CAIE A-Level Physics 8.3.4 Double-Slit Interference Equation

Practise using λ = ax/D to calculate fringe spacing, wavelength, slit separation or screen distance with consistent units and geometry.

Exam points

calculate fringe spacing, wavelength, slit separation or screen distance using λ = ax/D
rearrange λ = ax/D and use measurements from a double-slit pattern with consistent units
interpret the assumptions and geometry of the double-slit relationship when analysing fringes

[Maximum number: 3]

A laser emitting light of a single wavelength is used to illuminate slits $\mathrm{S}_{1}$ and $\mathrm{S}_{2}$ , as shown in Fig. 6.1.

Fig. 6.1 (not to scale)

An interference pattern is observed on the screen A B. The separation of the slits is 0.48 mm . The slits are 2.4 m from AB . The distance on the screen across 16 fringes is 36 mm , as illustrated in Fig. 6.2.

Fig. 6.2

Calculate the wavelength of the light emitted by the laser.
wavelength = m

$x=\lambda D / a$

C1

$\lambda=\left(36 \times 10^{-3} \times 0.48 \times 10^{-3}\right) /(16 \times 2.4)$

C1

$=4.5 \times 10^{-7} \mathrm{~m}$

A1

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8.3.4—Λ = ax / D for double-slit interference using light question 1