Waves

Wave Energy Transfer

• Waves travel through a medium, causing particles to oscillate

• Energy is transferred between particles

• Particles remain in the same place overall (no matter is transferred)

Labelling a wave

• Amplitude: maximum displacement of a point on a wave from its undisturbed position

• Wavelength: distance between the same point on two adjacent waves

• Frequency: number of complete waves passing a point per second (measured in Hz, where 1 Hz = 1 wave per second

Transverse Waves

• Oscillations (vibrations) are perpendicular (90°) to the direction of energy transfer

• All electromagnetic waves are transverse

• Examples: ripples and waves in water, waves on a string

Longitudinal Waves

• Oscillations (vibrations) are parallel to the direction of energy transfer

• They have regions of compression and rarefaction

• Examples include sound waves in air, ultrasound, and shock waves

Using an Oscilloscope to measure the speed of sound

• Attach a signal generator to a speaker to produce sound at a specific frequency

• Use two microphones connected to an oscilloscope to detect the sound waves

• Set up the oscilloscope so each microphone’s wave is shown separately

• Start with both microphones next to the speaker, then slowly move them apart until the two waves are aligned on the display but have moved exactly one wavelength apart

• Measure the distance between the microphones — this gives one wavelength

• Use speed = frequency × wavelength to calculate the speed of sound in air

• The speed of sound in air is approximately 330 m/s

• Repeat the experiment and check results are similar for reliability

How to measure the speed of water ripples using a lamp? RP

• Use a signal generator attached to a ripple tank dipper to produce waves at a set frequency

• Use a lamp to project wave crests onto a screen below the tank, making sure the wave shadows are the same size as the waves

• Measure the distance between shadow lines (each gap = one wavelength)

• Measure across 10 wavelengths and divide by 10 to find the average wavelength

• Use v = f × wavelength to calculate wave speed

• This method is useful for investigating wave speed and measuring wavelength without disturbing the waves

Standing Waves on a String RP

• Set up the apparatus with a signal generator, vibration transducer, string, pulley, and hanging masses

• Turn on the signal generator so the vibration transducer makes the string vibrate

• Adjust the frequency until a clear standing wave pattern is formed on the string

• The required frequency depends on the length of string between the pulley and transducer and the masses used

• Measure the wavelength by measuring the length of 4 or 5 full wavelengths as accurately as possible in one go

• If measuring half wavelengths, double the value to get a full wavelength

• The frequency of the wave is whatever you set on the signal generator

• Use v = f × wavelength to calculate the wave speed

Waves at a Boundary

• When waves arrive at a boundary between two different materials, three things can happen:

  1. The waves can be absorbed by the material they are trying to enter, transferring energy to the material’s energy stores (e.g. microwaves heating food)

  2. The waves can be transmitted, continuing to travel through the new material (often changing direction due to refraction)

  3. The waves can be reflected

• What happens depends on the wavelength of the wave and the properties of the materials involved

Reflection of a Wave

• Angle of incidence = angle of reflection

• The angle of incidence is the angle between the incoming wave and the normal

• The angle of reflection is the angle between the reflected wave and the normal

• The normal is a line drawn perpendicular (90°) to the surface at the point where the wave hits the boundary

• The normal is usually shown as a dotted line

Reflection of Waves

-Specular

-Diffuse

• Specular reflection happens when a wave is reflected in a single direction by a smooth surface. For example, when light is reflected by a mirror you get a clear reflection.

• Diffuse reflection is when a wave is reflected by a rough surface (like paper). The reflected rays are scattered in lots of different directions.

• This happens because the normal is different for each incoming ray, so the angle of incidence is different for each ray. The rule angle of incidence = angle of reflection still applies.

• When light is reflected by a rough surface, the image appears blurred or less shiny and you do not get a clear reflection of objects.

