Reflection, Absorption & Transmission (AQA GCSE Physics)

• Depending on the densities of the materials on either side of a boundary, a wave may be:
  • Reflected
  • Transmitted
  • Absorbed

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 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

Absorption

• Absorption occurs when:

Energy is transferred from the wave into the particles of a substance

• Waves can be partially or completely absorbed
  • Sound waves are absorbed by brick or concrete in houses

• Light will be absorbed if the frequency of light matches the energy levels of the electrons
  • The light will be absorbed, and then reemitted over time as heat

• If an object appears red, this means:
  • Only red light has been reflected
  • All the other frequencies of visible light have been absorbed

The object is seen as red since the red light is reflected whilst the other colours are absorbed

• Angles are measured between the wave direction (ray) and a line at 90 degrees to the boundary
  • The angle of the wave approaching the boundary is called the angle of incidence (i)
  • The angle of the wave leaving the boundary is called the angle of reflection (r)

• The law of reflection states that these angles are the same:

Angle of incidence (i) = Angle of reflection (r)

Reflection of a wave at a boundary

• When drawing a ray diagram an arrow is used to show the direction the wave is travelling
  • An incident ray has an arrow pointing towards the boundary
  • A reflected ray has an arrow pointing away from the boundary

• The angles of incidence and reflection are usually labelled i and r respectively