The Physics
Hypertextbook
Opus in profectus

# Interfaces and Barriers

## 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
• absorbent if it prevents both the reflection and the transmission of a wave
• Wave Characteristics at an Interface
• Amplitude is related to energy.
• Ereflected + Etransmitted = Eincident
conservation of energy, no absorption
• Ereflected + Etransmitted < Eincident
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.)
• Diffraction
• is the bending or spreading of a wave around an obstacle or through an opening.
• is most apparent when the size of the obstacle or opening (a) is on the same order as the wavelength of the wave (λ).

• A shadow is a region behind an obstacle into which a wave does not easily diffract.
• Umbra: a region of total shadow; the source of the wave is completely obscured.
• Penumbra: a region of partial shadow; the source of the wave is partially obscured.
• Sound vs. Light
• Sound waves are millions of times longer than light waves.
• Diffraction is more easily observed for sound than light in everyday experience.
• You can hear around corners, but you can't see around them.
• Light sources cast noticeable shadows.