EM waves

• All electromagnetic (EM) waves are transverse waves that transfer energy from a source to an absorber

• Hot objects transfer energy by emitting infrared radiation, which is absorbed by the surrounding air

• All EM waves travel at the same speed through a vacuum or air

• EM waves form a continuous spectrum and are grouped into 7 types: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays

• Mnemonic: Raging Martians Invaded Venus Using X-ray Guns (Radio → Gamma order)

• Radio waves have the lowest frequency and the longest wavelength → increasing frequency and decreasing wavelength

• The large range in EM waves exists because they are produced by different changes in atoms and nuclei

• Nuclear changes produce high-energy waves like gamma rays, while less energetic changes produce lower-frequency waves like radio waves

• This explains why different materials absorb different frequencies of EM waves

• Each type of EM wave is used for different purposes depending on its energy and properties

Refraction- waves changing direction at a boundary

• When a wave crosses a boundary between materials at an angle, it is refracted (changes direction)

• Refraction happens because the wave speed changes in different materials

• Generally, waves travel slower in more optically dense materials

• If a wave slows down → it bends towards the normal

• If a wave speeds up → it bends away from the normal

• The frequency stays the same, but the wavelength changes

• Optical density is how fast light travels in a material (higher optical density = slower light)

• Mnemonic: TAGAGA

  • Towards the normal = Air → Glass (slower medium)

  • Away from the normal= Glass → Air (faster medium)

How to construct ray diagrams?

• Start by drawing the boundary between two materials and the normal line (90°) at the point where the wave hits the boundary

• Draw the incident ray meeting the normal at a point — the angle between the ray and normal is the angle of incidence

• If given this, draw it carefully with a protractor

• Now draw the refracted ray in the second material

• If the second material is more optically dense than the first, the ray bends towards the normal

• The angle of refraction is smaller than the angle of incidence

• If the second material is less optically dense, the ray bends away from the normal

• The angle of refraction is larger than the angle of incidence

• Key rule:

  • Into denser medium → towards the normal

  • Into less dense medium → away from the normal

For Experiments what to always do involving light rays?

Do experiments in dim rooms so clearly see light rays, then use either a ray box or laser to produce thin rays of light so you can easily trace the ray and measure angles

Refraction in Different Materials RP

• The boundary between different substances refracts light by different amounts

• You can investigate this by looking at how much light is deviated when it passes from air into different materials

• Place a rectangular block on a piece of paper and trace around it

• Use a ray box or laser to shine a ray of light at the middle of one side of the block

• Trace the incident ray and mark where the ray emerges from the other side

• Remove the block and use a ruler to join the incident point to the emerging point to show the path of the refracted ray through the block

• Draw the normal at the point where the ray enters the block

• Use a protractor to measure the angle of incidence and angle of refraction

• Repeat with different rectangular blocks made of different materials, keeping the incident angle the same throughout

How different materials reflect light by different amounts? RP

• Take a piece of paper and draw a straight line across it

• Place a mirror on the line so one edge lines up with it

• Shine a ray of light at the mirror surface and trace the incident and reflected rays

• Draw the normal at the point where the ray hits the mirror

• Use a protractor to measure the angle of incidence and angle of reflection and record the values in a table

• Repeat the experiment with different angles of incidence

• Smooth surfaces like mirrors give specular reflection, producing a clear, thin, bright reflected ray

• Rough surfaces cause diffuse reflection, where the reflected rays are scattered and less clear

• The angle of incidence = angle of reflection

EM Waves and Radio waves

• EM waves are made up of oscillating electric and magnetic fields

• Alternating current (AC) is made up of oscillating charges

• As charges oscillate, they produce oscillating electric and magnetic fields, forming electromagnetic waves

• The frequency of the waves produced = frequency of the alternating current

• Radio waves can be produced using an alternating current in an electrical circuit

• The device where charges oscillate to create radio waves is called a transmitter

• When transmitted radio waves reach a receiver, they are absorbed

• The energy carried by the wave is transferred to the electrons in the material of the receiver

• This causes electrons to oscillate, producing an alternating current

• The circuit produces a current with the same frequency as the original radio wave

Radio Waves and Diffraction

• Radio waves are electromagnetic radiation with wavelengths longer than about 10 cm

• Long-wave radio waves (1–10 km) can travel long distances because they diffract (bend) around the curved surface of the Earth

• Long wavelengths can also diffract around hills, tunnels, and other obstacles

• This allows signals to be received even when the receiver is not in direct line of sight

Radio Waves and the Ionosphere / Uses

• Short-wave radio signals (10–100 m) can travel long distances because they are reflected by the ionosphere (a charged layer in Earth’s upper atmosphere)

• The ionosphere reflects signals back to Earth, depending on atmospheric conditions and time of day

• Medium-wave signals may also reflect from the ionosphere depending on conditions

• Very short wavelength waves (e.g. TV and FM radio) travel by line of sight, so they need a direct path between transmitter and receiver

