5.30 Transmission & Reflection of Waves

• When waves are incident on the interface between two different media, they are either transmitted or reflected
  • 'Incident on' simply means 'to meet'
  • The interface is also called the boundary between media
  • Transmitted means to pass through

• When the media have similar densities the energy of the wave is mostly transmitted
• When the media have different densities most of the energy is reflected

Reflected waves in use

• Uses of reflected waves include:
  • Medical x-rays
  • Sonar
  • Ultrasound scans

Transmitted waves in use

• In the above examples the waves have to be transmitted through one medium first, before they are reflected
  • X-rays are transmitted through soft tissue
  • Sonar is transmitted through air or water
  • Ultrasound is transmitted through a gel of similar density to the skin so that it reaches the tissues inside the body

Reflection

• Reflection occurs when:

A wave hits a boundary between two media and does not pass through, but instead stays in the original medium

• The law of reflection states:

The angle of incidence = The angle of reflection

Reflection of a wave at a boundary

• Some of the wave may also be absorbed or transmitted
  • Echos are examples of sound waves being reflected off a surface

• Flat surfaces are the most reflective
  • The smoother the surface, the stronger the reflected wave is

• Rough surfaces are the least reflective
  • This is because the light scatters in all directions

• Opaque surfaces will reflect light which is not absorbed by the material
  • The electrons will absorb the light energy, then reemit it as a reflected wave

Transmission

• Transmission occurs when:

A wave passes through a substance

• For light waves, the more transparent the material, the more light will pass through
  • Transmission can involve refraction but it is not exactly the same
  • For the process to count as transmission, the wave must pass through the material and emerge from the other side

• When passing through a material, waves are usually partially absorbed
  • The transmitted wave may have a lower amplitude because of some absorption
    • For example, sound waves are quieter after they pass through a wall

    

    When a wave passes through a boundary it may be absorbed and transmitted