Wave behaviours—reflection, refraction and diffraction Flashcards
Mechanical waves transfer energy through a medium, but there will be times when that medium physically ends. A change in the physical characteristics in the same medium, such as density and temperature, can act like a change in medium. When the medium ends, or changes, the wave doesn’t just stop. Instead, the energy that the wave is carrying will:
Reflection
Refraction
Absorption
Reflection
When a transverse wave pulse reaches a hard surface, the wave is bounced back or reflected.
Reflection characteristic
Wave pulse moves along string and hits post
String exerts upwards force on post
Due to newtons third law, the fixed post exerts equal opposite force on string which inverts the pulse and sends its reflection back to left on bottom side of string
Free Boundary
When a wave pulse hits the end of a rope thats free to move, the pulse returns with no change of phase, reflected pulse is same as incident pulse
When the transverse wave pulse is reflected, the amplitude of the reflected wave is not the same as the original, why?
Part of the energy of the wave is absorbed by the post and some is transformed into heat energy and some travels through the post
When a transfer wave pulse is sent from a light rope to a heavy rope
Part of the wave is reflected and part is transmitted to heavier rope
As the second rope is heavier, a smaller part is transmitted and more is reflected back
The more rigid and/or dense a wall is
the more energy will be reflected and less is absorbed (some energy will always be absorbed or transferred to the second medium)
Wave fronts
when close to source = curved
when far from source = straight = plane wave
Ray
The direction of motion of any wave front can be represented by a line drawn perpendicular to the wave front and in the direction the wave is
Law of reflection
The law of reflection states that the angle of reflection (θr), measured from the normal, equals the angle of incidence (θi) measured from the normal
θi = θr
Law of reflection is true for an surface whether its
straight, curved or irregular, normal is drawn perpendicular to the surface at the point of contact of the rays
diffuse reaction
when wave fronts meet an irregular , rough surface, the resulting reflection can be spread over a broad area bc each point on the surface reflects the portion of the wave front reaching it in a diff direction
echo
when a sound wave thats reflected at a wall reaches us in a delayed time, u can hear an echo and its heard as a seperate sound
reverberation
in smaller rooms, the waves are reflected several times at diff walls and they overlap and produce reverberation which sounds like a longer sound
echoes typically occur in
large rooms with bare walls and hard surfaces
how to reduce echo (acoustic engineering)
acoustic foam sheets, rugs, textiles on wall to absorb sound and objects to diffuse the reflections of the sound waves
refraction
when a wave crosses the boundary and theres a change in velocity and wavelnegth, the transmitted wave travels in a different direction to the original wave
why do wave fronts refract
the part of the wave front that reaches the boundary first slows down (or speeds up) while the remainder of the wave front still in the original medium continues at its original speed
speed of sound depends on
stiffness and elasticity
(inertia or density)
elasticity
a measure of the tendency of a medium to return to its original shape after it has been bent compressed or stretched
when a sound propagates through a medium in a series of compression and rarefactions (longitudinal)
it travels faster in a more elastic medium
solid -> liquid -> gas
if theres a change in state as a sound wave travels from one medium to the next
there will be a significant effect on the speed
density
the more dense the medium is, the slower the speed because the greater mass per unit volume makes it more difficult for a given force to move the medium so the vibrations take more time
temperature
increasing temp, reduces density (no change elasticity), increases speed of sound
decreasing temp, increase density, decreasing speed of sound
refraction can occur between hot and cold medium
refraction of sound when entering hotter medium (wave front speed up)
v2 > v1
λ2 > λ1
f2 = f1
∠r > ∠i
refraction of sound when entering colder medium
v1 > v2
λ1 > λ2
f1 = f2
∠i > ∠r
