Interference & DiffractionInterference & Diffraction

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 slits(Young’s Double-Slit narrow parallel slits(Young’s Double-Slit Experiment)Experiment)

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 equal traveled by the light from each source equal a whole wavelength a whole wavelength

sin sin θθ = m ( = m ( λλ/d) m=0,1,2,3…/d) m=0,1,2,3…

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

sin sin θθ = (m + 1/2) ( = (m + 1/2) (λλ/d) m=0,1,2,3…/d) m=0,1,2,3…

Thin-Film InterferenceThin-Film Interference

Occurs when light strikes a thin film over Occurs when light strikes a thin film over another mediumanother medium

Some rays are reflected back as they Some rays are reflected back as they strike the film while others refract through strike the film while others refract through the film and reflect back at the boundary the film and reflect back at the boundary with the next mediumwith the next medium

These refracted and then reflected waves These refracted and then reflected waves now travel a different distance then those now travel a different distance then those that simply reflected off of the filmthat simply reflected off of the film

Thin-Film InterferenceThin-Film Interference

Since the light rays travel different Since the light rays travel different distance both constructive and destructive distance both constructive and destructive interference can occurinterference can occur

The wavelength that is important for thin-The wavelength that is important for thin-film interference is the film interference is the wavelength within wavelength within the filmthe film!!

λλfilmfilm = = λλvacuumvacuum/n/n

Thin-Film InterferenceThin-Film Interference

Phase change upon reflection must be Phase change upon reflection must be taken into account as welltaken into account as well

When light travels from a smaller refractive When light travels from a smaller refractive index to a higher one the phase change is index to a higher one the phase change is equivalent to ½ equivalent to ½ λλ

When light travels from a larger toward a When light travels from a larger toward a smaller refractive index there will be no smaller refractive index there will be no phase changephase change

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

Dark Fringes for single slit diffractionDark Fringes for single slit diffractionsin sin θθ = m( = m(λλ/W) m = 1,2,3…./W) m = 1,2,3….

W = width of slitW = width of slit

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