Reflection, Absorption & Transmission

Reflection of Waves

Reflection occurs when a wave bounces off a surface instead of passing through it. This happens with both sound waves and light waves travelling through air, fluids, or solids.

The key rule for reflection is that the angle of incidence (the angle between the incoming wave and the normal to the surface) is equal to the angle of reflection (the angle between the reflected wave and the normal).

This means if a sound wave or light wave hits a flat surface at 30°, it will reflect away at 30° on the other side of the normal line.

For example, when you shout towards a cliff, the sound waves reflect back, creating an echo. The waves bounce off the cliff surface at the same angle they arrived.

Reflection is important in many everyday situations:

• Sound reflection helps us hear echoes in valleys or caves.
• Reflection of light waves allows us to see images in mirrors.

The law of reflection applies regardless of the wave type, as long as the surface is smooth compared to the wavelength.

For instance, if a sound wave hits a flat wall at an angle of 45°, it reflects off at 45°:

Angle of incidence = 45° = Angle of reflection

Absorption of Waves

Absorption happens when the energy of a wave is taken in by the material it hits. Instead of bouncing off or passing through, the wave’s energy is absorbed and converted into other forms, usually heat.

The amount of absorption depends on the properties of the material:

• Dark, rough, or soft surfaces tend to absorb more wave energy.
• Light, shiny, or smooth surfaces reflect more and absorb less.

For example, a thick curtain absorbs sound waves, reducing echo in a room, while a hard tiled floor reflects sound, making the room noisier.

Absorption reduces the intensity of waves that continue after hitting a surface, which is why materials like carpets or curtains are used to dampen sound in theatres or studios.

In everyday life, absorption affects how warm or cool a surface feels. Dark surfaces absorb more infrared radiation (heat), making them warmer in sunlight.

Transmission of Waves

Transmission occurs when waves pass through a material or medium. The wave continues travelling but may change speed and direction depending on the properties of the material.

The amount of transmission depends on:

• The type of medium (solid, liquid, gas)
• The density of the medium
• How transparent or opaque the material is to that wave

For example, sound waves travel faster in solids than in air because particles are closer together, allowing vibrations to pass more quickly.

When waves transmit through a new medium, their speed changes, which can cause a change in direction (refraction). However, refraction is covered in another topic, so here we focus on the fact that transmission means waves pass through rather than bounce back or get absorbed.

For instance, when sound travels from air into water, it transmits through the water but at a different speed.