6. As it gets hotter it glows orange and then yellow, and at very high temperatures it looks white because it emits strongly across all the visible colours.
7. Stars follow the same pattern, so the colour of a star tells astronomers about the temperature of its surface.
8. Cool stars, with surfaces at about $3000K$, look red, the Sun at about $5800K$ looks yellow-white, and the hottest stars, at $20000K$ or more, look blue.

8.3.5 The solar system and satellites

The Sun and the planets

1. At the centre of our solar system is the Sun, a main sequence star that contains more than $99\%$ of all the mass in the solar system.
2. The Sun's gravity holds everything in the solar system in orbit around it.
3. Eight planets orbit the Sun, in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
4. The planets' orbits are ellipses that are very close to circles, and they all lie in nearly the same flat plane.
5. All eight planets orbit in the same direction, which is a clue that they formed from the same spinning disc of dust and gas.
6. Planets do not give out their own visible light, so we see them only because they reflect sunlight.
7. The further a planet is from the Sun, the longer it takes to complete one orbit, so a year on Mercury lasts 88 Earth days while a year on Neptune lasts about 165 Earth years.

Inner and outer planets

1. The four inner planets, Mercury, Venus, Earth and Mars, are small rocky planets with solid surfaces made of rock and metal.
2. The inner planets are close to the Sun, so they are much warmer than the outer planets.
3. The four outer planets, Jupiter, Saturn, Uranus and Neptune, are much larger and are known as the giant planets.
4. Jupiter and Saturn are gas giants, made mainly of hydrogen and helium, with no solid surface to land on.
5. Uranus and Neptune are ice giants, containing large amounts of water, ammonia and methane ices beneath their atmospheres.
6. All four giant planets have ring systems, although Saturn's rings are by far the brightest.
7. Jupiter is the largest planet, with a diameter about eleven times that of the Earth.
8. The planets therefore share some features, such as orbiting the Sun and being nearly spherical, but they differ in size, composition, temperature and number of moons.

Minor planets and comets

1. The solar system also contains many smaller bodies that orbit the Sun, called minor planets.
2. Dwarf planets are big enough for their own gravity to pull them into a nearly round shape, but they have not cleared other objects from around their orbits.
3. Pluto was called the ninth planet until 2006, when it was reclassified as a dwarf planet because it shares its region of space with many similar icy bodies.
4. Ceres, the largest object in the asteroid belt between Mars and Jupiter, is also a dwarf planet.
5. Asteroids are smaller, irregularly shaped lumps of rock and metal, and most of them orbit the Sun in the asteroid belt.
6. Comets are lumps of ice, dust and rock that follow very stretched elliptical orbits, spending most of their time far beyond Neptune.
7. When a comet passes close to the Sun, some of its ice turns to gas and streams away with dust to form a glowing tail that points away from the Sun.

Moons

1. A satellite is any object that orbits a larger body.
2. Moons are natural satellites, and most of them orbit planets.
3. Mercury and Venus have no moons, the Earth has one, and Mars has two small moons called Phobos and Deimos.
4. Each of the giant planets has dozens of moons, and new small moons are still being discovered, so the totals keep changing.
5. Some moons are larger than the planet Mercury, such as Ganymede, which orbits Jupiter.
6. Like planets, moons do not give out their own light, so we see them by the sunlight they reflect.
7. Our Moon orbits the Earth about once every 27 days and always keeps the same face turned towards us.

Artificial satellites

1. Artificial satellites are human-made objects carried into orbit by rockets.
2. Thousands of artificial satellites now orbit the Earth.
3. They are used for communications, such as satellite television and phone links, and for navigation, such as the GPS signals used by phones and car sat-navs.
4. They are also used for weather forecasting, for monitoring the Earth's surface by tracking floods, crops and ice sheets, and for military surveillance.
5. Some carry telescopes, such as the Hubble Space Telescope, which observes space from above the blurring effect of the Earth's atmosphere.
6. The International Space Station is a large artificial satellite where astronauts live and carry out experiments.
7. The orbit chosen for each satellite depends on the job it has to do.

Geostationary and polar orbits

1. A satellite in a geostationary orbit circles the Earth directly above the equator.
2. It takes one day to complete each orbit and moves in the same direction as the Earth spins, so it stays above the same point on the Earth's surface.
3. Geostationary satellites orbit at a height of about $36000km$, which is a very high orbit.
4. Because they never move across the sky, a fixed dish aerial on a house can stay pointed at one, which makes them ideal for communications and satellite television.
5. Weather satellites in geostationary orbit can also watch the same large area of the Earth continuously.
6. A satellite in a polar orbit passes over, or close to, both the North Pole and the South Pole on each orbit.
7. Polar orbits are low, typically between a few hundred and about a thousand kilometres above the surface, and one orbit takes roughly 100 minutes.
8. The Earth spins beneath the satellite as it travels, so on each orbit the satellite passes over a different strip of the surface.
9. Within about a day the satellite can scan the whole surface of the Earth, which makes polar orbits ideal for mapping, weather monitoring and surveillance.
10. Being close to the Earth also lets a polar satellite take much more detailed images than a geostationary one.

