topic 6 - waves

What is a wave?

A disturbance that transfers energy from one place to another without transferring matter overall.

What are the two main types of wave?

Transverse waves and longitudinal waves.

What is a transverse wave?

A wave where oscillations are perpendicular to the direction of energy transfer.

What is a longitudinal wave?

A wave where oscillations are parallel to the direction of energy transfer.

Give examples of transverse waves.

Electromagnetic waves and water surface waves.

Give examples of longitudinal waves.

Sound waves in air and seismic P-waves.

What is an oscillation?

A repeated movement or variation about a central or equilibrium position.

What is amplitude?

The maximum displacement of a point on a wave from its undisturbed position.

What is wavelength?

The distance between two corresponding points on adjacent waves, such as crest to crest or compression to compression.

What is frequency?

The number of complete waves or oscillations passing a point each second.

What is the unit of frequency?

Hertz (Hz).

What is the time period of a wave?

The time taken for one complete wave or oscillation, measured in seconds.

What is the equation linking frequency and time period?

f = 1 ÷ T.

What does each symbol mean in f = 1 ÷ T?

f = frequency in hertz (Hz), T = time period in seconds (s).

What is wave speed?

The distance travelled by a wave per unit time.

What is the wave speed equation?

v = f × λ.

What does each symbol mean in v = fλ?

v = wave speed in metres per second (m/s), f = frequency in hertz (Hz), λ = wavelength in metres (m).

How do you calculate wavelength?

λ = v ÷ f.

How do you calculate frequency using wave speed and wavelength?

f = v ÷ λ.

How does increasing frequency affect wavelength if wave speed stays constant?

Wavelength decreases because v = fλ.

What happens to wave speed when a wave enters a different medium?

Its speed may change depending on the properties of the new medium.

What happens to frequency when a wave crosses into a different medium?

Frequency remains the same because it is set by the source.

What happens to wavelength when wave speed changes but frequency stays constant?

Wavelength changes in proportion to wave speed.

What is reflection?

The bouncing back of a wave from a surface or boundary.

What is refraction?

The change in direction of a wave when it enters a different medium because its speed changes.

What happens to a wave's frequency during refraction?

Frequency stays constant.

What happens to a wave's wavelength during refraction?

Wavelength changes because wave speed changes while frequency remains constant.

What happens when a wave enters a medium where it travels more slowly at an angle?

It bends towards the normal.

What happens when a wave enters a medium where it travels faster at an angle?

It bends away from the normal.

What is the normal in a reflection or refraction diagram?

An imaginary line perpendicular to the boundary at the point where the wave meets it.

What is diffraction?

The spreading out of waves as they pass through a gap or around an obstacle.

When is diffraction greatest?

When the gap size is similar to the wavelength.

What happens to diffraction when the gap becomes much larger than the wavelength?

There is less spreading out.

What is a sound wave in air?

A longitudinal wave made of compressions and rarefactions.

What is a compression?

A region where particles are close together, producing higher pressure.

What is a rarefaction?

A region where particles are spread further apart, producing lower pressure.

Can sound travel through a vacuum?

No. Sound needs a medium with particles to transfer vibrations.

How is sound produced?

Vibrating objects make nearby particles vibrate, transferring energy through the medium.

How does sound travel through air?

Air particles vibrate back and forth parallel to the direction of wave travel, passing on the disturbance.

What determines the pitch of a sound?

Frequency; higher frequency means higher pitch.

What determines the loudness of a sound?

Amplitude; greater amplitude generally means a louder sound.

What is the audible frequency range for a typical human?

Approximately 20 Hz to 20,000 Hz.

What is ultrasound?

Sound with a frequency above 20,000 Hz, beyond the typical human hearing range.

What is infrasound?

Sound with a frequency below 20 Hz.

What is ultrasound used for in medicine?

It can create images of internal structures using reflected sound waves.

How does an ultrasound scan create an image?

Ultrasound pulses are reflected at boundaries between tissues; the time taken for echoes to return helps determine the depth of boundaries.

How is distance to a reflecting boundary calculated using ultrasound?

Distance = speed of sound × time for the echo ÷ 2, because the pulse travels to the boundary and back.

Why is the ultrasound echo time divided by two?

The measured time includes the journey to the boundary and the journey back.

What is the approximate speed of sound in air?

About 330–340 m/s, depending on conditions.

What is the electromagnetic spectrum?

The continuous range of electromagnetic waves arranged by wavelength or frequency.

What are the electromagnetic waves in order of increasing frequency?

Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays.

What happens to wavelength as electromagnetic frequency increases?

Wavelength decreases.

What do all electromagnetic waves have in common?

They are transverse waves, transfer energy and travel at the same speed in a vacuum.

