Topic C - Wave Behaviour

Equilibrium position

The position where the object would rest if all energy was removed from the system.

Oscillations

Periodic back and forth motion.

Where does equilibrium lie in an oscillating system?

In the middle/centre.

Amplitude

The distance between the crest (or trough) of a wave and its equilibrium position.

Time period

The time taken to complete a full cycle.

Frequency

The rate at which something occurs over a particular period of time or in a given sample.

Angular frequency

The angle covered by a simple harmonic motion oscillation per second which is also known as angular speed/velocity.

Simple harmonic motion

Repeated motion around an equilibrium position where the acceleration of the object is proportional to its displacement, but in the opposite direction.

Simple pendulum

A body suspended from a fixed support such that it freely swings back and forth under the influence of gravity. It follows simple harmonic motion.

Mass-spring system

Consists of a mass attached to a spring that oscillates back and forth when displaced from its equilibrium position following simple harmonic motion.

Phase angle

The angular difference between two oscillations, or two cycles.

Damping

Technique used to restrain and reduce oscillations and vibrations.

Wave

A disturbance that transfers energy through a medium or space without a net movement of the particles of the medium.

Mechanical wave

A wave that needs a medium to travel through.

Transverse waves

A wave whose particles oscillate parallel to the direction of energy transfer.

Longitudinal waves

A wave whose particles oscillate parallel to the direction of energy transfer.

Wavelength

The distance between adjacent identical points on a wave, for example, two crests or two compressions.

Compressions

Areas of a longitudinal wave where the particles have a higher density.

Rarefactions

Areas of a longitudinal wave where the particles have a lower density.

Adiabatic processes

A thermodynamic process in which no thermal energy is transferred between the system and its surrounding.

Describe Sound waves

Longitudinal waves where the compressions and rarefactions move through the air, as the particles vibrate around their fixed positions.

Properties of a sound wave

Sound waves are mechanical, needing a medium to transfer energy and can bounce off walls and hard surfaces causing echoes.

Electromagnetic waves

An oscillation in electric and magnetic fields. It is not a mechanical wave and does not need a medium to travel through.

Electromagnetic spectrum

Continuous spectrum of all the wavelengths (frequencies) of electromagnetic waves from gamma rays to radio waves.

Wavefronts

Imaginary surfaces that connect points on a wave that are in phase.

Rays

Imaginary lines that show the direction of energy transfer in the wave as well as the direction of propagation.

Plane wave

In a plane wave, all the rays are parallel to each other, and we track a single ray.

Spherical wave

In a spherical wave, the rays are along radial directions and are divergent.

Reflection

A wave encounters a boundary between two different media and bounces back.

Angle of reflection

Equal to the angle of incidence (angle between the ray and the normal to the surface).

Refraction

The change in direction of a wave when it moves from one material to another at an angle, due to the difference in velocity of the waves in the two materials.

Transmission

When a wave passes across a boundary between two different media and continues to travel through the new medium.

Diffraction

The spreading of waves as they travel around a body or through an aperture.

Refractive index

A measure of how much a medium can slow down light waves. The ratio of the speed of light in a vacuum to the speed of light in the medium.

Critical angle

The angle of incidence at which light is refracted along the boundary between a medium with a greater refractive index and a medium with a smaller refractive index.

Total internal reflection

When the angle of incidence is greater than the critical angle as light changes medium, it is completely reflected back into the medium.

Superposition

Two waves meet and their displacements add together at every point in space to produce a combined wave with a resultant displacement.

Constructive Interference

Occurs when two waves meet in phase (crests meet crests and troughs meet troughs), resulting in a wave with increased amplitude.

Destructive Interference

Occurs when two waves meet out of phase (crests meet troughs), resulting in a wave with decreased or zero amplitude.

Path difference

The difference in distance that two waves travel from a source to a given point.

Single-slit diffraction

Occurs when light passes through a narrow slit and spreads out, creating an interference pattern on a screen.

Interference pattern for a double-slit

A series of modulated peaks within the envelope of the single slit diffraction pattern.

What happens to the maxima in an interference pattern as the number of slits increases?