Natural and artificial satellites compared

1. Moons and artificial satellites are both satellites, so both orbit a larger body.
2. Both are kept in orbit by the gravitational attraction of the body they orbit.
3. Both reflect sunlight, which is why the International Space Station can be seen from the ground as a bright point of light moving across the night sky.
4. A moon is natural, having formed alongside its planet or been captured by it, while an artificial satellite is human-made and placed in orbit deliberately.
5. Most moons are far larger than any artificial satellite, and a few are larger than the smallest planet.
6. Artificial satellites are placed in orbits chosen for a purpose, such as geostationary or polar orbits, while moons stay in the orbits they formed in or were captured into.
7. Moons orbit planets and dwarf planets across the solar system, while almost all artificial satellites orbit the Earth.
8. Planets orbit the Sun directly, while moons orbit planets, so our Moon is a satellite of the Earth rather than a planet in its own right.

8.3.6 Gravity and circular orbits

Speed and velocity

1. Speed is a scalar quantity, so it tells you only how fast an object is moving.
2. Velocity is a vector quantity, so it tells you both how fast an object is moving and in which direction.
3. Two objects can have the same speed but different velocities if they are moving in different directions.
4. The velocity of an object therefore changes if its speed changes, if its direction changes, or if both change.
5. An object moving at a constant speed can still have a changing velocity, as long as its direction of motion is changing.

Moving in a circle

1. A planet in a circular orbit stays the same distance from its star and moves at a constant speed.
2. At every instant, the planet's velocity points along a tangent to the circle, at right angles to the line joining the planet to the centre.
3. As the planet moves round, the direction of this tangent keeps turning, so the direction of the velocity keeps changing.
4. The size of the velocity, which is the speed, stays the same, but its direction is never the same from one moment to the next.
5. Because the velocity is changing, the planet is accelerating, even though it is not speeding up or slowing down.

Gravity provides the inward force

1. By Newton's first law, an object's velocity changes only if a resultant force acts on it.
2. Without any force, a planet would carry on in a straight line along the tangent and drift off into space.
3. The force that stops this is the gravitational force of attraction between the star and the planet.
4. This force acts along the line joining the planet to the star, so it always points towards the centre of the orbit.
5. The force continuously pulls the planet away from its straight-line path and bends its motion into a circle.
6. The planet's acceleration is in the same direction as the force, so the planet is accelerating towards the centre of the circle.
7. Whirling a ball on a string shows the same idea, because the tension in the string pulls the ball towards your hand and keeps it moving in a circle.
8. If the string snaps, the inward force disappears and the ball flies off in a straight line along the tangent.
9. The same reasoning applies to the Moon orbiting the Earth and to artificial satellites, with the Earth's gravity providing the inward force.

Why the speed does not change

1. In a circular orbit, the gravitational force is always at right angles to the direction in which the planet is moving.
2. A force at right angles to the motion has no part pulling forwards or backwards along the path, so it cannot speed the planet up or slow it down.
3. The force does no work along the direction of motion, so the planet's kinetic energy stays the same and its speed stays constant.
4. The only effect of the force is to change the planet's direction, which is why the velocity changes while the speed does not.

8.3.7 Orbital radius and orbital speed

Stable orbits

1. An object in a stable orbit stays at the same distance from the body it is orbiting, orbit after orbit.
2. The orbital radius is measured from the centre of the central body, so for a satellite it is the Earth's radius of about $6400km$ plus the satellite's height above the surface.
3. Gravity provides the inward force that keeps changing the direction of the orbiting object, bending its path into a circle.
4. For the orbit to be stable, the gravitational force must be exactly the size needed to bend the path into a circle of that radius at that speed.
5. A faster object needs a larger inward force to keep it on the same circle, because its direction has to change more quickly.

Gravity gets weaker with distance

1. The gravitational force between two objects becomes weaker as the distance between them increases.
2. A satellite in a small orbit, close to the Earth, therefore feels a much stronger pull than an identical satellite in a large orbit.
3. A strong pull can bend the path of a fast-moving object into a circle, so a satellite close to the Earth must travel fast to stay in a stable orbit.
4. A weak pull can only bend the path of a slower object into a circle, so a satellite far from the Earth must travel more slowly.
5. For stable orbits around the same body, a smaller orbital radius therefore goes with a higher orbital speed, and a larger orbital radius goes with a lower orbital speed.

Changing speed means changing radius

1. If an orbiting object's speed changes, its old radius no longer matches its new speed, so it cannot stay in the same orbit.
2. If the object is moving too fast for its radius, gravity is not strong enough to bend its path into that circle, so the object moves outwards to a larger radius.
3. If the object is moving too slowly for its radius, gravity pulls it inwards more than is needed, so it falls closer to the central body.
4. For the orbit to be stable at a new speed, the radius must change, with a higher stable speed needing a smaller radius and a lower stable speed needing a larger radius.
5. A satellite that is moved up to a higher stable orbit therefore ends up travelling more slowly than it did in its lower orbit.
6. The same pattern applies to the planets orbiting the Sun, so Mercury, the closest planet, moves fastest and Neptune, the furthest, moves slowest.
7. Low satellites such as the International Space Station feel a tiny drag from the thin upper atmosphere, so they slowly lose height and must be boosted back up by rocket engines.

8.3.8 Radiation balance and the temperature of the Earth

Absorbing and emitting radiation

1. Every object absorbs some of the radiation that falls on it and emits radiation of its own at the same time.
2. Absorbing radiation transfers energy to the object's thermal store, and emitting radiation transfers energy away from it.
3. The temperature of the object depends on the balance between the rate at which it absorbs radiation and the rate at which it emits radiation.
4. If an object absorbs radiation faster than it emits it, its thermal store increases and its temperature rises.
5. If an object emits radiation faster than it absorbs it, its thermal store decreases and its temperature falls.
6. If the two rates are equal, there is no overall change in the thermal store, so the temperature stays constant.
7. A constant temperature does not mean that radiation has stopped, because the object is still absorbing and emitting, only at equal rates.

Reaching equilibrium

1. A hotter object emits radiation at a greater rate, so the rate of emission depends on the object's temperature.
2. This makes the balance self-correcting.
3. When an object absorbs more than it emits, it warms up, so it emits faster and faster until emission catches up with absorption.
4. When an object emits more than it absorbs, it cools down, so it emits more and more slowly until emission falls to match absorption.
5. Either way, the object settles at an equilibrium temperature where the two rates are equal.
6. Anything that changes the rate of absorption or the rate of emission upsets the balance, so the object warms or cools until it reaches a new equilibrium temperature.

Everyday examples

1. A mug of hot tea in a room emits radiation faster than it absorbs radiation from the cooler walls around it, so it cools.
2. As the tea cools, it emits radiation more slowly, so it cools more and more slowly until it reaches room temperature and the two rates balance.
3. A car parked in sunshine absorbs radiation from the Sun faster than it emits radiation, so its temperature rises until its own emission balances the sunlight it absorbs.
4. A dark-coloured car reaches a higher equilibrium temperature than a white car in the same sunshine, because a dark, matt surface absorbs a greater share of the radiation falling on it.
5. On a clear night, the ground emits infrared into space but receives little radiation back, so it cools quickly and frost can form.
6. On a cloudy night, the clouds absorb infrared from the ground and emit some of it back down, so the ground cools more slowly.

Radiation reaching the Earth

1. The Earth is heated by radiation from the Sun, which arrives mainly as visible light, together with some infrared and ultraviolet.
2. The amount of radiation reaching each square metre of the Earth depends on how much radiation the Sun emits and on the distance between the Earth and the Sun.
3. About $30\%$ of the incoming sunlight is reflected straight back into space by clouds, ice, snow and the atmosphere, so it never heats the Earth.
4. Light-coloured surfaces such as ice and snow have a high albedo, so they reflect most of the radiation falling on them.
5. Dark surfaces such as oceans and forests have a low albedo, so they absorb most of the radiation falling on them.
6. When ice melts, the darker land or sea underneath is uncovered, so more radiation is absorbed and the Earth warms further.
7. The atmosphere also changes the Sun's radiation as it passes through, because ozone high in the atmosphere absorbs much of the harmful ultraviolet, while most visible light passes through to the surface.

Radiation leaving the Earth

1. The Earth's surface is much cooler than the Sun, so it emits radiation at much longer wavelengths, mainly in the infrared.
2. Visible light from the Sun passes through the atmosphere easily, but some gases in the atmosphere absorb the longer-wavelength infrared emitted by the surface.
3. These greenhouse gases, such as carbon dioxide, methane and water vapour, then emit infrared in all directions, including back down towards the surface.
4. Less energy escapes into space each second than would escape without these gases, so the surface warms until it emits enough radiation to restore the balance.
5. This greenhouse effect keeps the Earth's average surface temperature at about $15{}^{\circ }C$, rather than the $-18{}^{\circ }C$ it would be without any greenhouse gases.
6. Burning fossil fuels has increased the amount of carbon dioxide in the atmosphere, so less infrared escapes and the Earth now absorbs slightly more energy each second than it emits.
7. As a result, the Earth's average temperature is rising towards a new, higher equilibrium temperature.

