1. Interference & Diffraction

2. 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

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

4. 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

5. 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

6. 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…

7. 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

8. 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! • λfilm = λvacuum/n

9. 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

10. Diffraction • 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

11. 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