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.
