Interference & DiffractionInterference & Diffraction

Light Part 4Light Part 4

InterferenceInterference

Like other forms of wave energy, light Like other forms of wave energy, light waves also combine with each otherwaves also combine with each other

Interference only occurs between waves Interference only occurs between waves with the same wavelengthwith the same wavelength

Monochromatic – Single coloredMonochromatic – Single colored light light waves of the same wavelengthwaves of the same wavelength

InterferenceInterference

ConstructiveConstructive – brighter light than from the – brighter light than from the contributing wavescontributing waves

DestructiveDestructive – dimmer light or dark spots – dimmer light or dark spots

InterferenceInterference

Waves must have a constant phase Waves must have a constant phase difference for interference to be observeddifference for interference to be observed

Coherence – occurs when the phase Coherence – occurs when the phase difference is constant and does not shift difference is constant and does not shift over timeover time

Demonstrating InterferenceDemonstrating Interference

Light from a single source is passed Light from a single source is passed through a narrow slit and then through two through a narrow slit and then through two narrow parallel slitsnarrow parallel slits

If monochromatic light is used then a If monochromatic light is used then a series of bright and dark bands, called series of bright and dark bands, called fringesfringes can be viewed can be viewed

Double-slit InterferenceDouble-slit Interference

The position of the bright fringes requires The position of the bright fringes requires that the difference between the distance that the difference between the distance traveled by the light from each source traveled by the light from each source equal a whole wavelengthequal a whole wavelength

The position of the dark fringes require The position of the dark fringes require that the distance traveled by the light differ that the distance traveled by the light differ by a half of a wavelengthby a half of a wavelength

InterferenceInterference

Sin Sin θθ = m = mλλ/d/d

θθ angle angle

m m order # ( first dark fringe is ½ , order # ( first dark fringe is ½ , then add ½ to order #for destructive)then add ½ to order #for destructive)

λλ wavelength wavelength

d d slit separation slit separation

DiffractionDiffraction

The divergence of light from its initial The divergence of light from its initial direction of traveldirection of travel

The bending of light waves as it passes The bending of light waves as it passes through small openings, around obstacles, through small openings, around obstacles, or by sharp edgesor by sharp edges

This bending causes a diffraction pattern This bending causes a diffraction pattern because of the light waves interfering with because of the light waves interfering with each othereach other

DiffractionDiffraction

Becomes more evident as the width of the Becomes more evident as the width of the slit is narrowedslit is narrowed

Diffraction Grating – uses diffraction and Diffraction Grating – uses diffraction and interference to disperse light into its interference to disperse light into its component colorscomponent colors

like a prismlike a prism

CoherenceCoherence

LasersLasers – devices that convert light, – devices that convert light, electrical energy, or chemical energy into electrical energy, or chemical energy into coherent lightcoherent light

Active Medium – substance to which Active Medium – substance to which energy is added to produce coherent lightenergy is added to produce coherent light

composition determines the composition determines the wavelength, or color of light producedwavelength, or color of light produced

LasersLasers

Energy is input to the active medium which Energy is input to the active medium which causes its atoms to be excited to a higher causes its atoms to be excited to a higher energy levelenergy level

When light of a certain wavelength is When light of a certain wavelength is applied to excited atoms, they can be applied to excited atoms, they can be stimulated to release light waves that have stimulated to release light waves that have the same properties – the same properties – stimulated stimulated emissionemission

LasersLasersOnce one atom spontaneously releases its Once one atom spontaneously releases its energy in the form of a light wave it causes energy in the form of a light wave it causes other energized atoms to release light other energized atoms to release light waves with the same wavelength, phase waves with the same wavelength, phase and directionand directionMirrors will then stimulate more emissions Mirrors will then stimulate more emissions of coherent light which increases the of coherent light which increases the intensity of the lightintensity of the lightOne mirror is slightly transparent which One mirror is slightly transparent which allows some of the intense coherent light allows some of the intense coherent light to be emittedto be emitted