Light travels as waves. Waves can be described by their amplitude,wavelength and frequency. The speed of a wave can be calculated from its frequency and wavelength.

What are waves?

Waves are vibrations that transfer energy from place to place without matter (solid, liquid or gas) being transferred. Think of a Mexican wave in a football crowd. The wave moves around the stadium, while each spectator stays in their seat only moving up then down when it's their turn.

Some waves must travel through a substance. The substance is known as the medium, and it can be solid, liquid or gas. Sound waves and seismic waves are like this. They must travel through a medium. It is the medium that vibrates as the waves travel through.

Other waves do not need to travel through a substance. They may be able to travel through a medium, but they do not have to. Visible light, infrared rays, microwaves and other types of electromagnetic radiation are like this. They can travel through empty space. Electrical and magnetic fields vibrate as the waves travel.

Amplitude

As waves travel, they set up patterns of disturbance. The amplitude of a wave is its maximum disturbance from its undisturbed position. Take care, the amplitude is not the distance between the top and bottom of a wave.

Wavelength

The wavelength of a wave is the distance between a point on one wave and the same point on the next wave. It is often easiest to measure this from the crest of one wave to the crest of the next wave, but it doesn't matter where as long as it is the same point in each wave.

Frequency

The frequency of a wave is the number of waves produced by a source each second. It is also the number of waves that pass a certain point each second. The unit of frequency is the hertz (Hz). It is common for kilohertz (kHz), megahertz (MHz) and gigahertz (GHz) to be used when waves have very high frequencies. For example, most people cannot hear a high-pitched sound above 20kHz, radio stations broadcast radio waves with frequencies of about 100MHz, while most wireless computer networks operate at 2.4GHz.

You should know and be able to use the relationship between wave speed, frequency and wavelength.

How fast do waves travel?

The speed of a wave - its wave speed - is related to its frequency and wavelength, according to this equation:

wave speed (metre per second) = frequency (hertz) × wavelength (metre)

For example, a wave with a frequency of 100Hz and a wavelength of 2m travels at 100 × 2 = 200m/s.

Check your understanding of the equation by having a go at this activity.

Electromagnetic radiation travels as waves and transfers energy from one place to another. All electromagnetic waves can travel through a vacuum, and they all travel at the same speed in a vacuum.

The electromagnetic spectrum is a continuous range of wavelengths. The types of radiation that occur in different parts of the spectrum have different uses and dangers, which depend on their wavelength and frequency.

What is a spectrum?

The visible spectrum

Refraction from a prism

White light can be split up using a prism to form a spectrum. A prism is a block of glass with a triangular cross-section. The light waves are refracted as they enter and leave the prism. The shorter the wavelength of the light, the more it is refracted. As a result, red light is refracted the least and violet light is refracted the most, causing the coloured light to spread out to form a spectrum.

The electromagnetic spectrum

Visible light is just one type of electromagnetic radiation. There are various types of electromagnetic radiation, some with longer wavelengths than visible light and some with shorter wavelengths than visible light.

The electromagnetic spectrum

You should know the different types of electromagnetic radiation and their typical uses.

The main types of electromagnetic radiation

frequencytype of electromagnetic radiationtypical usewavelength highest gamma radiation killing cancer cells shortest X-rays medical images of bones ultraviolet detecting forged bank notes by fluorescence visible light seeing infrared optical fibre communication microwaves cooking lowest radio waves television signals longest

Gamma radiation and X-rays

You should know some of the properties and uses of gamma radiation and X-rays.

Gamma radiation

Gamma waves have a very high frequency. Gamma radiation cannot be seen or felt. It mostly passes through skin and soft tissue, but some of it is absorbed by cells.

Gamma radiation is used, among other things, for the following purposes.

• to sterilise surgical instruments
• to kill harmful bacteria in food
• to kill cancer cells (note that lower doses of gamma radiation could lead to cells becoming cancerous)

X-rays

Chest X-ray

X-rays have a lower frequency than gamma radiation. Like gamma rays, they cannot be seen or felt. X-rays mostly pass through skin and soft tissue, but they do not easily pass through bone or metal.

X-rays are used to produce photographs of bones to check for damage such as fractures. They are also used in industry to check metal components and welds for cracks or other damage.

Lower doses of X-rays can cause cells to become cancerous, so precautions are taken in hospitals to limit the dose received by patients and staff when X-ray photographs are taken.

Ultraviolet radiation and infrared radiation

You should know some of the properties and uses of ultraviolet radiation andinfrared radiation.

