Photographs of moving rods

[1morethicko]

A rod of length L is parallel to the surface of a photographic plate a distance d above it, and moves in a direction along its length. A light flash, of negligible duration, illuminates the plate at normal incidence while the rod is in front of it. How long is the photograph of the shadow according to obsevers in both the reference frame of the plate and that of the rod? How does the observer moving wtih the rod reconcile this shadow with the known length of the rod?

[Tweed]

Unless I'm missing something here, it would be the same length for both observers, same length as the rod.

[Dagman]

Doesn't it depend on the distance of light source from rod? Why have you asked this question? What will it solve? 

[jake-the-peg]

I think an important piece of information is missing here which is the speed of motion of the rod.

If the speed of the rod is an appreciable fraction of the speed of light relativistic length contraction becomes an issue and the question gets more interesting

[landie]

The shadow length will be a function of the distance between the light source, rod and the background. As the shadow willl move at the same speed as the rod I don't believe that the shadow will be affected by the speed of the rod as it is infinitesimal compared to the speed of light.

[slimfandango]

COOOOL QUESTION!
I think it's actually a trick question, one of these ones that fills out lots of detail to take you down one path, which is a red herring. This red herring is contraction along a length relative to another length in a parallel plane.
Mini-caveat (1) : ignore the light position, casting a larger shadow, etc. It can't be about that. Light diffuses outwards (in an area the square of the distance from the source). I think since this information is missing, you can assume that the flash is itself just 'diffuse'.
Mini-caveat (2): Ignore any thoughts of 'oh, the rod is moving, so by the time the light gets to the plate, it will create a shadow of different length on the plate due to the movement of the rod.' The instantaneity of the light flash rules this out.

Sooo...you get thinking about relativity. The length of the rod will contract, with movement, relative to a length in a parallel plane that is moving at a different speed in the parallel plane. So you measure the length of the rod from 'rest' position, ie as an observer on the rod. You compare it to the length of the 'rod' on the other plane: the plate. Therein lies the red herring. You are comparing the rod's length NOT between reference frames, rod to plate, but within the same reference frame: the rod's length is always the same on the rod and its shadow: the shadow moves just at the same speed as the rod, it just happens to be projected on the plate. The plate is a different reference frame to the rod, but so what? The speed properties of the shadow relate directly to the speed of the rod, and are nothing to do with the plate.
The length is the same on the plate and the rod.

[1morethicko]

Sorry, forgot to mention to that the speed of the rod is assumed to be comparable to that of light...