Diffraction and interferenceIB MYP Physics: Revision notes
Section 1
Diffraction: waves spreading out
Diffraction is the spreading out of waves when they pass through a gap or move past the edge of an obstacle. The waves bend into the region behind the gap or obstacle, called the shadow region.
Diffraction happens to all waves: water waves, sound and light. The wavelength, frequency and speed of the wave do not change. Only the direction of the waves (and so the pattern of the wavefronts) changes.
Section 2
The size of the gap matters
The amount of diffraction depends on the width of the gap compared with the wavelength.
- A gap about the same as the wavelength gives the greatest diffraction. The waves spread out in almost semicircles.
- A gap much wider than the wavelength gives very little diffraction. The waves mostly carry straight on, with only slight bending at the edges.
- A narrower gap (compared with the wavelength) gives more spreading.
The same idea works for obstacles. Waves with a long wavelength bend round an obstacle much more than waves with a short wavelength.
Diffraction does not change the wavelength. The waves spread out, but each wave is the same length as before.
Section 3
Ripple tank diagrams
A ripple tank shows diffraction clearly. A vibrating bar makes straight water waves. In diagrams, the waves are drawn as parallel lines (wavefronts) that are one wavelength apart.
When drawing a ripple tank diagram:
- Draw straight, evenly spaced wavefronts before the gap.
- Keep the spacing between wavefronts (the wavelength) the same after the gap.
- For a narrow gap, draw wavefronts after the gap as curved, circular arcs spreading in all directions.
- For a wide gap, draw wavefronts that stay straight in the middle and curve only at the edges.
In a diffraction diagram, the gap between wavefronts must look the same before and after the gap. Examiners check this.
Section 4
Diffraction of sound and light
Sound has a wavelength of about 0.02 m to 17 m, similar in size to doors and windows. So sound diffracts a lot through doorways and you can hear someone talking round a corner.
Visible light has a very tiny wavelength (about 5 × 10⁻⁷ m), far smaller than everyday gaps. So light diffracts hardly at all through a door and you cannot see round the corner. Light only shows diffraction through gaps as tiny as the wavelength itself.
Low-pitched sounds (long wavelength) diffract more than high-pitched sounds (short wavelength). This is why you hear the deep thump of music from a neighbour more than the high notes.
Section 5
Superposition and interference
When two waves meet, their displacements add together. This is called superposition. The result is called interference.
- Constructive interference: a crest meets a crest (or a trough meets a trough). The waves are in step and the displacements add, giving a larger displacement (a louder sound or a brighter light).
- Destructive interference: a crest meets a trough. The waves are out of step and the displacements cancel, giving a smaller displacement or none (a quiet place or a dark place).
Two loudspeakers playing the same note can make loud and quiet places in a room. At the loud places the waves arrive in step. At the quiet places they arrive out of step. Noise-cancelling headphones make a second wave that is out of step with the noise, so the two cancel.
Destructive interference does not destroy the energy of the waves. The energy is just not delivered at that point; it goes to the places where there is constructive interference.
Must know
- Diffraction is the spreading of waves at a gap or edge.
- It is greatest when the gap is about the same size as the wavelength.
- Wavelength, speed and frequency do not change in diffraction.
- Sound diffracts easily; light hardly diffracts through everyday gaps.
- Superposition: displacements add. Crest and crest give constructive interference; crest and trough give destructive interference.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Diffraction and interference
- A harbour is protected by a long concrete breakwater with a single gap 15 m wide. Ocean swell with a wavelength of about 20 m arrives at the breakwater with its wavefronts parallel to the wall.Describe what happens to the waves as they pass through the gap, and state what happens to their wavelength.2 marks
- Noise-cancelling headphones have a tiny microphone that detects the steady low hum of an aeroplane engine. A circuit then makes a loudspeaker inside each ear cup produce a second sound wave with the same frequency and the same amplitude as the hum, timed carefully against the engine noise.Explain how the headphones reduce the noise heard by the passenger.2 marks
- A class uses a ripple tank to investigate how the width of a gap affects the spreading of water waves. A vibrating bar produces straight waves of wavelength 2 cm. Two barriers leave a gap in the middle, and the students change the width of the gap between trials. Beyond the gap they measure the angle through which the waves spread out from the straight-on direction. They find that a gap of 8 cm gives a spread of 15°, a gap of 4 cm gives 35° and a gap of 2 cm gives 80°.Identify the independent variable, the dependent variable and one control variable in this investigation.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).