Wave Behaviour
Section: Waves | Syllabus: Cambridge IGCSE Physics (0625)
Reflection, Refraction and Diffraction
When a wave meets a barrier, or crosses a boundary into a different medium, it can change direction. Waves can undergo reflection, refraction and diffraction.
Reflection
- Reflection occurs when a wave bounces back after meeting a plane (flat) surface
- Wavelength, frequency and speed are unchanged by reflection
- Formula:
- i = r
- Where:
- i = angle of incidence
- r = angle of reflection
- both measured from the normal - a line drawn at right angles to the surface at the point where the wave strikes it
Reflection at a plane surface: angle of incidence equals angle of reflection
Refraction
- Refraction is the change in direction of a wave caused by a change in its speed as it crosses a boundary into a different medium
- When a wave slows down, it bends toward the normal; when it speeds up, it bends away from the normal
- Wavelength changes in proportion to the change in speed; frequency stays the same
- A wave crossing the boundary along the normal changes speed and wavelength but does not change direction
Refraction: a wave bends toward the normal as it slows down
Diffraction
- Diffraction is the spreading out of waves as they pass through a narrow gap, or as they pass the edge of an obstacle
- Wavelength, frequency and speed are unchanged by diffraction - only the direction in which the wave energy spreads changes
Diffraction of waves through a gap and around an edge
Investigating Wave Behaviour with a Ripple Tank
A ripple tank is a shallow, transparent tank of water used to generate and observe water waves. A lamp above the tank shines through the water onto a screen or paper below: wave crests act like converging lenses and produce bright lines, while troughs act like diverging lenses and produce dark lines, so the pattern of wavefronts can be seen on the screen.
A ripple tank set-up
Reflection at a Plane Surface
- Setup: a straight barrier is placed in the tank and straight (plane) wavefronts are directed at it
- Observation: the wavefronts reflect off the barrier so that the angle of incidence equals the angle of reflection; wavelength and frequency stay the same
Reflection of plane wavefronts at a straight barrier
Refraction Due to a Change in Depth
- Setup: a glass or plastic sheet is placed under the water on one side of the tank, creating a shallower region
- Observation: waves travel more slowly in the shallower region, so the wavelength decreases there; if the wavefronts meet the boundary at an angle, they bend toward the normal on entering the shallow region
Refraction as waves cross from deep to shallow water
Diffraction Due to a Gap
- Setup: a barrier with a gap is placed in the tank and straight wavefronts are directed at the gap
- Observation: the wavefronts spread out on the far side of the gap; wavelength and frequency are unchanged
Diffraction of waves through a gap
Diffraction Due to an Edge
- Setup: a single straight barrier, with no gap, is placed part-way across the tank so that it has one edge in the open water
- Observation: the wavefronts bend around the edge of the barrier and spread into the region directly behind it, instead of continuing in a straight line only
Diffraction of waves around the edge of a barrier
Extended Only
Effect of Wavelength and Gap Size on Diffraction Through a Gap
- When the gap width is similar to the wavelength, diffraction is greatest - the waves spread out strongly into a semicircular pattern beyond the gap
- When the gap is much wider than the wavelength, diffraction is much less noticeable - the waves mostly continue straight through, spreading only slightly at the edges
Effect of Wavelength on Diffraction at an Edge
- A longer wavelength diffracts (bends) more strongly around an edge, spreading further into the region behind the obstacle
- A shorter wavelength diffracts less, producing a sharper-edged shadow behind the obstacle
Summary: Reflection, Refraction and Diffraction
| Behaviour | What Happens | Key Point |
|---|---|---|
| Reflection | Wave bounces back off a surface | Angle of incidence = angle of reflection |
| Refraction | Wave changes direction at a boundary | Caused by a change in speed; frequency unchanged |
| Diffraction | Wave spreads through a gap or around an edge | Greatest when gap size is similar to the wavelength |
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