Tuesday, November 8, 2011
The acousto-optical effect that uses lasers to see sound: As pressure waves move through the air, they change the air density. That shift in density changes the air's refractive index. When light hits a high-density section, it will be slowed down. When it hits a low-density section, it will shoot ahead. These regular high-density and low-density sections can also act as a diffraction grating, causing the light to splay out and cause interference. By bouncing laser light off a reflective sheet on the far side of the speaker, light makes two trips through the acoustic waves. It's then taken up by a detector. The acoustic waves will have caused the light to move at different angles and different speeds, causing phase shift.