• Bluetooth uses short-range radio waves to transfer data wirelessly over small distances

Uses of Microwaves-satellites

• Communication to and from satellites uses microwaves because they pass easily through the Earth’s atmosphere

• The signal from a transmitter is sent into space

• It is picked up by a satellite receiver dish orbiting thousands of kilometres above Earth

• The satellite then transmits the signal back to Earth in a different direction

• The signal is received by a ground-based dish

• There is a time delay between sending and receiving the signal because of the long distance it travels

Uses of Microwaves- Ovens

• In a microwave oven, microwaves are absorbed by water molecules in food

• The microwaves penetrate a short distance into the food before being absorbed

• This transfers energy to the water molecules, making them vibrate faster

• The water molecules then transfer this energy to other particles in the food by heating

• This causes the food to heat up and cook quickly

Uses of Infrared radiation

• Infrared (IR) radiation is given out by hot objects

• The hotter the object, the more IR radiation it emits

• Infrared cameras detect IR radiation and convert it into an electrical signal

• The signal is displayed as an image

• Hotter objects appear brighter on the image

• Absorbing IR radiation increases temperature

• Food can be cooked using IR radiation because it increases the thermal energy of the food

Uses of IR Radiation - heaters

Electric heaters

• Electric heaters warm a room using a long wire (heating element)

• When current flows, the wire heats up

• The wire emits infrared radiation (and some visible light)

• This radiation is absorbed by objects and air in the room

• Energy is transferred, increasing the temperature of the surroundings

Uses of Visible light

• Optical fibres are thin glass or plastic fibres that carry data over long distances using visible light

• They work because of total internal reflection

• Light rays are reflected repeatedly inside the fibre until they reach the end

• They use visible light as the signal carrier

• Light is not easily absorbed or scattered as it travels along the fibre

• The fibre has cladding, which helps keep light inside the core and protects the signal - cladding protects the fibre

Uses of UV Radiation

• Fluorescence is a property of some chemicals where UV radiation is absorbed and then visible light is emitted

• This is why fluorescent colours look very bright — they are actually emitting light

• In a fluorescent lamp, UV radiation is produced and absorbed by a phosphor coating inside the tube

• The phosphor then re-emits the energy as visible light

• These lamps are energy efficient and useful for long periods of lighting

• Security pens can be used to mark property

• The marks are invisible in normal light but glow under UV light

• Sunbeds use UV radiation to give an artificial tan

• However, UV exposure can be dangerous to skin

Uses of X-Rays and Gamma Rays

• Radiographers use X-rays to check for broken bones

• X-rays pass through soft tissue (flesh) but are absorbed by dense materials like bone and metal

• This creates an image showing the inside of the body

• X-rays and gamma rays are used to treat cancer

• Their high energy can kill living cells, including cancer cells

• They are carefully directed to target tumour cells

• Gamma rays can also be used as a medical tracer

• A gamma-emitting substance is injected into a patient

• It is tracked as it moves through the body to show how organs are working

• Both X-rays and gamma rays are harmful

• Protection includes lead aprons, lead screens, and limiting exposure to reduce risk

EM Radiation that is harmfull

• The effect of electromagnetic waves depends on how much energy they transfer

• Low-frequency waves (e.g. radio waves) transfer very little energy

• They can pass through soft tissue without being absorbed much

• High-frequency waves (e.g. X-rays and gamma rays) transfer large amounts of energy

• This can cause damage to cells and tissues

• Ultraviolet (UV) radiation can damage skin cells

• It can cause sunburn, premature skin ageing, and increase the risk of skin cancer

• X-rays and gamma rays are ionising radiation

• They have enough energy to remove electrons from atoms

• This can cause ionisation, leading to mutations in DNA and cancer

How to measure the risk of Radiation?

• Radiation can be useful, for example in medicine and communication

• Radiation dose is measured in sieverts (Sv)

• A sievert is a measure of the risk of harm to the body from radiation exposure

• It is not a measure of the total amount of radiation energy absorbed

• The risk depends on:

  • The total dose of radiation absorbed

  • How harmful the type of radiation is

• A large dose in sieverts means a higher risk of damage to the body

• Small doses are less harmful, but repeated exposure can still build up risk

How does risk of radiation vary with body parts?

