Waves and boundaries (AQA GCSE Physics): Revision Notes
Waves and boundaries
When waves travel from one substance to another, several things can happen at the boundary (the place where two materials meet). Understanding these effects is crucial for explaining how waves behave in different situations.
What happens at boundaries
When any type of wave (sound, light, or water) reaches the boundary between two materials, there are four possible outcomes that can occur. Each of these processes plays an important role in how we experience waves in everyday life.
The Four Boundary Effects:
- Reflected - the wave bounces back
- Refracted - the wave bends as it passes through
- Transmitted - the wave passes straight through
- Absorbed - the wave is taken in and doesn't pass through
Transmission happens when waves successfully pass through a material without being blocked or significantly altered. For example, radio waves can pass through walls and reach your radio receiver, allowing you to listen to music even when you're inside buildings.
Absorption occurs when waves are taken in by a material and don't continue travelling. The material converts the wave energy into other forms, such as heat. Microwaves are absorbed by food in a microwave oven, which is exactly how your food gets heated.
Reflexion at a surface
All types of waves (light, water, and sound) can reflect when they hit a surface. This surface acts as the boundary between two different materials, and the reflexion follows a predictable pattern.
The Law of Reflection
The fundamental rule governing reflexion states that: angle of incidence = angle of reflection (or angle i = angle r)
This law applies to all types of waves and all reflecting surfaces.
Understanding reflexion requires knowing several key terms that describe the geometry of the process:
- Incident ray - the wave coming towards the surface
- Reflected ray - the wave bouncing off the surface
- Normal - an imaginary line at 90° to the surface
- Both angles are measured from the normal line
It's important to remember that both the incident angle and reflected angle are always measured from the normal line, not from the surface itself.
Refraction
Refraction is the bending of a wave when it travels from one transparent material into another. This fascinating phenomenon happens because the wave changes speed as it moves between different substances with different properties.
How Refraction Works
When light moves between materials with different densities:
- The ray bends towards the normal as it enters denser material (like glass)
- The ray bends away from the normal when it leaves denser material and enters less dense material (like air)
The amount of bending depends on how different the two materials are.
Special cases of refraction
There are two important situations where the normal bending behaviour of refraction doesn't occur:
Special Refraction Cases
-
No bending at 90°: When light rays hit a surface at exactly 90° (along the normal), they continue straight through without bending direction.
-
Moving along the normal: When light, sound, or water waves travel along the normal line between materials, their direction doesn't change - they continue straight through.
Real-world applications
Understanding wave behaviour at boundaries helps explain many everyday phenomena that we might otherwise find puzzling.
Worked Example: Infrared Remote Controls
Have you ever noticed that infrared remote controls sometimes work when pointed at walls but not through walls?
What happens:
- Some infrared waves reflect off the wall surface and reach the TV
- The waves that enter the wall are absorbed by the wall material and don't pass through to the other side
Result: You can sometimes control your TV by bouncing the remote signal off a wall, but you can't control it through a wall from another room.
Key Points to Remember:
- Four things can happen at boundaries: reflexion, refraction, transmission, and absorption
- Law of reflection: angle of incidence equals angle of reflexion (i = r)
- Refraction is the bending of waves when they change materials and speeds
- No bending occurs when waves hit at 90° or travel along the normal
- Real examples help explain wave behaviour - like remote controls and microwaves