Physical Science Enhancement — Physics 5 — Electromagnetic Radiation & Astrophysics

PS5.1 — Introduction to the Universe

The video below shows the relative sizes of the planets, moons, stars and galaxies in the known universe.

Objectives

• Describe the scale of the universe from planets to galaxies
• Define stars, galaxies, and the observable universe
• Recognise the solar system as part of a galaxy within the universe

Starter

Show a short video such as Powers of Ten or NASA scale of the universe. Ask: “How small are we compared to the universe?”

Explanation

• Planet: a body orbiting a star
• Star: a massive ball of gas producing energy through nuclear fusion
• Solar system: a star and the objects orbiting it
• Galaxy: billions of stars held together by gravity
• Universe: all space, time, matter, and energy

Discuss the observable universe — the part we can detect with telescopes and radiation.

Plenary

Exit ticket: write down three facts about the universe they did not know before this lesson. Share some answers with the class.

PS5.2 — The Electromagnetic Spectrum

Electromagnetic energy consists of oscillations (vibrations) of an electrical and perpendicular magnetic field. This electromagnetic energy propagates (moves) through empty space at a velocity of 3 x 10⁸ ms^-1.

Objectives

• Identify the main regions of the electromagnetic (EM) spectrum
• Describe the properties of EM waves (wavelength, frequency, speed)
• Recognise that all EM waves travel at the same speed in a vacuum

Explanation

• EM spectrum ranges from radio waves (longest wavelength) to gamma rays (shortest wavelength).
• All travel at the speed of light in a vacuum (3 × 10⁸ m/s).
• Frequency and wavelength are inversely related: c = fλ.

Mini-quiz

Quick class quiz: Which EM wave is used in mobile phones? Which has the highest frequency? Which has the longest wavelength?

Plenary

Summarise: All EM waves travel at the same speed in a vacuum but differ in wavelength, frequency, and energy. They have a wide range of uses in everyday life and astronomy.

PS5.3 — EM Radiation and Space Observation

Objectives

• Explain how telescopes use different regions of the EM spectrum to observe space
• Recognise that different wavelengths provide different information about stars and galaxies
• Match observatories to the type of EM radiation they detect

Starter

Show pictures of telescopes (optical, radio, space-based). Ask: “Why do astronomers need different types of telescope?”

Explanation

• Visible light telescopes show stars and galaxies as seen by the eye.
• Radio telescopes detect cold hydrogen clouds and map galaxy structures.
• Infrared telescopes see through dust clouds to reveal star formation.
• X-ray and gamma telescopes detect high-energy events like black holes and supernovae.

Plenary

Class discussion: “Why can’t all telescopes be built on Earth’s surface?” → Atmosphere absorbs many EM wavelengths. Summarise key advantages of space telescopes vs ground-based.

PS5.4 — Introduction to Circular Motion

Objectives

• Define circular motion and centripetal force
• Recognise that circular motion requires acceleration toward the centre
• Give everyday and astronomical examples of circular motion

Starter

Ask: “When a car turns a corner, why don’t the passengers keep moving in a straight line?” Use this to introduce the idea of circular motion needing an inward force.

Demonstration

Spin a bucket of water in a vertical circle. Ask: “Why does the water not fall out at the top?” → The centripetal force (gravity + tension) keeps the water moving in a circle.

Explanation

• Circular motion means the object is constantly changing direction, so it is accelerating.
• This requires a resultant force directed toward the centre (centripetal force).
• Examples: Moon orbiting Earth, car on a bend, ball on a string, planets orbiting the Sun.

Plenary

Discussion: “Why does circular motion always require a force? What provides the centripetal force in different situations (gravity, tension, friction)?”

PS5.5 — Gravity and Orbits

Objectives

• Explain gravity as the force causing circular motion in planetary orbits
• Apply Newton’s law of gravitation qualitatively
• Calculate orbital speed using a simplified formula

Starter

Ask: “If the Earth is moving around the Sun at 30 km/s, why doesn’t it fly off into space?” Use this to introduce gravity as the centripetal force.

Explanation

• Gravity provides the centripetal force that holds planets in orbit.
• Newton’s law of gravitation: F = Gm₁m₂ / r².
• Closer objects → stronger gravitational attraction.
• Balance between forward motion of the planet and inward pull of gravity creates stable orbits.

Discussion

Consider why outer planets take longer to orbit the Sun. Link to decreasing gravitational force with increasing distance (inverse square law).

Plenary

Summarise: Orbits are a balance between velocity and gravity. The further from the Sun, the weaker the force and the slower the orbital speed.

PS5.6 — The Life Cycle of Stars

Objectives

• Identify the main stages in the life cycle of a star
• Describe how mass determines a star’s final stage
• Recognise how stars produce energy by nuclear fusion

Starter

Show an image sequence of stars at different stages. Ask: “Do stars live forever? What happens when they ‘die’?”