• CT scans use X-rays + computer processing to produce detailed body images

• They show how much radiation is absorbed by different tissues

• Typical doses:

  • Head scan ≈ 2 mSv

  • Chest scan ≈ 8 mSv

• A chest scan gives about 4× more radiation than a head scan

• Higher dose means a higher risk of damage to body cells

Lenses

Convex lens

• A convex lens bulges outward

• It causes parallel rays of light to be brought together (they converge)

• The point where they meet is the principal focus

Concave lens

• A concave lens curves inward

• It causes parallel rays of light to spread out (they diverge)

• The rays appear to come from a virtual principal focus

Key terms

• The principal axis is a line through the centre of the lens

• The principal focus is the point where rays meet (or appear to come from)

• The focal length is the distance from the centre of the lens to the principal focus

Rules for refraction in Convex lens

• A ray parallel to the principal axis refracts through the lens and passes through the principal focus on the other side

• A ray passing through the principal focus refracts through the lens and travels parallel to the principal axis

• A ray passing through the centre of the lens continues in a straight line without changing direction

Rules for refraction in Concave lens

• A ray parallel to the principal axis refracts through the lens and passes through the principal focus (or appears to come from it if traced backwards)

• A ray directed towards the principal focus refracts through the lens and travels parallel to the principal axis

• A ray passing through the centre of the lens continues in the same direction (undeviated)

Describing images

• A real image is formed when light rays from an object actually meet (converge)

• It can be projected onto a screen (e.g. image formed on the retina of the eye)

• A virtual image is formed when light rays do not actually meet

• The rays only appear to come from a point behind the object

• Example: a mirror image, where the object appears to be behind the mirror

• A magnifying glass produces a virtual image that is often larger than the object

Image descriptions

• Magnified / diminished = larger or smaller

• Upright / inverted = same way up or upside down

• Real / virtual = rays actually meet / rays only appear to meet

How to draw a ray diagram through a convex lens>>?

• Draw a convex lens (thicker in the middle, thinner at the edges)

• Draw the principal axis (straight horizontal line through the centre of the lens)

• Draw the object (arrow) above the axis

Ray rules:

• Draw a ray from the top of the object parallel to the axis

  • After refraction, it goes through the principal focus (F) on the other side

• Draw a ray from the top of the object through the centre of the lens

  • This ray continues in a straight line without bending

Image formation:

• Where the rays meet on the other side of the lens, mark the top of the image

• Draw the image down to the principal axis for the bottom point

• This gives the full real image position and size

How does distance from the lens affect the image?

• If the object is at 2F (twice the focal length):

  • The image is real

  • Inverted

  • Same size as the object

  • Formed at 2F on the other side

• If the object is between F and 2F:

  • The image is real

  • Inverted

  • Magnified (larger than the object)

  • Formed beyond 2F

• If the object is closer than F (inside the focal length):

  • The image is virtual

  • Upright

  • Magnified

  • Appears on the same side of the lens as the object

How to draw ray diagram for a concave lens?

• Draw a concave lens (thin in the middle, thicker at the edges)Concave- line points inwards

• Draw the principal axis through the centre of the lens

• Draw the object (arrow) above the axis

Ray rules:

• Draw a ray from the top of the object parallel to the principal axis

  • After refraction, it diverges and appears to come from the principal focus (F)

  • Extend this refracted ray backwards (dotted line) to the focus

• Draw a ray from the top of the object through the centre of the lens

  • This ray continues in a straight line without bending

Image formation:

• The refracted rays diverge, so extend them backwards (dotted lines)

• Where the backward extensions meet, mark the top of the image

• Repeat for the bottom of the object

• The image is:

  • Virtual

  • Upright

  • Diminished (smaller than the object)

  • Formed on the same side of the lens as the object

Concave lens always produce a…

Virtual image. The image is the right way up, smaller that the object and on the same side of the lens as the object.

How Magnifying glasses work?

• A magnifying glass uses a convex lens

• The object must be placed closer than the focal length (inside F)

• This produces a virtual image

  • Upright

  • Magnified (larger than object)

  • Appears on the same side of the lens as the object

• A virtual image cannot be projected onto a screen

  • Because light rays do not actually meet

  • Magnification > 1 → image is bigger

  • Virtual image = no real light convergence

  • Works because rays are refracted and appear to diverge from a point

• The visible spectrum(a range if wavelengths we perceive as different colours) is a small part of the EM spectrum -

• Visible light ranges from about 400 nm (violet) to 700 nm (red)

• Different wavelengths correspond to different colours

• Colours of light can be mixed

• Primary colours of light: red, green, blue (cannot be made by mixing)

• Mixing all colours of visible light produces white light

?How do colour and transparency depend on absorbed wavelength?

