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Relate fringe spacing to the apparatus

For Young’s double-slit interference, fringe separation is $s=\lambda D/d$ , where $\lambda$ is wavelength, $D$ is slit-to-screen distance and $d$ is slit separation.

Rearrange before substituting

Use $\lambda=sd/D$ , $d=\lambda D/s$ , or $D=sd/\lambda$ as needed. Measure the separation between adjacent bright or dark fringe centres; if several fringes are measured, divide the total width by the number of intervals.

Check the trends

Fringes spread farther apart when wavelength or screen distance increases, and become closer when slit separation increases. The small-angle model assumes $D\gg d$ and approximately plane wavefronts normal to the slits.

Common trap

Do not use the total width across several fringes as s without dividing by the number of fringe spacings, and do not confuse slit separation d with screen distance D.

[ 3 ]

Calculate, in $\mathrm{nm}, \lambda$ .

C.3 Wave phenomena is secure when you can connect the physical picture to the equation and its limits.

Wavefronts and rays
Reflection, refraction and Snell’s law
Refractive index and total internal reflection
Superposition, coherent sources and interference
Young’s double-slit pattern

C.3.9—Young double-slit equation

Use Young’s Double-Slit Equation

Retrieve the Core C.3 Wave Phenomena Model

C.3.9 Exam Analysis