IB Physics SL C.3.9 Young double-slit equation

Practise applying the Young double-slit equation to calculate wavelength, fringe spacing or angular separation from screen data.

Exam points

rearrange s = λD/d to calculate wavelength or fringe spacing, converting mm, nm and m correctly
choose a graph of s against 1/d, or D/d, so the data table gives a straight-line relationship
use s = λD/d to predict how increasing λ or decreasing d changes fringe spacing on the screen

[Maximum number: 3]

This question is about interference.

Light from a laser is incident on two identical parallel slits. The light from the two slits produces a fringe pattern on a screen.

Figure for Question C.3.9—Young double-slit equation question 1 — IB Physics SL

A central bright fringe is produced at C . The next bright fringe is produced at A . There is a dark fringe at B .

The distance from the two slits to the screen is 1.5 m . The distance BC is 1.8 mm and the distance between the slits is 0.30 mm .

Show that the laser produces light of wavelength equal to 720 nm .

use of $\lambda=\frac{s d}{D}$ ;
$s=\underline{2} \times 1.8(\mathrm{~mm})$ ; (award this mark for any evidence for the factor of 2 )
$\lambda=\frac{2 \times 1.8 \times 10^{-3} \times 0.30 \times 10^{-3}}{1.5} /$ OWTTE;
( $=7.2 \times 10^{-7} \mathrm{~m}$ )

Exact answer is given, award marks for correct working.

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C.3.9—Young double-slit equation question 1