Waves 1.2 Flashcards
(16 cards)
Behaviour of Waves at Boundaries
When wave is incident at a boundary between two media…
- Some of the wave transmitted
- Some of the wave reflected
Proportion of wave reflected depends on…
- Relative difference between acoustic characteristics
- Relative difference between impedance of the two media
Greater difference in acoustic impedance = greater amplitude of reflected wave
Phase Change at Boundaries
Reflected from a medium of greater impedance –> 180 phase change
Reflected from a medium of lower impedance –> No phase change
Reflection and Transmission in Strings:
Less dense to more dense
- 180 phase change
- Transmitted slower than reflected
- Transmitted has smaller wavelength than reflected
- Speed and wavelength of reflected same as speed and wavelength of incident
Reflection and Transmission in Strings:
More dense to less dense
- No phase change
- Transmitted pulse travelling faster than reflected pulse
- Transmitted pulse has larger wavelength than reflected pulse
- Speed and wavelength of reflected pulse same as speed and wavelength of incident pulse
Reflection, Transmission and Absorption of Sound
Obeys laws of reflection similar to light:
- Angle of incidence equal to angle of reflection
- Incident ray, reflected ray and normal all lie in the one plane
Sound reflected from concave surface –> converges towards focal point where sound energy concentrated
Sound reflected from convex surface –> diverges
Regular Reflection
Occurs at a flat surface and is responsible for echoes and reverberation
Refraction (Definition)
The bending of a wave as it passes from one medium into another with different acoustic impedance (sound) or difference optical densities (light)
Occurs due to a change in speed of the wave as it passes obliquely into a new medium
Refraction
Speed reduced –> Bend towards normal
Speed increases –> Bend away from normal
Frequency of wave is constant and does not change
v = f 𝜆
Wavelength directly proportional to speed of wave
Effect of Temperature
Speed of sound in air increases with temperature
Temperature inversion
A region where the temperature is increasing with height
Diffraction (Definition)
The spreading of a wave about a barrier (or through an aperture)
Effect of Wavelength on Diffraction
Diffraction increases with increase in wavelength
Effect of Aperture Size on Diffraction
Diffraction increases with decrease in aperture size
Effect maximised when aperture is approximately the same size as the wavelength
Huygens’ Principle
When a wave moves through a medium, the particles in that medium act as secondary sources
Particles oscillate with same frequency generating circular or spherical wavefronts
Wavefronts overlap and combine to form resultant wavefront
Circular waves are able to spread out around the obstacle
Intensity of diffracted wavefronts decrease as fewer wavelets contribute to this part of wave
Diffraction of Sound
Audible sound has relative long wavelength –> readily diffracted about obstacles and through apertures
To diffract visible light, apertures comparable to wavelength of light must be used
Diffraction of Electromagnetic Radiation
Degree of diffraction of electromagnetic radiation depends upon wavelength of respective bands of radiation
e.g. Low frequency AM radio stations (1000kHz) can be detected over long distances due to long wavelength (300m)
High frequency FM stations (96MHz) have shorter wavelengths (3m)
