1. Interference  When two light waves meet, their amplitudes add (by principle of superposition) and the resulting disturbance can be either reinforced (constructive interference) or eliminated (destructive interference). Constructive interference. When the crest of one wave passes through, or is super positioned upon, the crest of another wave, we say that the waves constructively interfere. Constructive interference also occurs when the trough of one wave is super positioned upon the trough of another wave

2. Phenomena of constructive interference During any wave interference the shape of the medium is determined by the sum of the separate amplitudes of each wave. We often say that when waves interfere, amplitudes add. Using the arrow buttons step through the above animation to see how the amplitudes add during constructive interference. Notice that in this animation the crest of the red wave constructively interferes with the crest of the blue wave.

9. Destructive Interference When the crest of one wave passes through, or is super positioned upon, the trough of another wave, we say that the waves destructively interfere.

10. Young’s Double Slit Experiment The narrow slits, S 1 and S 2 act as sources of waves The waves emerging from the slits originate from the same wave front and therefore are always in phase

11. Resulting interference  The light from the two slits form a visible pattern on a screen  The pattern consists of a series of bright and dark parallel bands called fringes  Constructive interference occurs where a bright fringe occurs  Destructive interference results in a dark fringe

12. Interference Patterns The upper wave has to travel farther than the lower wave The upper wave travels one wavelength farther Therefore, the waves arrive in phase A bright fringe occurs

13. Interference Patterns The upper wave travels one-half of a wavelength farther than the lower wave The trough of the bottom wave overlaps the crest of the upper wave (180  phase shift) This is destructive interference A dark fringe occurs

14. Application of Interference in CD`s Data is stored digitally A series of ones and zeros read by laser light reflected from the disk Strong reflections correspond to constructive interference These reflections are chosen to represent zeros Weak reflections correspond to destructive interference These reflections are chosen to represent ones

15. Applications Michelson-Morley: Evidence of Special Relativity Lab work: Density of Air Detection of gravitational waves