Wave Behaviour

Waves transfer energy without transferring matter. When waves meet surfaces, change depth or medium, or pass gaps, they change direction and shape. This section explains reflection, refraction, and diffraction using water, sound, and light.

Key Idea

Wave speed, frequency, and wavelength are linked by \\( v = f\,\lambda \\). Frequency $f$ is set by the source. When a wave enters a new medium (or different water depth), $f$ stays the same, but speed $v$ and wavelength $\lambda$ can change.

Reflection

Reflection happens when a wave bounces off a surface (like a wall or mirror). The angle of incidence equals the angle of reflection, measured from the normal (an imaginary line at right angles to the surface). This rule works for water waves, light, and sound.

• Sound reflection creates an echo.
• Speed and frequency do not change on reflection in the same medium; only direction changes.

Refraction (bending due to speed change)

Refraction is the bending of waves when they move into a region where they travel at a different speed (for water waves, a change in depth changes speed).

• If a wave slows down, it bends towards the normal. If it speeds up, it bends away from the normal.
• Frequency stays the same; wavelength changes because $v = f\lambda$.
• Example you know: a straw in water looks “bent” because light refracts.

Diffraction (spreading)

Diffraction is the spreading of waves when they pass through a gap or around an edge.

• Diffraction is greatest when the wavelength is similar to the gap width.
• Wide gap (many wavelengths across): little spreading. Narrow gap (about one wavelength): strong spreading.
• Low-pitch (long-wavelength) sound bends round doorways better than high-pitch sound. Light has a very short wavelength, so it rarely diffracts noticeably through doorways.

Ripple Tank Demonstrations

• Reflection: Straight wavefronts hit a barrier and bounce off; angles in and out are equal.
• Refraction (change in depth): Over a shallower plate, waves slow and wavelengths become shorter; wavefronts bend towards the normal.
• Diffraction through a gap: A narrow slit makes circular spreading; a wide slit gives a less spread-out beam.
• Diffraction at an edge: Wavefronts curl into the shadow region past a single barrier edge.

Common Misconceptions

• “Frequency changes in refraction.” It does not; speed and wavelength change.
• “Bigger amplitude means faster waves.” Amplitude affects energy/loudness, not speed.
• “Diffraction needs a tiny gap only.” It needs a gap comparable to the wavelength.