Explanation

• Nebula → cloud of gas and dust collapses under gravity.
• Protostar → heat and pressure increase until fusion begins.
• Main sequence star → long stable period where hydrogen fuses to helium.
• Low-mass star → red giant → planetary nebula → white dwarf.
• High-mass star → red supergiant → supernova → neutron star or black hole.

Extension

Introduce the Hertzsprung–Russell (HR) diagram briefly as a way to classify stars by brightness and temperature. Place the Sun as an example of a main sequence star.

Plenary

Think–pair–share: “What is the difference between the fate of a star like the Sun and a star much more massive?” Share a few responses with the class.

PS5.7 — Properties of Stars and the HR Diagram

Objectives

• Describe the key properties of stars: luminosity, temperature, colour, and size
• Interpret the Hertzsprung–Russell (HR) diagram
• Classify stars into main sequence, giants, supergiants, and white dwarfs

Starter

Show images of stars of different colours. Ask: “Why do some stars look red, while others look blue?” → colour linked to surface temperature.

Explanation

• Luminosity: the total energy output of a star per second.
• Temperature: hotter stars emit more blue light; cooler stars emit more red light.
• Size: affects brightness; larger stars radiate more energy at the same temperature.
• HR diagram: plots luminosity vs surface temperature.

Worksheet practice

Students label an HR diagram with regions for: main sequence, red giants, supergiants, and white dwarfs. Answer guided questions: “Where are the hottest stars? Which stars are most luminous?”

Plenary

Discuss: “What can astronomers learn about a star from its position on the HR diagram?” Summarise: temperature, luminosity, stage in life cycle.

PS5.8 — The Big Bang and Expansion

Objectives

• Explain the Big Bang theory as a model for the origin of the universe
• Describe evidence supporting the Big Bang: redshift of galaxies and cosmic microwave background (CMB)
• Recognise that the universe is still expanding today

Starter

Balloon demo: Blow up a balloon with dots on the surface. As it expands, the dots move apart. Ask: “How is this like galaxies in the universe?”

Explanation

• The Big Bang model suggests the universe began from a very hot, dense state ~13.8 billion years ago.
• Redshift: light from distant galaxies is shifted toward longer wavelengths, showing they are moving away.
• CMB radiation: faint microwave background left over from the early universe, evidence of its hot dense origin.
• The universe is still expanding; distant galaxies move away faster (Hubble’s Law).

The Doppler effect

Discussion

Ask: “How do we know the universe had a beginning?” → Both redshift and CMB point to an expanding universe with an origin. Contrast with steady state theory (optional extension).

Plenary

Summarise: The Big Bang model explains expansion, redshift, and CMB. The universe is still expanding, and this expansion gives clues to its age and future.

PS5.9 — Space Exploration

Objectives

• Describe modern technologies used in space exploration
• Recognise the benefits and risks of human space travel
• Evaluate the role of telescopes, probes, and satellites in advancing knowledge

Starter

Ask: “What do you think is the most important discovery from space exploration so far?” Collect a few quick responses to introduce the theme.

Explanation

• Space telescopes (e.g. Hubble, James Webb) extend our view beyond Earth’s atmosphere.
• Space probes (e.g. Voyager, New Horizons) explore planets and outer solar system.
• Satellites support communication, navigation, and Earth monitoring.
• Human missions (e.g. ISS, Apollo) test survival and expand exploration capacity.
• Risks: radiation, isolation, cost, and danger of launch/landing.

Debate

Split class: “Should we spend billions on space exploration when there are problems on Earth?” Each side prepares arguments and a short closing statement.

Plenary

Class reflection: “How has space exploration changed the way humans see our place in the universe?” Collect final thoughts before the assessment lesson.

PS5.10 — Review and Assessment

Objectives

• Review and consolidate key concepts from the unit
• Apply knowledge of EM radiation, circular motion, and astrophysics to questions
• Reflect on progress and identify areas for further study

Starter

Quick-fire revision: students work in pairs to recall as many terms as possible from the unit in 2 minutes (e.g. redshift, main sequence, centripetal force).

Review stations

Set up small stations around the room: - EM radiation & telescopes - Circular motion & orbits - Stars & HR diagram - Big Bang & expansion Students rotate in groups, answering one key task at each station.

Plenary

Reflection task: “What is one idea you understand clearly now, and one idea you want to revise further?” Teacher collects a few responses and closes the unit.

The Big Bang theory

The video below shows the relative sizes of the planets, moons, stars and galaxies in the known universe.

The big bang

Once the observation was made that everything in the universe appears to be moving away from everything else the logical conclusion was that all matter must have started out closer together.

The idea that everything started from one point in the universe and has been expanding ever since became known as the Big Bang.

The term originated during a radio broadcast in 1949 in which the astronomer Fred Hoyle was describing this theory of the origin of the universe.