Ultraviolet radiation

Ultraviolet radiation is found naturally in sunlight. We cannot see or feel ultraviolet radiation, but our skin responds to it by turning darker. This happens in an attempt to reduce the amount of ultraviolet radiation that reaches deeper skin tissues. Darker skins absorb more ultraviolet light, so less ultraviolet radiation reaches the deeper tissues. This is important because ultraviolet radiation can cause normal cells to become cancerous.

Ultraviolet radiation is used in:

• sun beds
• security pens
• fluorescent lights (coatings inside the tube or bulb absorb the ultraviolet light and re-emit it as visible light)

Infrared radiation

Infrared radiation is absorbed by the skin and we feel it as heat. It is used in heaters, toasters and grills. It is also used for television remote controls and inoptical fibre communications.

The foremost theory of the origin of the universe is the Big Bang theory. It suggests that the universe began several billion years ago in an explosion that caused it to expand, and to continue expanding. Some of the evidence for the Big Bang comes from studying the red shift of light received from distant galaxies. Telescopes allow us to observe the universe.

The Big Bang

Scientists have gathered a lot of evidence and information about the universe. They have used their observations to develop a theory called the Big Bang.

The theory states that originally all the matter in the universe was concentrated into a single incredibly tiny point.

This began to enlarge rapidly in a hot explosion, and it is still expanding today. This explosion is called the Big Bang, and happened about 13.6 billion years ago (that's 13,600,000,000 years using the scientific definition of 1 billion = 1,000 million).

The Mil

Evidence of the Big Bang

There are two key pieces of evidence for Big Bang theory. These are red shift and the Cosmic Microwave Background radiation.

Red shift This happens with light too. Our sun contains helium. We know this because there are black lines in the spectrum of the light from the sun, where helium has absorbed light. These lines form the absorption spectrum for helium.

Spectrum of the sun

When we look at the spectrum of a distant star, the absorption spectrum is there, but the pattern of lines has moved towards the red end of the spectrum, as you can see below.

Spectrum of a distant star

This is called red shift. It is a change in frequency of the position of the lines.

Astronomers have found that the further from us a star is the more its light is red shifted. This tells us that distant galaxies are moving away from us, and that the further a galaxy is the faster it is moving away.

Since we cannot assume that we have a special place in the universe this is evidence for a generally expanding universe

. It suggests that everything is moving away from everything else. The Big Bang theory says that this expansion started billions of years ago with an explosion.

A summary of some of the evidence of the Big Bang and its interpretation

EvidenceInterpretation The light from other galaxies is red-shifted. The other galaxies are moving away from us. The further away the galaxy, the more its light is red-shifted. The most likely explanation is that the whole universe is expanding. This supports the theory that the start of the universe could have been from a single explosion. Cosmic Microwave Background The relatively uniform background radiation is the remains of energy created just after the Big Bang.

Telescopes

Distant stars and galaxies are too far away for us to reach. We cannot go to them to study them. So everything we know about distant stars and galaxies comes from analysing the radiation they produce.

Telescopes are devices used to observe the universe. There are many different types and some are even sited in space.

Optical telescopes

Optical telescopes observe visible light from space. Small ones allow amateurs to view the night sky relatively cheaply but there are very large optical telescopes sited around the world for professional astronomers to use.

Optical telescopes on the ground have some disadvantages:

• they can only be used at night
• they cannot be used if the weather is poor or cloudy

Other telescopes

Radio telescopes detect radio waves coming from space. Although they are usually very large and expensive, these telescopes have an advantage over optical telescopes. They can be used in bad weather because the radio waves are not blocked by clouds as they pass through the atmosphere. Radio telescopes can also be used in the daytime as well as at night.

X-rays are partly blocked by the Earth's atmosphere and so X-ray telescopes need to be at high altitude or flown in balloons.

Electricity is a very convenient form of energy that can be generated using different energy resources. Some of these resources are renewable and some are non-renewable. Each resource has advantages and disadvantages.

Fossil fuels

The fossil fuels are coal, oil and natural gas. They are fuels because they release heat energy when they are burned. They are fossil fuels because they were formed from the remains of living organisms millions of years ago.

About three-quarters of the electricity generated in the UK comes from power stations fuelled by fossil fuels. To the right is an energy transfer diagram for the generation of electricity from a fossil fuel such as coal.

Disadvantages of using fossil fuels

Fossil fuels are non-renewable energy resources. Their supply is limited and they will eventually run out. Fossil fuels do not renew themselves, while fuels such as wood can be renewed endlessly.

