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Physics OnlineLaser Diffraction

Introduction

Diffraction occurs when light waves interfere with each other. A diffraction gratingconsists of a thin piece of opaque glass or plastic with a pattern of equally-spaced,transparent slits. If a beam of monochromatic light (that is, light of only onewavelength) is incident on a diffraction grating at !", then a series of bright and dar#bands (called fringes) will emerge. $he location of the bright bands is given by %nell&slaw of diffraction'

d*sin() = m* (m = 0, 1, 2,....).

$he variable dis the distance between the centers of the grooves in the diffractiongrating, is the angle between the line connecting the center of the grating and thebright band and the line perpendicular to the grating, mis called the order of the bandandis the wavelength of the light.

otice that the angle of diffraction depends on the wavelength. ed light (longerwavelength) is diffracted through a larger angle than blue light (shorter wavelength).$his means that %nell*s law can be used to calculate the wavelength ofmonochromatic light, such as that from a laser.$he purpose of this lab is to test the theoretical value for the wavelength of a redlaser.

Equipment You Procure

$ape measure or meter stic#

$ape

An assistant

Equipment from Kits

Diffraction grating

+aser (do not aim at anyone*s eyes)

Eperiment!" Procedures

) %olve the above equation for, the wavelength of the light.) alculate d, the distance between grooves (in meters), given that there are /!!

grooves0mm.1) $ape the diffraction grating to a hori2ontal surface so that the grating is vertical.

$he grating should be a large distance from a wall and parallel to the wall.3) 4easure the distance between the diffraction grating and the wall, L./) $urn off the lights.5) Aim the laser at the diffraction grating. 6ou should notice an odd number of dots

on the wall, typically /. $he one in the center should be the brightest (m 7 !).

x

L

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8) 4easure the distance between the center dot and one near it,x. $his is a locationfor m 7 .

9) :se the definition of sin(;),x, and + to calculate sin(;).) alculate the wavelength of the light and compare it to the theoretical value of

5/! < ! nm.!) epeat steps 8 through using a dot that is dots away from the center. $his

will be m 7 .) epeat steps 1 through ! using a different location for the diffraction grating.

$his will give you a total of 3 calculations of the wavelength.