Waves - Edexcel International A level Unit 2

Wave

A means of transferring energy via oscillations

Mechanical waves

Waves that require a material medium to oscillate through

EM Waves

Waves that do not need a material medium to transfer energy and can travel through both vacuums and material mediums. They travel fastest in vacuums. Their transfer of energy slows down when oscillating through material mediums

Amplitude (A)

The magnitude of the maximum displacement by an oscillation in the wave

Frequency (f)

The amount of complete wave cycles passing a point per second. Measured in Hertz (Hz)

Period (T)

The time taken for one complete oscillation at one point on the wave. Measured in second

Wave Speed

The distance traveled by a wave per unit time. Measured in meters per second.

Longitudinal Waves

Waves in which the particles oscillate parallel to the propagation of the wave and direction of energy transfer. Longitudinal waves have areas of high pressure called compressions, as well as areas of low pressure called rarefactions.

Transverse Waves

Waves in which the particles oscillate perpendicular to the propagation of the wave and the direction of energy transfer. Transverse waves have peaks and troughs.

Wavefront

Lines connecting points on the wave that are at exactly the same phase position

Coherence

Waves are coherent if they have the same frequency and a constant phase difference

Superposition

When two or more waves are in the same position, the overall effect is the vector sum of their individual displacements at each point where they meet.

Constructive Interference

The superposition effect of two waves that are in phase, producing a larger amplitude resultant wave.

Destructive Interference

The superposition effect of two waves that are out of phase, producing a smaller amplitude resultant wave.

Phase

The position of a certain point on a wave cycle. Measured in radians or angles.

Phase Difference

How much a wave lags behind another wave. Measured in angles, radians or fractions of wavelength.

Standing waves

Produced by the superposition of two waves with the same speed, frequency and amplitude travelling in opposite directions. The two waves must be coherent. Standing waves normally occur due to a wave interacting with its reflected wave.

Nodes

regions on a stationary wave where the amplitude of oscillation is 0.

Antinodes

regions on a stationary wave where the amplitude of oscillation is at its maximum.

Total Internal Reflection

occurs when the angle of incidence is greater than the critical angle and the incident refractive index is greater than the refracted refractive index (going from an optically denser medium to a less optically dense medium).

Polarised waves

waves that can only oscillate in one direction perpendicular to the direction of energy transfer.

Diffraction

the spreading out of waves when they pass an obstruction, typically a slit.

Hyugen's Construction

A principle that allows us to predict the future movement of waves if we know the current position of a wave's wavefront. The idea is that every point on a wavefront can be considered a new source of circular waves travelling forwards from that point. The superposition of these new wavelets create a new wavefront travelling in the same direction. This is how waves are able to diffract and continue travelling even when faced with obstacles such as slits. The peaks and crests of the waves superimpose to form areas of minima and maxima, creating an interference pattern.

What causes the greatest amount of diffraction

The greatest amount of diffraction occurs when the gap size is the same length as the wavelength

Evidence of the wave nature of electrons

Electrons are accelerated through a high potential difference in an electron gun and directed at a thin graphite film. The graphite consists of regularly spaced atoms with tiny gaps between them. Instead of forming a simple shadow or passing straight through, the electrons create an interference pattern on a detecting screen. This pattern is characteristic of wave behavior, demonstrating that electrons can diffract and interfere like waves. This experiment provides evidence for the wave-particle duality of electrons.

Transmission

When waves are incident on an interface between two mediums, the wave is mostly transmitted (passed through) if the two mediums have similar optical densities. Amplitude may decrease, as waves are only partially transmitted most of the time.

Reflection

When waves are incident on an interface between two mediums, the wave is mostly reflected if the two mediums have different optical densities.

Photoelectric effect

The photoelectric effect is the emission of electrons from a metal surface when light of a high enough frequency shines on it. It shows that light behaves as particles (photons), because:

Each electron absorbs one photon.

Emission only occurs if the photon's energy (E = hf) exceeds the work function (minimum energy needed to release an electron).

Increasing intensity increases the number of emitted electrons, not their energy.

Step by Step Pulse Echo Technique

Short pulse ultrasound waves are targeted into the body.

These waves are reflected when they are incident on an interface between mediums. E.g. woman's skin to baby's bone. The greater the difference in densities, the more the wave is reflected.

The intensity of the reflected waves determines the structure of the baby and the time taken shows the position of each point of the baby. For the best amount of detail, short wavelengths are used.

Why are shorter wavelengths better or pulse echo techniques?

Shorter wavelengths have better resolutions

Shorter wavelengths show better detail since they diffract less

One con is that shorter wavelengths require more energy due to higher frequencies

Pulse duration and its effect on pulse echo techniques

Pulses are relatively short and are produced at intervals. This is because the transducer cannot produce and detect ultrasound waves at the same time, as this causes an overlap and therefore loss of data. Therefore, a combination of short pulses followed by comparatively long intervals make for the clearest images.

How does higher frequency affect pulse echo imaging?

Higher frequency gives smaller wavelengths.

Small wavelength leads to high level of detail

Smaller wavelength can detect smaller objects and gaps

Photoelectric Effect Step by Step

Zinc plate attached to negatively charged gold leaf electroscope

Leaf and rod repel each other. Leaf stays raised

UV light is shone on metal plate

Photoelectrons are emitted

Negative charge is lost in plate and electroscope

Reduced repulsion causes gold leaf to fall

What does the photoelectric effect prove

Light can behave as a particle, and the light travels in discrete packets of energy called photons.