The maxima remain in the same place but become narrower.

How does increasing the number of slits affect the intensity of the peaks in an interference pattern?

It increases the intensity of the peaks due to more light incident on the screen.

What occurs due to the additional distance traveled by light from more distant slits in an interference pattern?

Destructive interference occurs closer to the maximum.

What is observed between the maxima as the number of slits increases in an interference pattern?

Small secondary maxima modulations appear between them.

In phase

When the interference is crest meeting crest or trough meeting trough.

Standing wave

When a wave in a string reflects off a hard boundary, it reverses direction and undergoes a π radian/ 180° phase change. The incoming and reflected waves interfere constructively when they meet in phase and destructively when anti-phase. Important property of a standing wave: No transfer of energy.

Antinode

The point of a standing wave where amplitude of oscillation is at a maximum.

Node

A point of a standing wave where amplitude of oscillation is zero. Node never oscillates.

Boundary conditions

The conditions at the ends of a string or pipe. For a string, an end can be fixed or free. For a pipe, an end can be open or closed.

Natural frequency

The frequency at which a system vibrates or oscillates when it is disturbed.

Driving frequency

The frequency of the oscillator supplying energy to the system.

Resonance

When the frequency of the energy incident on a system is equal to that of the system's natural frequency, it vibrates at maximum amplitude.

Light damping

A system continues to oscillate and the amplitude of the oscillations decreases exponentially over time.

Heavy damping

When a system dissipates all its energy without oscillating. It returns at a slow rate to its equilibrium position.

Critical damping

The system returns to its equilibrium state as quickly as possible, without oscillating.

Why is damping used to avoid resonance?

Damping reduces the frequency of an oscillator. As damping is increased, the resonant frequency decreases, and the amplitude decreases.

Shock absorbers

Shock absorbers contain a piston mounted in oil that moves so the vehicle does not oscillate up and down when the wheels hit a bump in the road.

Degree of damping of a shock absorber

Heavy damping since light damping causes oscillation and critical damping degrades the components of the suspension system over time.

Reference frame

The specific perspective from which an observer analyses a situation.

Doppler effect

The change in the observed frequency of a wave as the result of the relative motion between two bodies.

What does a moving source cause in terms of wave perception?

The observer perceives a change in the frequency and the wavelength of the sound wave, but no change in the speed.

What does a moving observer cause in terms of wave perception?

The observer perceives a change in the frequency and the speed of the sound wave, but no change in the wavelength.

Double doppler effect

When a moving source produces a wave that reflects off a moving object and is received by the source again.

Spectral lines

The wavelengths of light emitted by different elements losing electrons.

Blue shift

A decrease in the electromagnetic wavelength of light caused by the motion of an object getting closer to the observer. Shorter wavelengths of visible light are shifted towards the blue region of the visible spectrum.

Red shift

An increase in the electromagnetic wavelength of light caused by the motion of an object getting further away from the observer. Longer wavelengths of visible light are shifted towards the red region of the visible spectrum.

Resonance in Analogue Radios

When the incoming radio wave’s frequency matches the natural frequency of the electrons in the aerial, resonance occurs. An amplifier is used to increase the strength of the signal, and information carried in the radio wave can be decoded.

Resonance in MRI (Magnetic Resource Imaging)

Uses radio waves and oscillating magnetic fields to excite water molecules in the body. The radio waves have the same frequency as the oscillation of the magnetic field, causing resonance to occur. When the water molecules ‘relax’, they emit energy that can be detected and mapped by a computer.

Resonance in Analogue watches

The work by exposing the quartz crystal to a electric potential difference which causes the crystal to vibrate at its natural frequency. The vibration is constant which allows a proper measurement of time.

Resonance in Greenhouse Effect

As the Earth warms, infrared radiation is emitted, which matches the natural frequency of greenhouse gases, causing the greenhouse gases to resonate. The gas molecules re-emit the energy in all directions. Some of the re-emitted energy is directed back towards the Earth, heating it up.

Resonance in bridges and structures

If the driving frequency (caused by wind, traffic or footfall) matches the structure’s natural frequency, resonance can occur, with dangerous results.