# 6.1.7 Reflection, Absorption & Transmission

### Reflection, Absorption & Transmission

• 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

### Reflection Ray Diagrams

• 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

#### Exam Tip

When drawing ray diagrams, a simple line with an arrow is enough to represent the wave.

You do not need to draw the wavefronts unless asked to do so! ### Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.
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