I agree, in part, with jnanian here. Photographs are reactive to light. I would add, however, the time element. Photographic materials are reactive to light over time. This time element is what distinguishes it from the eye. The eye sees things at about 1/60th of a second. Photography records things from a few thousands of a second to multi-year long exposures. This time element, combined with the action of light, allows photography to be unique.
The eye can see a horse running. Photography can freeze the animal so we can see that all four legs are off the ground at the same time. A long exposure allows blur, which cannot be seen by the eye.
Photographs do not need to be of any tangilble thing. The subject can be light. For example, take a flashlight and turn it on and off rapidly at varying distances from a sheet of photo paper while moving it around. You will end up with cirlces of light of varying size and brightness, some of which will over lap. The subject of the photo is light, and can only be seen as a photograph. The eye will see each circle of light as the flashlight is turned on. But the accumulation of the cirlces can only be seen once the paper is developed.