A monochromatic laser with an unknown wavelength is directed through a pair of narrow…

PHYSICS · Multi-part · A Level · Physics · Waves · Waves

Problem

A monochromatic laser with an unknown wavelength is directed through a pair of narrow slits separated by

d=0.400\,\text{mm}

. A screen is positioned

L=2.50\,\text{m}

from the slits. The positions of several bright interference fringes are measured relative to the central maximum and recorded in the table below.

Fringe order $m$	$y_\text{upper}$ (cm)	$y_\text{lower}$ (cm)
$+1$	$3.38$	$-3.34$
$+2$	$6.80$	$-6.77$
$+3$	$10.26$	$-10.20$

(The negative values correspond to fringes below the central axis.) (a) Using the data, determine the average fringe spacing

\Delta y

between adjacent bright fringes. Show your method clearly. (b) Employing an appropriate approximation, calculate the wavelength

\lambda

of the laser light. Express your answer in nanometres to three significant figures and include all working steps. (c) State whether the calculated wavelength lies within the visible spectrum and briefly justify your conclusion.

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