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Practical skills in core practicals 4 to 6

Edexcel IAL Physics · WPH13 Practical Skills in Physics I

Applying Unit 3 skills to the Unit 2 wave practicals: the speed of sound with an oscilloscope, the vibrating string and the diffraction grating. Revise each method in the Unit 2 lessons first.

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Core practical 4: speed of sound

Reading an oscilloscope: period T = (number of horizontal divisions for one cycle) x time-base setting; f = 1/T. Measure across several cycles to reduce the percentage uncertainty.
Move the microphone through several wavelengths (traces back in phase n times) and divide the distance by n.
Plot λ against 1/f for several frequencies: gradient = v.

Core practical 5: vibrating string

Find resonance by approaching from below and above, and average the two frequencies.
Measure the vibrating length between the bridges with a metre rule; find μ by weighing a measured length of wire (mass / length).
Plot f against 1/L (gradient (1/2) sqrt(T/μ)), or f² against T.

Core practical 6: diffraction grating

Use a large screen distance D (1 to 2 m) to make the spacing large and its percentage uncertainty small.
Measure the distance between the first-order maxima on both sides and halve it: this doubles the measured distance and removes errors in locating the centre.
tan θ = x / D, then λ = d sin θ / n with d = 1 / (lines per metre).
Safety: do not look into the beam or its reflections.

Worked example

A grating has 300 lines per mm. On a screen 1.000 m away, the first-order maxima are 0.380 m apart. Find λ.

x = 0.380 / 2 = 0.190 m; θ = tan^-1(0.190 / 1.000) = 10.76°
d = 1 / (300 x 10³) = 3.33 x 10^-6 m
λ = d sin θ = 6.22 x 10^-7 m

Answer: λ = 622 nm

Key idea: CP4: T = divisions x time base, measure over many cycles and wavelengths; λ against 1/f gives v. CP5: approach resonance from both sides. CP6: large D, measure between maxima on both sides, tan θ = x/D.

Check yourself

One cycle on an oscilloscope covers 4.0 divisions with the time base on 0.50 ms/div. Find the frequency in Hz.
Show answer
500 Hz. T = 4.0 x 0.50 = 2.0 ms; f = 1 / 0.0020 = 500 Hz.
The microphone in core practical 4 is moved 1.72 m while the traces come back into phase 5 times. Find the wavelength in m.
Show answer
0.344 m. λ = 1.72 / 5 = 0.344 m.
Using λ = 0.344 m at a frequency of 1000 Hz, find the speed of sound in m/s.
Show answer
344 m/s. v = fλ = 1000 x 0.344 = 344 m/s.
A grating has 600 lines per mm. On a screen 1.50 m away the two first-order maxima are 1.20 m apart. Find the wavelength in nm, to 3 s.f.
Show answer
619 nm. x = 0.60 m; θ = tan^-1(0.60 / 1.50) = 21.80°; d = 1.67 x 10^-6 m; λ = d sin θ = 619 nm.
A 1.20 m length of wire has a mass of 0.96 g. Find its mass per unit length, in units of 10^-4 kg/m.
Show answer
8.0 x 10^-4 kg/m. μ = 0.96 x 10^-3 / 1.20 = 8.0 x 10^-4 kg/m.
Why measure the distance between the first-order maxima on opposite sides of the central maximum?
- It makes the maxima brighter
- It doubles the measured distance and avoids locating the exact centre
- It changes the wavelength
- It removes the need for a large screen distance
Show answer
It doubles the measured distance and avoids locating the exact centre. A larger measurement has a smaller percentage uncertainty, and the centre of the central maximum is hard to locate.
Why should a period on an oscilloscope be measured over several cycles?
- It increases the frequency
- It removes the need for a time-base setting
- It makes the trace brighter
- It reduces the percentage uncertainty in the period
Show answer
It reduces the percentage uncertainty in the period. The reading uncertainty (about a fraction of a division) is shared over more cycles.

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Last updated 8 November 2026. Spotted a mistake? Tell us.