Waves Revision Notes

Reflection at Interfaces

Reflection is the bouncing back of a wave when it strikes a boundary between two mediums. The angle of incidence (angle between the incoming wave and the normal to the surface) is equal to the angle of reflection (angle between the reflected wave and the normal). This applies to light, sound, and other types of waves. Examples include echoes (sound reflection) and mirrors (light reflection).

Refraction at Interfaces

Refraction is the change in direction of a wave as it passes from one medium to another. This change in direction is caused by a change in the wave's speed as it enters the new medium. When a wave enters a denser medium at an angle, it bends towards the normal; when it enters a less dense medium, it bends away from the normal. This is significant for lenses and prisms. Some of the wave energy is still reflected.

Transmission through Interfaces

Transmission occurs when a wave passes through the boundary into the new material. If a wave is fully transmitted, it means it has passed through without reflection or absorption. In reality, some reflection and absorption almost always occur alongside transmission. The amount of transmission depends on how transparent or permeable the second medium is to that specific type of wave.

Absorption at Interfaces

Absorption is the process by which a material takes in the energy of a wave, converting it into another form, typically thermal energy (heat). When a wave is absorbed, its amplitude decreases, and the wave loses energy. Materials that absorb a lot of light appear dark or opaque. For sound, absorption reduces echoes and reverberation. The absorbed energy is not reflected or transmitted.

Water Wave Evidence

When a floating object, like a duck or a toy boat, is observed on the surface of water where waves are present, it bobs up and down or moves in small circular paths, but it does not travel with the wave across the water. This demonstrates that the water molecules themselves are oscillating in place rather than moving linearly as the wave passes underneath. The energy is transferred, but the medium remains largely in position.

Sound Wave Evidence

Sound waves involve compressions and rarefactions of a medium like air. If air molecules were to travel with the sound, a strong sound source would generate a continuous breeze towards the listener, which does not happen. Instead, tuning forks or loudspeakers cause air particles to vibrate back and forth, transferring energy from one particle to the next. This oscillation, rather than bulk movement, confirms that the air itself does not travel with the sound wave.

Energy Transfer, Not Mass Transfer

A key concept for all waves, including water and sound waves, is that they primarily transfer energy from one point to another without transferring matter over significant distances. The medium particles themselves undergo oscillatory or vibratory motion around a fixed equilibrium position. This is the fundamental difference between wave propagation and the physical movement of a substance.

Understanding Refraction

Refraction is the phenomenon where a wave changes its direction of travel when it passes from one medium to another. This occurs because the wave's speed changes as it enters the new medium. If the wave hits the boundary perpendicular to its surface (along the normal), it will change speed but not direction.

Change in Speed and Direction

When a wave enters a medium where its speed decreases, it bends towards the normal (an imaginary line perpendicular to the surface at the point of incidence). Conversely, if a wave enters a medium where its speed increases, it bends away from the normal. This change in direction is a direct consequence of the wave's speed altering as it traverses the boundary.