1. Interference & Diffraction Light Part 4

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

7. Interference Sin θ = mλ/d θ  angle m  order # ( first dark fringe is ½, then add ½ to order #for destructive) λ  wavelength d  slit separation

8. 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

9. Diffraction Becomes more evident as the width of the slit is narrowed Diffraction Grating – uses diffraction and interference to disperse light into its component colors  like a prism

10. Coherence Lasers – devices that convert light, electrical energy, or chemical energy into coherent light Active Medium – substance to which energy is added to produce coherent light  composition determines the wavelength, or color of light produced

11. Lasers Energy is input to the active medium which causes its atoms to be excited to a higher energy level When light of a certain wavelength is applied to excited atoms, they can be stimulated to release light waves that have the same properties – stimulated emission

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