Wave

amplitude

Amplitude is the maximum distance of the disturbance (or the maximum displacement of a particle) from its equilibrium position.

anti-node

Anti-nodes are the positions in a stationary wave with the largest disturbance.

constructive interference

Consider two waves with the same frequency interfering with each other. At positions where the two waves are in phase, the vibration amplitudes add up according to the principle of superposition and the disturbance will be a maximum. For example, if the amplitudes of these two waves are the same, the resulting amplitude will be doubled and the energy will be four times (square of two) that of a wave on its own.

crest

Crests refers to the positions in a wave where the distance of the particles (or disturbance) from their equilibrium positions reaches the most positive value.

destructive interference

Consider two waves with the same frequency interfering with each other. At positions where the two waves are out of phase, the vibration has an amplitude equals to the difference in the amplitudes of the waves and the disturbance will be a minimum. For example, if the amplitudes of these two waves are the same, they will be cancelled out completely.

diffraction

Diffraction is a characteristic of waves. When wave meets obstacles, it will bend around the edges and disperse. This phenomenon is called diffraction. Only when the size of the obstacle is comparable to the wavelength will the effect of diffraction significant.

electromagnetic wave

Electromagnetic waves (EM waves) are transverse waves. They are the oscillations of electric field and magnetic field in space. EM waves can propagate in vacuum or media. The speed of EM waves in vacuum is a constant. It is the fastest speed in the universe. Visible light, microwave and gamma ray are examples of EM waves.

frequency

Frequency is the number of complete oscillations performed by a particle or the disturbance in a medium per unit time. The relation between the frequency and the period is . The unit of frequency is Hertz (Hz), .

intensity

Intensity is a measure of the energy transported by a wave. The intensity received by an area is the energy received per unit time (power) per unit area . The intensity of wave is usually directly proportional to the square of its amplitude. The unit of intensity is ( ).

interference

Interference is a characteristic of waves. When two or more waves reach the same location through different paths (or from different sources), the distribution of disturbances and energy in space will change according to the principle of superposition.

longitudinal wave

For longitudinal waves, the direction of disturbance is parallel to the direction of wave propagation. Sound waves in a gas are transverse waves.

node

Nodes are the positions in a stationary wave with zero disturbance.

period

The time required for a particle (or disturbance) to complete one cycle of motion is called period. (see frequency)

phase, phase difference

Phase is a measure of the state of vibration. It can be represented by an angle ( ). A complete cycle of vibration is . Phase difference represents the difference of vibration states between two waves with the same frequency (or between different positions of a wave). For example, if the crests of two waves meet at one point, then the waves are in phase at that point (phase difference is ). If the crest of one wave meets the trough of another wave, then the two waves are out of phase at that point (phase difference is ).

principle of superposition

When two or more waves exist at the same time, the disturbance (or the distance between a particle and its equilibrium position) at any location will be the algebraic sum of the disturbance (or the displacement of a particle from equilibrium) that each wave would produce on its own.

resonance

When a vibrating system is disturbed by an external source which has a frequency coincides with the natural frequency of the system, the efficiency of energy input will be greatly enhanced and the amplitude and energy of vibration will increase continuously by a large extent. This phenomenon is called resonance.

sound wave

A longitudinal wave transmitted by alternating compression and rarefaction of regions in a material, where the density and pressure are higher or lower than normal. Sound waves can travel in air, as well as in liquids and solids. Human ears can distinguish sound of frequency from about 20 Hz to 20,000 Hz.

stationary wave

The superposition of two waves of equal amplitude, frequency and wavelength, but traveling in opposite directions. At some fixed locations in a stationary wave (anti-nodes), the two waves are in phase and added up, resulting in maximum disturbances. At some fixed locations in stationary wave (nodes), the two waves are out of phase and cancelled out. resulting in minimum disturbances.

transverse wave

For transverse waves, the direction of disturbance is perpendicular to the direction of wave propagation. Electromagnetic waves and water waves are examples of transverse waves

travelling wave

Waves in which the disturbances can travel from one region in space to another.

trough

Troughs refers to the positions in a wave where the distance of the particles (or disturbance) from their equilibrium positions reaches the most negative value.

ultrasonic wave

The nature of ultrasonic waves is the same as sound waves. However the frequency of an ultrasonic wave is above 20,000 Hz so that human beings cannot hear it.

wave

Motions or disturbances propagating in space that carry energy. Some waves travel in media (e.g. sound waves), while others can travel in vacuum (e.g. electromagnetic waves). During propagation, the part of the media involved in the wave is only vibrating along its equilibrium position. It does not move away permanently from the equilibrium position.

wave velocity

The velocity of propagation of a wave. It is equal to the velocity of a wavefront. The relation between the velocity , the frequency , and the wavelength of a wave is .

wavelength

The distance between two neighbouring similar points (e.g. two neighbouring crests) in a wave is called wavelength.