Interference & Diffraction

Interference Like other forms of wave energy, light waves also combine with each other Interference only occurs between waves with the same wavelength Monochromatic – Single colored  light waves of the same wavelength

Interference Constructive – brighter light than from the contributing waves Destructive – dimmer light or dark spots

Interference Waves must have a constant phase difference for interference to be observed Coherence – occurs when the phase difference is constant and does not shift over time

Demonstrating Interference Light from a single source is passed through a narrow slit and then through two narrow parallel slits(Young’s Double-Slit Experiment) If monochromatic light is used then a series of bright and dark bands, called fringes can be viewed

Double-slit Interference The position of the bright fringes requires that the difference between the distance traveled by the light from each source equal a whole wavelength sin θ = m ( λ/d) m=0,1,2,3… The position of the dark fringes require that the distance traveled by the light differ by a half of a wavelength sin θ = (m + 1/2) (λ/d) m=0,1,2,3…

Thin-Film Interference Occurs when light strikes a thin film over another medium Some rays are reflected back as they strike the film while others refract through the film and reflect back at the boundary with the next medium These refracted and then reflected waves now travel a different distance then those that simply reflected off of the film

Thin-Film Interference Since the light rays travel different distance both constructive and destructive interference can occur The wavelength that is important for thin- film interference is the wavelength within the film! λ film = λ vacuum /n

Thin-Film Interference Phase change upon reflection must be taken into account as well When light travels from a smaller refractive index to a higher one the phase change is equivalent to ½ λ When light travels from a larger toward a smaller refractive index there will be no phase change

Diffraction The divergence of light from its initial direction of travel The divergence of light from its initial direction of travel The bending of light waves as it passes through small openings, around obstacles, or by sharp edges This bending causes a diffraction pattern because of the light waves interfering with each other

Diffraction Becomes more evident as the width of the slit is narrowed Dark Fringes for single slit diffraction sin θ = m(λ/W) m = 1,2,3…. W = width of slit Diffraction Grating – uses diffraction and interference to disperse light into its component colors  like a prism