shot cab are destroyed and vaporized. These vapors are accelerated to very high velocities, several tens of kilometers per second, faster than the shock front. However, this acceleration happens in a short period, so the material is trapped behind the shock front, even though it eventually travels faster than the shock front. The various light and dark patches are caused by the varying vapor density of the material splashing against the back of the shock front. The irregular variations in mass distribution around the bomb core create the mottled blob-like appearance. Sounding rockets After a few milliseconds, the energy of the shock front will no longer be great enough to heat the air into incandescence. At that point, the shock front becomes invisible, a process known as "breakaway". This makes the shock wave difficult to diagnose beyond this boundary. Photographs of nuclear tests often show numerous vertical rope-like lines to one side. These are typically created by small sounding rockets launched a few seconds before the firing, leaving smoke trails. The purpose of these trails is to record the passing of the now invisible shock wave, which causes an obvious visual effect on the smoke by compressing the air into a lens. This is not necessarily related to the rope trick effect in any physical way, but it is possible to confuse the two in some photographs. In the photograph of the Tumbler-Snapper test (at the top of this article), the vertical lines in the lower-right corner are blast line poles not smoke trails. Camera recording The photo was shot by a rapatronic camera (a high-speed camera invented by Harold Edgerton and colleagues) built by EG&G. Each camera was capable of recording only one exposure on a single sheet of film. To create time-lapse sequences, banks of four to ten cameras were set up to take photos in rapid succession. The average exposure time was three microseconds. Image gallery References Sources This article incorporates text from the National Nuclear Security Administration's "Rapatronic Photography" factsheet (August 2013). External links Rare Nuclear Bomb Footage Reveals Their True Power | WIRED Nuclear weapons testing Articles containing video clips