Why isn't it allowed to use a flash when taking pictures in a certain place? When I go to, for example, a museum I try to take some pictures.
Sometimes the museum staffs forbid me to use a flash. Do you know the reason? I don't think it is related to photo-electric effect, right?
 A: From Amateur Photographers in Art Galleries: Assessing the harm done by flash photography. by Martin H. Evans:

The flash built into a digital compact has, typically, a GN value of about 6 to 9 (though some manufacturers are rather coy about revealing the GN rating). If one extends the calculations used by Saunders and by Evans to these little units, then one finds that if one of these is fired at full power at about 2.5 metres (ca 8 feet), it exposes the object to about the same quantity of light as that falling on it every one-eighth of a second in a 200 lux (ca 18.6 footcandle) gallery, or every half second in a dark 50 lux (ca 4.6 footcandle) gallery. Is it worth getting steamed up about such a tiny extra quantity of light, as far as pigment fading is concerned? Several photographers have already suggested that any trifling damage done by a few hundred of these little flashes in a day could be fully offset by closing the gallery and turning off the lights a few minutes early. A ban would be justified in rare cases, where large numbers of photographers might be taking many flash photographs very close to something that could reasonably be considered photosensitive.

So it appears to me the main reason for the ban is not related to the photoelectric effect.
A: My local museum prohibits flash photography because other people in the museum find it annoying. It's nothing to do with any effect, real or imagined, that the flash may have on the exhibits.
Many organic pigments are slowly destroyed by light, so colours fade if exposed to light. I suppose in some special cases museums might be concerned about this. In some museums illustrated manuscripts are exhibited in rooms that are kept at low light levels - I seem to remember the Book of Kells in the Dublin Trinity Museum is exhibited in this way. I can understand why flash photography is banned in those cases.
A: Yes, as you guessed it is related to the photoelectric effect. The photons from the flash, certain frequencies there, can change the molecular composition of the surface paints and pigments.
The precaution is the same as in avoiding a valuable painting or rug to be illuminated by the sun. It is the photoelectric effect.
A: It really does depend on the place.
Sometimes it will be to minimise the (real or imagined) damaging effect of the light and/or heat from the flash, sometimes because it annoys or disturbs other visitors.
In a very popular place it may be to avoid triggering a photosensitive epileptic fit. Theoretically that's possible with a large number of flashbulbs going off in rapid succession in an enclosed space!
Quite often, in museums in particular, it's because the curator knows a bit about photography and is aware that with many of the exhibits being behind glass then flash photography is more likely to result in a rubbish, glare-filled image than a worthwhile shot.
Take your pick from those - or, if you really want to know, politely ask one of the staff for the reasons behind the ban.
A: As user9886 explained, the main reason is probably not physical. There are indeed cases where strong flashes can damage pigments that are fairly stable in daylight. I know that some modern documents use rhodopsin based ink that makes it impossible to use an ordinary photocopier to copy them without destroying them. I'm not sure if there are a lot of historically used pigments that are similarly senitive.
The main other reasons are simply:


*

*It might annoy people. Especially in museums with a lot of visitors, you would have flashing all the time. Also, in some places (churches etc.) it might be considered rude or, I don't know, not doing the holiness of the place justice or something.

*Intellectual property. Museums often ban the use of cameras altogether because they don't want anyone to take photos of their pieces. In cases where pictures are allowed, they often forbid flashes and/or tripods, so that you can't take "professional" pictures, but you can still take private pictures for yourself.

*It might also be a bit of "we don't know why, but we've been allways doing it like this". Museums could tend to err on the side of caution when forbidding flashes.
A: It's actually astonishing to see how damage much a camera flash can do to black/dark colored objects!
This is a pretty good demonstration.
So, I imagine that if a photographer takes a picture with a powerful flash gun close to a black object (e.g. a dark painting) it could cause the painting to undergo combustion and get irreversibly damaged. It makes sense why most museums and galleries are paranoid about such occurrences.