Evidence for the Big Bang theory

The acceleration of galaxies that could be observed from Earth was determined by their Doppler shift. That is the shift in light wavelength due to their moving away from us. An object move towards us exhibits a compression of light waves making it seem more blue. An object leaving us at a great speed shows a stretching of the light waves and seems more red.

This effect is similar to the noise a train makes as it approaches compared to when it is moving away. During the approach the sound waves are compressed and the sound has a higher pitch. When moving away it has a lower pitch due to expansion of the sound waves.

The life-cycle of stars

Stars are thought to form from condensed gas being slowly collected by gravity until a critical mass is reached, when the object's gravity is powerful enough to allow the hydrogen atoms to fuse together. Several hydrogen nuclei are fused together in a series of steps forming Helium.

This fusion reaction releases vast amounts of energy.

This is a simplified version. The video below explains about how stars go through different phases during their life cycle.

The life cycle of of stars.

What is a star made of

A star is made of gas, which consists mostly of Hydrogen and helium, the most commonly found elements in the Universe. The gas cloud is so dense its gravity holds it together.

Like the Earth, stars are made of many layers, each with distinctive properties.

The structure of the sun

Stellar classification

Stellar classification, means the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with spectral lines. Each line indicates a particular chemical element or molecule, with the line strength indicating the abundance of that element. The strengths of the different spectral lines vary mainly due to the temperature of the photosphere, although in some cases there are true abundance differences.

The element helium was first discovered by examining the spectral lines of the sun. New coloured lines were discovered in the sun's spectrum that did not correspond to any known element on Earth. For this reason its name is helium, which comes from the Greek word "helios", meaning sun.

The spectrum of helium

The Earth

Our planet

The Earth consists of several well-defined layers. Evidence for this is obtained using sound waves. The inner-core consists of a solid nickel/iron alloy surrounded by a liquid of similar composition. Outside of this there is a large region of soft hot rock, known as the mantle. On the very outside is the Earth's crust on which the continents stand.

The moon

Classification of rocks

The lithosphere is the solid part of the Earth's crust. It consists of three types of rock:

• Igneous rock
• Sedimentary rock
• Metamorphic rock

The original form of the Earth was a molten ball of different substances condensed from the gasous remenants of exploded stars.

The lime cycle

The carbon cycle

Carbon dioxide is intimately involved with the carbon cycle that you should already have studied.

Limestone

When the carbon cycle takes place in the sea, the organisms that die are decomposed by microorganisms and collect at the bottom of the sea as layers of skeletons made up of mainly calcium carbonate, CaCO₃.

Over many years of geological time these layers of calcium carbonate become compressed and form a kind of rock called limestone.

As the sea-level changed over millions of years these layers of limestone appear once again as rock formations that may be pushed to thousands of metres of altitude.

Other layers of limestone get so compressed by the huge mass of rocks on top of them and the heat of the earth that the limestone changes to another form of calcium carbonate, called marble.

When calcium carbonate is heated strongly it decomposes, making calcium oxide and carbon dioxide.

CaCO₃(s) → CaO(s) + CO₂(g)

The lime cycle

This shows how calcium ions are implicated into the eco and geo systems of the world and interact wit the carbon cycle.

Carbon dioxide in the air dissolves in rainwater makine a slightly acidic solution (pH < 7). This solution of carbon dioxide is sometimes called carbonic acid due to the equilibria:

CO₂(g) + H₂O(l) ⇌ H₂CO₃(aq)

Some of the carbonic acid dissociates (breaks apart into ions) giving free hydrogen ions, which are responsible for acidity:

H₂CO₃(aq) → H⁺(aq) + HCO₃^-(aq)

This acidic solution can react with limestone, CaCO₃, in the rocks making a dilute solution of calcium hydrogen carbonate, Ca(HCO₃)₂(aq)

CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq)

The resultant solution can pass through rivers to the sea, where it can be used by tiny sea organisms to build shell and bones. It is these creatures that over millions of years die, sink to the bottom of the sea and form layers of calcium carbonate that gets compressed into limestone.

If solutions of calcium hydrogen carbonate evaporate slowly, such as in caves, they can form stalactites and stalagmites in the caves, or even large crystals of calcite (see photo).

Ca(HCO₃)₂(aq) → CaCO₃(s) + H₂O(l) + CO₂(g)

Teaching notes and resources

Heating calcium carbonate (limestone) makes calcium oxide (quicklime). Adding water to calcium oxide (quicklime) makes calcium hydroxide (slaked lime). Dissolving calcium hydroxide in water makes limewater. Blowing carbon dioxide back through limewater makes a suspension of calcium carbonate at first which then dissolves with more carbon dioxide, making calcium hydrogen carbonate solution. Heating thin layers of calcium oxide to a high temperature causes it to glow with a greenish light. This used to be used (before electricity) to produce light for theatrical performance called limelight. Hence, the expression "to be in the limelight", meaning to be the focus of attention.

Now test yourself

Click on the button below to access the self-tests for MYP9 and MYP10.

MYP Self-test