Required Practicals 1–2: Waves and Interference

Use a vibration generator, signal generator, string, pulley and hanging masses. Measure the vibrating length between nodes and identify the fundamental or a known harmonic. Tension is approximately the hanging weight mg if pulley friction is small; measure linear density by weighing a known long length of the string.

For the fundamental, vary length while keeping tension and string type fixed, then plot f against 1/L. Alternatively fix L and μ and plot f² against T, or fix L and T and plot f² against 1/μ. Adjust frequency for a clearly defined resonance; record repeated judgements and avoid confusing harmonic number with a change in wave speed. Secure masses and keep hands clear of the driven string.

For double slits, use monochromatic light, known slit separation s, a screen at distance D and fringes of spacing w. Measure across several fringe intervals, then divide by the number of intervals, not the number of bright lines. Keep D much greater than s and measure D from slits to screen.

For a grating, determine spacing from the number of lines per metre, measure the angle from the central maximum to a chosen order, and use both left and right maxima to reduce zero-offset error. If screen offset x and distance D are measured, calculate θ from arctan(x/D); do not use x as the angle. Follow the school laser procedure, keep beams below eye level and avoid specular reflections.

Choose a graph whose gradient has a stated meaning. For f against 1/L, gradient $=\tfrac12\sqrt{T/\mu}$. A straight line near the origin supports the inverse-length model under the controlled conditions, but a limited range cannot prove it universally.

Fringe-spacing uncertainty often dominates a double-slit result. Measure a wider span of fringes, increase D within the available space, and repeat with independent realignment. Increasing the number of readings does not remove a systematic mistake in slit separation.

Try each problem before opening the worked solution. State the physical model, show substitutions with consistent units, and explain what your answer means.