What is the speed of electromagnetic waves in a vacuum?

Approximately 3.0 × 10⁸ m/s.

What is the wave equation for electromagnetic waves in a vacuum?

c = f × λ, where c is approximately 3.0 × 10⁸ m/s.

Can electromagnetic waves travel through a vacuum?

Yes; they do not need a medium.

What are radio waves used for?

Radio and television broadcasting and communications.

What are microwaves used for?

Satellite communications, mobile communications and cooking food.

How do microwaves heat food?

They are absorbed by water molecules in the food, increasing internal energy.

What are the dangers of microwaves?

At high intensities, they can cause internal heating and tissue damage.

What is infrared radiation used for?

Heaters, thermal imaging, remote controls and communications.

What are the dangers of infrared radiation?

Strong infrared exposure can cause skin burns or damage to the eyes.

What is visible light used for?

Vision, photography, illumination and optical instruments.

What is ultraviolet radiation used for?

Security marking, fluorescent lamps and sterilisation in some applications.

What are the dangers of ultraviolet radiation?

It can damage skin cells and DNA, causing sunburn and increasing the risk of skin cancer; it can also damage eyes.

What are X-rays used for?

Medical imaging and airport security scanners.

What are the dangers of X-rays?

They are ionising and can damage cells and DNA, increasing the risk of cancer.

What are gamma rays used for?

Sterilising medical equipment and treating cancer in radiotherapy.

What are the dangers of gamma rays?

They are ionising and can damage cells and DNA, increasing the risk of cancer.

Which electromagnetic waves are ionising?

Ultraviolet at sufficiently high energies, X-rays and gamma rays; X-rays and gamma rays are strongly associated with ionising radiation in GCSE contexts.

Which electromagnetic waves are generally non-ionising in GCSE examples?

Radio waves, microwaves, infrared and visible light.

What is ionising radiation?

Radiation with enough energy to remove electrons from atoms, forming ions.

Why can ionising radiation cause cancer?

It can damage DNA, causing mutations that may lead to uncontrolled cell division.

Why is ultraviolet radiation potentially dangerous?

It can damage skin and eyes, and higher-energy UV can ionise atoms or cause chemical changes that damage DNA.

How can exposure to harmful electromagnetic radiation be reduced?

Limit exposure time, increase distance where appropriate, and use suitable shielding or protection.

What is the required practical involving waves and a glass block?

Investigate refraction by tracing a ray through a rectangular glass block and measuring angles of incidence and refraction.

How do you investigate refraction through a glass block?

Place a glass block on paper, draw around it, shine a narrow ray into it, mark the incident and emergent rays, remove the block and draw the ray path.

What is the angle of incidence?

The angle between the incident ray and the normal.

What is the angle of refraction?

The angle between the refracted ray and the normal.

Why should angles be measured from the normal rather than the surface?

That is the standard convention used to describe reflection and refraction.

What happens when light travels from air into glass at an angle?

It slows down and bends towards the normal.

What happens when light travels from glass into air at an angle?

It speeds up and bends away from the normal.

What happens to a light ray entering a rectangular glass block and leaving through the opposite parallel face?

The emergent ray is parallel to the incident ray but displaced sideways.

What is a seismic wave?

A wave produced by earthquakes or other sudden movements within the Earth.

What are P-waves?

Primary seismic waves; longitudinal waves that can travel through solids and liquids.

What are S-waves?

Secondary seismic waves; transverse waves that travel through solids but not liquids.

How can seismic waves provide evidence about Earth's structure?

S-waves do not travel through liquids, and their absence in part of Earth's interior provides evidence that the outer core is liquid.

What is the difference between P-waves and S-waves?

P-waves are longitudinal and travel through solids and liquids; S-waves are transverse and travel through solids only.

What is the equation for wave speed using distance and time?

v = distance ÷ time.

How do you calculate the time taken for a wave to travel a distance?

time = distance ÷ wave speed.

How do you calculate the distance travelled by a wave?

distance = wave speed × time.

How should you answer a wave calculation question?

Write the equation, convert values into consistent units, substitute the numbers and give the answer with the correct unit.

What is the relationship between amplitude and energy transferred by a wave?

A greater amplitude generally means more energy is transferred.

What is the difference between a wave's frequency and its amplitude?

Frequency is the number of oscillations per second; amplitude is the maximum displacement from equilibrium.

What is the difference between a transverse and a longitudinal wave diagram?

In a transverse diagram, oscillations are perpendicular to wave travel; in a longitudinal diagram, compressions and rarefactions run along the direction of wave travel.

What should you remember about the electromagnetic spectrum order?

Radio, microwave, infrared, visible, ultraviolet, X-ray, gamma; frequency increases and wavelength decreases along this order.