Wave behaviour at boundaries

Waves, such as light or sound waves, act in specific ways when they reach a boundary between two different materials. A boundary is the point where one material ends and another begins, like where air meets glass. At these points, waves can be absorbed, transmitted, or reflected, and what happens depends on factors like the wave's wavelength (the distance between two wave crests) and the properties of the materials involved, such as their density or how well they let waves pass through.

This behaviour helps explain everyday events, like why you hear an echo or see your reflection in a mirror. Understanding these options shows how waves interact with the world around them.

Possible outcomes for waves at boundaries

• Absorption - The wave's energy is taken in by the material, converting it to another form like heat, so the wave stops moving forward.
• Transmission - The wave passes through the boundary into the new material, often changing speed or direction slightly.
• Reflection - The wave bounces back off the boundary into the original material, like light reflecting from a mirror.

The choice between these outcomes relates to how well the wavelength matches the material's properties. For example, a short wavelength might be absorbed by a dense material, while a longer one could be transmitted.

The law of reflection

When a wave is reflected at a boundary, it follows a specific rule called the law of reflection. This law states that the angle of incidence (the angle at which the incoming wave hits the surface) equals the angle of reflection (the angle at which the wave bounces away). Both angles are measured from a line called the normal, which is an imaginary line drawn perpendicular (at a 90-degree angle) to the reflecting surface at the point where the wave hits.

This law applies to all types of waves that reflect, helping predict where the wave will go after bouncing. It works because the wave's energy is conserved during reflection, leading to equal angles on either side of the normal.

Drawing ray diagrams for reflection

Ray diagrams are simple drawings that show the path of waves using straight lines called rays. These diagrams help visualise how waves reflect according to the law of reflection. To create one, you represent the wave as a ray (a straight arrow showing direction) and include the reflecting surface, the normal, and the angles.

Steps to draw a ray diagram for reflection:

1. Draw the reflecting surface as a straight line.
2. At the point where the ray will hit, draw the normal as a dashed line perpendicular to the surface.
3. Draw the incident ray approaching the surface at an angle to the normal.
4. Measure the angle of incidence between the incident ray and the normal.
5. Draw the reflected ray leaving the surface at the same angle on the other side of the normal, ensuring the angle of reflection matches the angle of incidence.

These steps ensure the diagram accurately shows the law of reflection in action, making it easier to understand wave paths.

Specular and diffuse reflection

Not all reflections are the same—they depend on the smoothness of the surface. Specular reflection happens on smooth surfaces, where waves bounce off in a single, predictable direction, creating clear images. In contrast, diffuse reflection occurs on rough surfaces, where waves scatter in multiple directions, preventing clear images from forming.

This difference explains why a polished mirror shows your face clearly, while a bumpy wall does not. The type of reflection affects how we see objects and how light spreads in a room.

Comparison of specular and diffuse reflection: