image credit: Weilong She; image source; larger image
Light Bends Glass
A long-standing issue in physics has been what happens to the momentum of a photon as it passes from a transparent material into the air.
-- Weilong She investigated this question by firing laser light through fine glass fibers, including the ones shown in the photo, which are only 0.5 µm wide and 1.5 mm long (a human hair is about 100 µm wide).
-- The change in the photon's momentum determines, according to Newton's third law, whether the fiber recoils backwards or stretches forwards.
-- As the photo shows, this experiment indicates the fiber recoils backwards, but the issue persists; to learn about it, see the American Physical Society's Physical Review Focus article Light Bends Glass.
Momentum Conservation Principle
Visit Momentum Conservation Principle to find out about the connection between action reaction forces and the conservation of linear momentum.
(This feature was updated on August 14, 2013.)