The speed of light is often called a"constant ". New experiments show that may not be true, even within a vacuum. Sometimes, light slows a bit. The new research is"very impressive work", Robert Boyd toldScienceNews.An optical physicist at the University of Rochester in New York, he did not work on the study. He speaks of the new finding:"It's the sort of thing that's so obvious; you wonder why you didn't think of it first." Light travels as particles and as waves. Photons are the name given to those particles that represent the smallest possible amount of light. At their fastest, photons travel nearly 300 million meters (almost 1 billion feet) in a single second. That's fast enough to get to the moon and back in about 2.6 seconds. Scientists had long known, though, that they could slow light photons down by shining them through a material such as glass or water. Still, light moving through a vacuum has usually been assumed to move at a peak speed. That would seem to make sense. After all, a vacuum is a space with nothing in it to slow light down. In a new study, however, scientists show that even racing through a vacuum, light can slow. So the speed of light is more of a universal speed limit than an   rule for how fast photons travel, the new data suggest. Miles Padgett led the new study. As an optical physicist, he works at the University of Glasgow in Scotland. He and his colleagues showed that the key to slowing light is to change the shape of a light wave. Padgett described how they did this in a paper published online January 22 inScience. His team raced a pair of photons. One traveled unchanged through a cable made from optical fibers. The other passed through a pair of devices that acted like lenses. A lens changes light: It can focus a spread-out beam of light on a point, for instance. The second photon passed through one device that changed the shape of its waveand another that changed it back to its original form. The first photon won the race. The second photon came in a tiny bit later, several quadrillionths  of a second late for every meter of the race. The difference wasn't huge, but it was enough to measure. Most technology would not pick up such a small change, but it could be important to scientists who study short pulses of light. "I'm not surprised the effect exists,"Boyd toldScienceNews."But it's surprising that the effect is so large and strong." The most important factor in changing the speed of light is to transform    .
the shape of a light wave the length of a light wave the direction of a photon the structure of a photon
Question: The speed of light is often called a"constant ". New experiments show that may not be true, even within a vacuum. Sometimes, light slows a bit. The new research is"very impressive work", Robert Boyd toldScienceNews.An optical physicist at the University of Rochester in New York, he did not work on the study. He speaks of the new finding:"It's the sort of thing that's so obvious; you wonder why you didn't think of it first." Light travels as particles and as waves. Photons are the name given to those particles that represent the smallest possible amount of light. At their fastest, photons travel nearly 300 million meters (almost 1 billion feet) in a single second. That's fast enough to get to the moon and back in about 2.6 seconds. Scientists had long known, though, that they could slow light photons down by shining them through a material such as glass or water. Still, light moving through a vacuum has usually been assumed to move at a peak speed. That would seem to make sense. After all, a vacuum is a space with nothing in it to slow light down. In a new study, however, scientists show that even racing through a vacuum, light can slow. So the speed of light is more of a universal speed limit than an   rule for how fast photons travel, the new data suggest. Miles Padgett led the new study. As an optical physicist, he works at the University of Glasgow in Scotland. He and his colleagues showed that the key to slowing light is to change the shape of a light wave. Padgett described how they did this in a paper published online January 22 inScience. His team raced a pair of photons. One traveled unchanged through a cable made from optical fibers. The other passed through a pair of devices that acted like lenses. A lens changes light: It can focus a spread-out beam of light on a point, for instance. The second photon passed through one device that changed the shape of its waveand another that changed it back to its original form. The first photon won the race. The second photon came in a tiny bit later, several quadrillionths  of a second late for every meter of the race. The difference wasn't huge, but it was enough to measure. Most technology would not pick up such a small change, but it could be important to scientists who study short pulses of light. "I'm not surprised the effect exists,"Boyd toldScienceNews."But it's surprising that the effect is so large and strong." The most important factor in changing the speed of light is to transform    .
A. the shape of a light wave
B. the length of a light wave
C. the direction of a photon
D. the structure of a photon
Answer: