Understanding Wave Motion: Types, Properties, and Phenomena
Wave Motion
Wave motion is the propagation of a disturbance of some physical magnitude through space. In wave motion, there is transportation of energy and momentum, and the particles successively reached by the wave vary their state.
Wave Types
Waves are classified by:
Nature
- Mechanical: These waves need a material medium for the disturbance to propagate (e.g., sound waves, water waves).
- Electromagnetic: These waves do not need a material medium and can propagate in a vacuum (e.g., light rays, X-rays).
Propagation
- Longitudinal: The direction of propagation coincides with the direction in which the disturbance occurs (e.g., sound waves).
- Transverse: The direction of propagation is perpendicular to the direction in which the disturbance occurs (e.g., seismic waves, waves on a rope).
Propagation Medium
Waves can propagate in one dimension (string), two dimensions (water), or three dimensions (sound, light).
Damping of Waves
Damping refers to the decrease in amplitude of a wave. A wave is damped as it moves due to two reasons:
- Absorption: Energy dissipation due to the medium, increasing its temperature. Loss of energy is associated with loss of range.
- Attenuation: In spherical waves, energy spreads in all directions, distributing energy among a greater number of particles as the wave progresses.
Sound
Sound is produced by the vibration of an emitting source. Sound waves are mechanical and longitudinal. Humans can hear frequencies between 16 and 20,000 Hz. Frequencies above 20 kHz are ultrasound.
Qualities of Sound
- Sound Intensity: The amount of auditory sensation produced by a sound, perceived as strong or weak. For the same frequency, greater intensity means greater wave amplitude.
- Tone: Allows us to distinguish between bass and treble and is related to frequency. Higher frequency is perceived as acute, lower frequency as serious. The human hearing range is approximately 16 Hz (frequency threshold) to 20 kHz (maximum perceptible frequency).
- Timbre: Differentiates sounds emitted by different sources. The difference is perceived by the varying complexity of the wave generated by each emitting source. The fundamental frequency (first harmonic) is accompanied by a set of vibrations of multiples of the fundamental frequency, called harmonics.
Huygens’ Principle and Diffraction
A wavefront is the surface formed by all points reached by a wave at the same time. All points on a wavefront have the same phase, i.e., the same state of vibration. The lines perpendicular to the wavefront at each point are called rays.
Huygens’ principle states that each point on a wavefront acts as a source emitting secondary waves whose envelope is the new wavefront. This principle applies to mechanical waves and was later shown to be applicable to electromagnetic waves as well.
Diffraction
Diffraction is a consequence of Huygens’ principle and states that waves bend around corners and edges of obstacles. Wave diffraction can be observed when the wave encounters an obstacle whose size is on the same order of magnitude as the wavelength. An obstacle of slits with an adequate width is called a diffraction grating.
Diffraction and interference often occur together. Waves interfere because they are diffracted by the obstacle. Diffraction is explained by Huygens’ principle, since each point of the obstacle, after being hit by the wave, becomes a secondary source emitting waves. The interference between these secondary waves produces diffraction patterns.
Refraction and Reflection of Waves
Wave phenomena can be studied from Huygens’ principle, considering each point reached by the wave as a new emitting source.
Refraction
Refraction is the change in the direction of wave propagation when passing from one medium to another. If a medium does not allow the transmission of a wave (opaque medium for that wave):
Laws of Refraction:
- The incident wave, the normal, and the refracted wave are in the same plane.
Reflection
Reflection is the change in the direction of wave propagation when it encounters the boundary separating two different media. After reflection, the wave continues its propagation in the same medium (e.g., light reflected in a mirror).
Laws of Reflection:
- The direction of incidence of the wave, the direction of reflection, and the normal to the reflecting surface are in the same plane, called the plane of incidence.
Doppler Effect
The Doppler effect is a wave phenomenon that occurs when there is relative motion between a wave emitter and an observer. The frequency perceived by the observer is different from the frequency emitted by the source.