Physics unit guide
Wave behaviour
Simple harmonic motion, wave properties, standing waves, Doppler, interference and diffraction.
Unit 3 asks you to move fluently between three representations of the same thing: an equation, a graph, and a physical picture. Most difficulty here is representational rather than conceptual - students who can read a displacement-time graph and say where the velocity is greatest find the unit straightforward, and those who cannot struggle regardless of how well they understand waves. Standing waves and interference reward drawing before calculating.
Topics in this unit
What gets examined
- Identifying where displacement, velocity and acceleration are maximum and zero in SHM, from a graph or an equation.
- Relating period, frequency, wavelength and speed, and knowing which change when a wave crosses a boundary.
- Drawing standing wave patterns for strings and for open and closed pipes, and counting harmonics.
- Path difference and phase difference, and the conditions for constructive and destructive interference.
- Double-slit and single-slit calculations, and how fringe spacing responds to changing slit separation or wavelength.
- Doppler shift for a source moving towards or away from a stationary observer.
How to revise it
Draw the standing wave before calculating
Mark the nodes and antinodes, count the wavelengths in the length, then substitute. Almost every error in this topic comes from an assumed pattern rather than a drawn one, particularly for closed pipes.
Remember what stays constant across a boundary
Frequency is fixed by the source and never changes; speed and wavelength change with the medium. Half the refraction questions in this unit hinge on that single fact.
Work interference in wavelengths
Constructive when the path difference is a whole number of wavelengths, destructive at half-integers. Converting to wavelengths first makes the condition obvious rather than something to reason about.
Reason about Doppler direction before using the equation
Approaching means a higher observed frequency. Deciding that first tells you which sign you need and catches an inverted answer immediately.
Where marks get lost
- Assuming a closed pipe supports the same harmonics as an open one - it only supports odd ones.
- Changing frequency rather than wavelength when a wave enters a new medium.
- Reading velocity off a displacement-time graph as the value rather than the gradient.
- Mixing up slit separation and slit-to-screen distance in the double-slit equation.
- Giving a Doppler answer that shifts the wrong way for the stated motion.
Practise this unit
Reading about a unit only goes so far. Work through real IB-style questions on it, with mark schemes.