Fossil fuels release carbon dioxide when they burn, which adds to the greenhouse effect and increases global warming. Of the three fossil fuels, for a given amount of energy released, coal produces the most carbon dioxide and natural gas produces the least.

Coal and oil release sulfur dioxide gas when they burn, which causes breathing problems for living creatures and contributes to acid rain.

Nuclear fuels

You should be able to outline how electricity is generated using nuclear fuels.

Uranium and plutonium

The main nuclear fuels are uranium and plutonium. These are radioactive metals. Nuclear fuels are not burnt to release energy. Instead, the fuels are involved in nuclear reactions in the nuclear reactor, which leads to heat being released.

The rest of the process of generating electricity is then identical to the process using fossil fuels. The heat energy is used to boil water. The kinetic energy in the expanding steam spins turbines, which then drive generators to produce electricity.

Advantages

Unlike fossil fuels, nuclear fuels do not produce carbon dioxide or sulfur dioxide.

Disadvantages

.Like fossil fuels, nuclear fuels are non-renewable energy resources. If there is an accident, large amounts of radioactive material could be released into the environment. In addition, nuclear waste remains radioactive and is hazardous to health for thousands of years. It must be stored safely.

Wind energy

You should be able to outline how electricity is generated using wind energy.

Big convection currents

The wind is produced as a result of giant convection currents in the Earth's atmosphere, which are driven by heat energy from the sun. This means that the kinetic energy in wind is a renewable energy resource: as long as the sun exists, the wind will too.

Wind turbines

A wind turbine

Wind turbines have huge blades mounted on a tall tower. The blades are connected to anacelle or housing that contains gears linked to a generator. As the wind blows, it transfers some of its kinetic energy to the blades, which turn and drive the generator. Several wind turbines may be grouped together in windy locations to form wind farms.

Advantages

Wind is a renewable energy resource and there are no fuel costs. No harmful polluting gases are produced.

Disadvantages

Wind farms are noisy and may spoil the view for people living near them. The amount of electricity generated depends on the strength of the wind. If there is no wind, there is no electricity.

The Earth has a layered structure, including the core, mantle and crust. The crust and upper mantle are cracked into large pieces called tectonic plates. These plates move slowly, but can cause earthquakes and volcanoes where they meet.

The Earth’s atmosphere has changed over billions of years, but for the past 200 million years it has been much as it is today.

Cross section showing structure of the Earth

The Earth is almost a sphere. These are its main layers, starting with the outermost:

1. crust - relatively thin and rocky
2. mantle - has the properties of a solid, but can flow very slowly
3. outer core - made from liquid nickel and iron
4. inner core - made from solid nickel and iron

Note that the radius of the core is just over half the radius of the Earth. The core itself consists of a solid inner core and a liquid outer core.

Plate tectonics

The Earth's crust and upper part of the mantle are broken into large pieces called tectonic plates. These are constantly moving at a few centimetres each year. Over millions of years the movement allows whole continents to shift thousands of kilometres apart. This process is called continental drift.

The plates move because of convection currents in the Earth's mantle. These are driven by the heat produced by the decay of radioactive elements and heat left over from the formation of the Earth.

Where tectonic plates meet, the Earth's crust becomes unstable as the plates push against each other, or ride under or over each other. Earthquakes and volcanic eruptions happen at the boundaries between plates, and the crust may ‘crumple’ to form mountain ranges.

Alfred Wegener

Alfred Wegener (1880 - 1930)

The theory of plate tectonics and continental drift were proposed at the beginning of the last century by a German scientist, Alfred Wegener. Before his time it was believed that the planet's features, such as mountains, were caused by the crust shrinking as the Earth cooled after it was formed.

It took more than 50 years for Wegener’s theory to be accepted. This was because it was difficult to work out what the mechanism was that could make whole continents move, and it was not until the 1960s that enough evidence was discovered to support the theory fully.

Evidence for plate tectonics

So what was the evidence for Wegener's theory?

1. Plate tectonics explained why earthquakes and volcanoes were concentrated in specific places - around the boundaries of moving plates.
2. The match in shape between the east coast of South America and the west coast of Africa suggests both were once part of a single continent. There are similar patterns of rocks and similarfossilson both sides of the Atlantic - including the fossil remains of land animals that would have been unable to swim across an ocean.

The animation shows how the original single continent - Pangaea - is thought to have broken up and drifted apart.Earth around 200 million years ago, at the time of Pangaea.