Required Practicals: Waves

The AQA A-Level Physics specification includes several required practicals related to waves. You must understand the experimental methods, be able to describe the apparatus, explain sources of uncertainty, and perform calculations from experimental data. These practicals are assessed in Paper 3 and the Practical Endorsement.

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Required Practical: Measuring Wavelength Using Young's Double Slits

Aim

To measure the wavelength of laser light using a double slit interference pattern.

Apparatus

• Laser (e.g., red He-Ne laser, λ ≈ 633 nm)
• Double slit (known slit separation s, typically 0.1–0.5 mm)
• Screen (white card or wall)
• Metre ruler or measuring tape
• Ruler with mm divisions or travelling microscope

Method

1. Set up the laser to shine through the double slit onto the screen.
2. Measure the distance D from the double slit to the screen using a metre ruler. Use a large D (1–2 m) to produce widely spaced fringes.
3. Observe the interference pattern on the screen — evenly spaced bright and dark fringes.
4. Measure the distance across several fringes (e.g., measure across 10 bright fringes).
5. Calculate the fringe spacing: w = total distance / number of fringe spacings.
6. Calculate the wavelength using w = λD/s, so λ = ws/D.

Safety

• Never look directly into the laser beam.
• Avoid directing the beam towards other people.
• Place a "Laser On" warning sign on the door.
• Use a screen to terminate the beam.
• Avoid reflective surfaces that could redirect the beam.

Sources of Uncertainty

• Slit separation s: the main source of uncertainty, as it is very small and may not be accurately known. Measure using a travelling microscope if possible.
• Screen distance D: measure from the slits to the screen, ensuring the ruler is parallel to the beam. Uncertainty ≈ ±1 mm.
• Fringe spacing w: measure across many fringes and divide to reduce percentage uncertainty.

Worked Example 1 — A student measures across 12 bright fringes and obtains a distance of 42.0 mm. The slit separation is 0.40 mm and the screen distance is 2.80 m. Calculate the wavelength.

Number of fringe spacings = 12 − 1 = 11

w = 42.0/11 = 3.818 mm = 3.818 × 10⁻³ m

λ = ws/D = (3.818 × 10⁻³ × 4.0 × 10⁻⁴)/2.80

λ = (1.527 × 10⁻⁶)/2.80 = 5.45 × 10⁻⁷ m = 545 nm

This is consistent with green laser light.

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Required Practical: Measuring Wavelength Using a Diffraction Grating

Aim

To measure the wavelength of light using a diffraction grating of known line spacing.

Apparatus

• Light source (laser or discharge lamp with colour filter)
• Diffraction grating (known number of lines per mm)
• Screen or spectrometer with angular scale
• Metre ruler
• Protractor or goniometer (for angle measurement)

Method

1. Set up the light source to shine through the diffraction grating.
2. Observe the diffraction pattern — sharp maxima at specific angles.
3. Measure the angle θ from the central maximum (zero order) to the first-order maximum.
4. For a screen: measure the distance x from the central maximum to the first-order maximum, and the distance D from the grating to the screen. Then tan θ = x/D, so θ = arctan(x/D).
5. Calculate: d sin θ = nλ, so λ = d sin θ/n.

Worked Example 2 — A grating with 300 lines per mm is used. The first-order maximum is observed at 10.2° from the central maximum. Calculate the wavelength.

d = 1/(300 × 10³) = 1/(3.00 × 10⁵) = 3.33 × 10⁻⁶ m

λ = d sin θ/n = (3.33 × 10⁻⁶ × sin 10.2°)/1

sin 10.2° = 0.1771

λ = 3.33 × 10⁻⁶ × 0.1771 = 5.90 × 10⁻⁷ m = 590 nm

This is consistent with the sodium yellow doublet.

Advantages of a Grating Over a Double Slit

• Sharper, more defined maxima — easier to measure angles precisely.
• Multiple orders available — can take measurements at n = 1, 2, 3 and average.
• Higher angular dispersion — better separation of closely spaced wavelengths.

Worked Example 3 — The same grating gives a second-order maximum at 21.0°. Calculate the wavelength and compare with the first-order result.

λ = d sin θ/n = (3.33 × 10⁻⁶ × sin 21.0°)/2

sin 21.0° = 0.3584

λ = (3.33 × 10⁻⁶ × 0.3584)/2 = (1.193 × 10⁻⁶)/2 = 5.97 × 10⁻⁷ m = 597 nm

Average of both measurements: (590 + 597)/2 = 594 nm

Using two orders improves the reliability of the result.

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Required Practical: Measuring the Speed of Sound

Aim

To determine the speed of sound in air using stationary waves.

Method 1: Resonance Tube (Closed Pipe)

Apparatus: Glass tube partially submerged in water, tuning fork of known frequency f.

1. Strike the tuning fork and hold it above the open end of the tube.
2. Raise the tube out of the water slowly, increasing the length of the air column.
3. Listen for the first resonance — a loud increase in sound volume. Record the air column length L₁.
4. Continue raising the tube to find the second resonance at length L₂.
5. The distance between successive resonances corresponds to half a wavelength:

λ/2 = L₂ − L₁, so λ = 2(L₂ − L₁)

6. Calculate the speed of sound: v = fλ