2.5 - Determination of wavelength using Young’s double slits

Theory:

The fringe spacing, Δ𝑦 is given by the equation ∆𝑦 = 𝜆D/𝑑 where 𝜆 is the wavelength of the light; D is the distance from the slits to the screen where the fringes are viewed and d is the distance between the slits. A graph of Δ𝑦 against D should be a straight line and the gradient can be used to determine the wavelength of the light.

Apparatus:

Laser pen

Stand and clamp

Double slit

Screen

Metre rule 30 cm ruler or digital callipers

Diagram:

Experimental Method:

Measure the fringe spacing Δ𝑦, the spacing between the slits, d, and the distance, D, from the slits to the screen using either the ruler or digital callipers. Vary the distance, D in equal intervals. Plot a graph of the fringe spacing Δ𝑦 (y-axis) against the slit-screen distance D (x-axis). This should be a straight line through the origin.

If the fringes are close together; Δ𝑦 can be determined by measuring the separation of a number of fringes. So determine by dividing the distance by the number of fringes measured.