Nothing happens when the black cloth is removed. Once the "picture" is taken, the gases within the tube react.
The UV radiation from the flash initiated the reaction between the H2 and Cl2 gas. The reaction proceeds via a chain-reaction mechanism.
The enthalpy for the overall reaction is -184.6 kJ per two moles of HCl. However, the enthalpy for the initial step is 243.36 kJ per mole of Cl2. This corresponds to the energy carried by photons with a wavelength of 491.5 nm This reaction is catalyzed by light toward the violet end of the visible spectrum.
It should be noted that, because a camera's flash bulb initiates the reaction, a certain amount of showmanship could be employed. For example, one could take the cloth off and, when nothing happens, take a photograph of the demonstration under the ruse that it actually failed and such a picture is a necessary bit of documentation in such an event.
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