• Objects interact with light by absorbing, reflecting, or transmitting different wavelengths

• Opaque objects do not transmit light

• Opaque objects absorb some wavelengths and reflect others

• The colour of an opaque object depends on the wavelengths it reflects

• Example: a red object appears red because it reflects red wavelengths and absorbs others

• Some colours (e.g. yellow objects) may reflect a single wavelength or a mix of primary colours (red + green)

• White objects reflect all wavelengths of visible light equally

• Black objects absorb all wavelengths of visible light (no reflection)

• Transparent objects transmit most light

• Translucent objects transmit some light but scatter it

• Transparent/translucent objects may still absorb or reflect some wavelengths

• The colour of transparent/translucent objects depends on transmitted and reflected wavelengths

Colour Filters

• Colour filters only allow certain wavelengths of light to pass through

• All other wavelengths are absorbed

• A primary colour filter only transmits its own colour

• White light entering a filter loses all other colours except the one transmitted

• If you view a blue object through a blue filter:

  • It appears blue because only blue light is transmitted

• If you view a red object through a blue filter:

  • It appears black/dark because no red light is transmitted

• The final colour seen depends on which wavelengths are transmitted by both the object and the filter

• All objects emit and absorb infrared (IR) radiation from their surface.

• Hotter objects emit more IR radiation per second than cooler ones.

• If an object is hotter than its surroundings:

  • It emits more IR than it absorbs

  • It cools down

• If an object is cooler than its surroundings:

  • It absorbs more IR than it emits

  • It warms up

• At constant temperature:

  • Emit IR radiation at the same rate they absorb IR radiation

  • Surface effects:

  • Black, matt surfaces → best emitters and absorbers

  • White, shiny surfaces → poor emitters and absorbers (reflect IR)

• Example:

  • A hot drink cools because it emits more IR radiation than it absorbs.

How to investegate IR emission using a Leslie cube? RP

Cube is boiling so don’t touch

1. Place a Leslie cube on a heat-proof mat.

2. Fill it with boiling water.

3. Wait for it to warm up evenly.

4. Check all faces with a thermometer to confirm they are the same temperature.

5. Place an infrared detector a fixed distance (e.g. 10 cm) from one face.

6. Record the IR radiation detected.

7. Repeat for each face, keeping the distance constant.

8. Repeat the whole experiment for reliability.

Expected Results

• Black surface → emits the most IR radiation

• Matt surfaces → emit more than shiny surfaces

• Shiny surface → emits the least IR radiation

Black Body

• A perfect black body absorbs all radiation that hits it (no reflection or transmission) and is the best possible emitter of electromagnetic (EM) radiation.

• All objects emit EM radiation due to their thermal energy store, not just infrared, but a range of wavelengths.

• Intensity is the power per unit area (energy transferred per second per unit area).

• As temperature increases, the intensity of all wavelengths increases, with shorter wavelengths increasing more rapidly than longer wavelengths.

• The intensity and distribution of the wavelengths emitted by an object depends on the objects temperature.

• The distribution of wavelengths shows how intensity is spread across different wavelengths and changes with temperature.

• The peak wavelength is the wavelength with the highest intensity, and it decreases as temperature increases.

• Overall, hotter objects have higher intensity and a shorter peak wavelength.

How does radiation affect the Earth’s temperature?

• The Earth’s overall temperature depends on how much radiation it reflects, absorbs, and emits.

• During the day, the Earth absorbs more radiation (e.g. sunlight), so local temperature increases.

• At night, the Earth emits more radiation than it absorbs, so local temperature decreases.

• Overall, the Earth’s temperature stays fairly constant because there is a balance between absorbed and emitted radiation.

• Changes in the atmosphere can affect temperature:

  • If the atmosphere absorbs more radiation than it emits, the Earth’s temperature increases

  • This continues until absorption = emission again (new equilibrium)

Sound Waves

• Sound waves are produced by vibrating objects.

• They travel as longitudinal waves, made of compressions and rarefactions through a medium.

• Sound travels fastest in solids, slower in liquids, and slowest in gases.

• In solids, sound is transmitted when particles vibrate and pass the vibration to neighbouring particles.

• In a speaker, a vibrating diaphragm moves air particles, creating compressions and rarefactions.