8.3.9 Seismic waves and exploring hidden structures

Using waves to explore hidden structures

1. Many structures cannot be observed directly, such as the inside of the Earth or the seabed beneath several kilometres of ocean.
2. Scientists explore them by sending waves into them and detecting the waves that come back or pass through.
3. When a wave meets the boundary between two materials, some of it may be reflected, some transmitted into the new material, and some absorbed.
4. The wave's velocity usually changes as it enters a new material, and if it crosses the boundary at an angle it changes direction, which is refraction.
5. How much of a wave is reflected or absorbed, and how fast it travels, depends on both the type of wave and the material it travels through.
6. Measuring when waves arrive, where they arrive and how strong they are therefore gives evidence about the materials and boundaries they have passed through.

Seismic waves

1. An earthquake releases energy that spreads out through the Earth as seismic waves.
2. Seismometers at monitoring stations all over the world record the vibrations when the waves arrive.
3. Comparing the records from many stations shows which waves reached each place, when they arrived and how strong they were.
4. The two types of seismic wave that travel through the inside of the Earth are P-waves and S-waves.

P-waves and S-waves

1. P-waves, or primary waves, are longitudinal, so the particles vibrate parallel to the direction in which the wave travels.
2. P-waves can travel through both solids and liquids, because both can be squashed and then spring back.
3. S-waves, or secondary waves, are transverse, so the particles vibrate perpendicular to the direction in which the wave travels.
4. S-waves can travel through solids but not through liquids, because the particles of a liquid slide past each other and cannot pass on a sideways vibration.
5. P-waves travel faster than S-waves through the same rock, so P-waves always reach a seismometer first.
6. The time gap between the arrival of the P-waves and the arrival of the S-waves gets longer the further a station is from the earthquake.
7. Combining the distances worked out by three or more stations pinpoints where the earthquake happened, which is how earthquakes are detected and located.
8. The speed of seismic waves increases gradually with depth in the mantle, so the waves refract gradually and travel along curved paths.

Evidence for a liquid outer core

1. After a large earthquake, S-waves are detected across the side of the Earth nearest the earthquake.
2. No S-waves are detected anywhere beyond about $105^{\circ }$ from the earthquake, measured around the Earth's centre, which leaves a huge S-wave shadow zone on the far side.
3. S-waves cannot pass through liquids, so the S-wave shadow zone shows that the Earth contains a liquid layer that blocks them.
4. The size of the shadow zone shows how big this liquid layer is, which reveals a liquid outer core with a radius of a little over half the Earth's radius.
5. P-waves can pass through the liquid outer core, but they slow down suddenly as they enter it and refract sharply at its boundary.
6. This sharp bending leaves a ring-shaped P-wave shadow zone, between about $105^{\circ }$ and $140^{\circ }$ from the earthquake, where almost no P-waves arrive.
7. The sharp refraction shows that there is a clear boundary where the solid mantle meets the liquid outer core.

Evidence for a solid inner core

1. Faint P-waves are detected inside the P-wave shadow zone, where none would arrive if the core were liquid all the way through.
2. These P-waves have been refracted or reflected at another boundary deep inside the core, where they speed up again.
3. This is evidence for a solid inner core at the centre of the Earth, which was first proposed by the seismologist Inge Lehmann in 1936.
4. Together, the seismic evidence gives a layered model of the Earth, with a solid crust, a solid mantle, a liquid outer core and a solid inner core.

Exploring deep water with sonar

1. Light cannot travel far through seawater because it is quickly absorbed, so the deep ocean floor cannot be seen.
2. Sound and ultrasound travel well through water, at about $1500m/s$, so they are used to explore deep water instead.
3. A ship's sonar transmitter sends a short pulse of ultrasound down into the water.
4. The pulse is reflected when it reaches the seabed or any object in the water, and a detector on the ship picks up the echo.
5. The time between sending the pulse and receiving the echo is the time taken for the sound to travel down and back.
6. Using $distance=speed\times time$ and then halving the result gives the depth of the seabed or the distance to the object.
7. Sonar is used to map the shape of the seabed, find shipwrecks, detect submarines and locate shoals of fish.
8. Hard rock reflects a strong echo, while soft mud absorbs more of the sound and reflects a weaker echo, so the strength of the echo shows what the seabed is made of.
9. Some of the sound passes into the seabed and is reflected from the boundaries between layers of rock and sediment beneath it, which helps geologists search for oil and gas.