Reflection, Transmission, Absorption

The Physics Hypertextbook™
© 1998-2008 by Glenn Elert -- A Work in Progress
All Rights Reserved -- Fair Use Encouraged

prev | up | next

---

Discussion

Sound, for example …

• Acoustic impedance, Z = ρv, is important when calculating the amount of reflection and transmission at an interface. For the reflected portion, notice that when Z₂ < Z₁, the sign of the reflected portion is negative, which indicates a change of phase.

  R/A0 = (Z2 - Z1)/(Z2 + Z1)		(reflected pressure amplitude)
  T/A0 = 2Z2/(Z2 + Z1)		(transmitted pressure amplitude)

Summary

• Boundary & Interface
  • A boundary is the end, edge, or face of a finite medium
    • A "rigid" boundary is known as a fixed end or closed end.
    • A "loose" boundary is known as a free end or open end.
  • An interface is a boundary shared by two media.
    • To a wave entering a medium with a slower wave speed,the interface is more like a fixed end than a free end.
    • To a wave entering a medium with a faster wave speed, the interface is more like a free end than a fixed end.
• Reflection, Transmission, Absorption
  • When a wave is incident on a boundary or interface it is partially reflected, partially transmitted, and partially absorbed.
    • Energy and momentum are conserved in the process
    • An echo is a reflected wave, especially a reflected sound wave
• Properties of Materials
  • The degree of reflection, transmission, and absorption depends upon the two media and the frequency of the incident wave.
  • A material is …
    • opaque if it prevents the transmission of a wave,
    • transparent if it allows the transmission of a wave, or
    • absorbent if it prevents both the reflection and the transmission of a wave.
• Wave Characteristics at an Interface
  • Amplitude is related to energy.
    • E_reflected + E_transmitted = E_incident,
      conservation of energy, no absorption
    • E_reflected + E_transmitted < E_incident,
      work-energy theorem, absorption present
  • Speed is affected by medium.
    • The reflected wave speed is the same as the incident wave speed.
    • The transmitted wave speed may be different from incident wave speed.
  • Frequency is determined at the source.
    • Frequency and period remain constant upon transmission and reflection.
  • Wavelength is affected by speed and frequency.
    • Wavelength is directly proportional to speed.
    • Wavelength is inversely proportional to frequency.
  • Phase …
    • remains constant on transmission, but
    • may be affected by reflection.
      • Reflection from a free end leaves phase unchanged. (The wave is reflected in phase.)
      • Reflection from a fixed end inverts the wave. (The wave is reflected out of phase.)

Problems

practice

1. Write something.
   • Answer it.
2. Write something else.
   • Answer it.
3. Write something different.
   • Answer it.
4. Write something completely different.
   • Answer it.

numerical

1. A sound wave traveling through air at STP has a frequency of 1000 Hz.
   1. What is its period?
   2. What frequency would this same wave have in water (speed of sound in water, 1500 m/s)?
   3. What frequency would this same wave have in steel (speed of sound in steel, 5000 m/s)?

Resources

• no resources for this topic

---

prev | up | next

Another quality webpage by Glenn Elert

home | contact bent | chaos | eworld | facts | physics