Thursday, January 12, 2012

Watching real wavefronts in slow motion

..John Wallace
..Senior Editor
..Laser Focus World

A team at the MIT Media Lab, led by Ramash Raskar, has developed the best way yet to visualize the passage of light as it hits and passes around or through objects. They use streak cameras, femtosecond-laser pulses, and a million or more repeated measurements of a stationary scene to capture the actual laser pulse as it passes through a transparent bottle, or is intercepted by an apple or other object.

Two points I’d like to make. First, is that Senior Editor Gail Overton’s news story on this will appear in the February issue of Laser Focus World; don’t miss it, as she has talked to the researchers and describes in detail how the whole thing works.

The second point is -- this is pretty amazing. They’ve created videos of wavefronts as they propagate through an everyday scene . . . and these are no simulations; they are the real thing. The colorized gray-scale photo here shows a couple of wavefronts in passage through a bottle.

(Photo: MIT Media Lab)

The bottle is perpendicular to the camera, and the pulses propagate from left to right (with no toward- or away-from-the-viewer component). I stared at this for a moment before I realized why the wavefronts look tilted -- it’s because the light from the farther-away points on the wavefront take longer to get to the camera, so these spots appear to have not progressed as far as those nearer to the camera.

This realization made me feel strangely relativistic, as if I were flying by the bottle in a spaceship at half lightspeed. (And I won’t even try to explain this.)

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