• When sound hits a solid, air particles transfer pressure to the solid’s particles, causing them to vibrate and pass the wave through.

• Sound cannot travel in space because there are no particles in a vacuum to carry vibrations.

• If the sound reaches an ear, it makes the eardrum vibrate, allowing it to be heard

How do you hear sound?

• Sound waves reaching the eardrum cause it to vibrate.

• Vibrations are passed through the ossicles and then to the cochlea.

• The cochlea converts vibrations into electrical signals sent to the brain, allowing us to hear.

• Humans hear frequencies from 20 Hz to 20 kHz.

• Different materials and systems respond to different frequency ranges.

• Microphones can detect frequencies outside human hearing, but we cannot hear them directly.

• Human hearing is limited by the size and structure of the eardrum and inner ear components that transfer vibrations.

• Microphones work similarly:

  • A diaphragm vibrates when sound waves hit it

  • This vibration is converted into an electrical signal

How sound waves reflect and refract?

• Sound waves can be reflected by hard, flat surfaces, producing echoes.

• An echo is simply a reflected sound wave.

• Sound waves can also refract (bend) when they pass between different media.

• When sound enters a denser medium, it travels faster.

• This happens because:

  • The frequency stays constant

  • The wavelength changes

  • So the speed must change (since speed = frequency × wavelength)

• In normal conditions, refraction of sound is hard to notice because the change in direction is usually small.

Ultrasound

• Electrical devices can produce electrical oscillations at different frequencies.

• These oscillations can be converted into mechanical vibrations, creating sound waves.

• Sound waves with frequencies above 20,000 Hz are called ultrasound.

• Ultrasound is beyond the range of human hearing.

• Ultrasound is widely used in many applications because it can be generated and detected by electronic devices.

How Ultrasound waves get reflected at boundaries?

• When a wave passes between two different media, part of it is reflected and part is transmitted (and refracted). This is called partial reflection.

• Ultrasound pulses can be directed at objects or materials.

• At each boundary between different substances, some of the ultrasound is reflected back.

• The time taken for the reflected waves to return can be used to calculate the distance to the boundary.

Uses of Ultrasound

• Medical imaging (e.g. foetal scanning):

  • Ultrasound passes through the body but is partly reflected at boundaries between different tissues.

  • The reflected echoes are detected and timed.

  • A computer uses this data to create a video image of the foetus.

  • Ultrasound is generally considered safer than X-rays, which are definitely harmful.

• Industrial imaging (flaw detection):

  • Ultrasound is used to detect cracks or flaws in materials like metal or wood.

  • Waves are normally reflected from the far side of the object.

  • If there is a defect, reflections return sooner than expected, revealing the flaw.

• Echo sounding:

  • Uses high-frequency sound (including ultrasound).

  • Used by boats and submarines to measure water depth or detect objects underwater.

What can waves be used for?

• Waves have different properties (e.g. speed) depending on the material they travel through.

• When a wave reaches a boundary between materials, it can:

  • Be completely reflected or partially reflected (e.g. ultrasound imaging)

  • Be transmitted but change speed, which may cause refraction

  • Be absorbed

• These effects also apply to different wave types, such as ultrasound waves and seismic waves (e.g. S-waves).

• By studying how waves travel, reflect, and refract, scientists can investigate hidden or invisible structures that cannot be seen directly.

How earthquakes and explosions cause seismic waves?

• Earthquakes produce seismic waves that travel through the Earth.

• These waves are detected worldwide using seismometers.

• Seismologists:

  • Measure the time taken for waves to reach different locations

  • Identify areas where waves are not detected at all

• When seismic waves meet boundaries between Earth’s layers (with different properties like density):

  • Some waves are absorbed

  • Some are refracted (change direction due to speed change)

• If wave speed changes gradually, the path becomes curved.

• If wave speed changes suddenly, the wave path shows a sharp bend (kink).

P-waves and S-waves

• There are two main seismic waves: P-waves and S-waves.

• By studying how waves are absorbed and refracted, scientists have worked out the internal structure of the Earth, including the core.

P-waves

• Longitudinal waves

• Travel through solids and liquids

• Faster than S-waves

• Pass through the Earth’s core, but some regions show no detection

S-waves

• Transverse waves

• Travel only through solids (not liquids or gases)

• Slower than P-waves

• Cannot pass through the liquid outer core, so they are not detected on the opposite side of the Earth