When the Camera is facing toward the Sun and a person is standing to and We're capturing his photo , all we get is a black shadow even when we capture with the flash light or anything. Why does it happen ?
Her face is black because the number of photons reflected from her face is negligible relatively to the huge number of photons that are coming directly from the Sun next to her. So the camera chooses a very short exposure time because there seem to be lots of photons (those from the Sun) so that the Sun looks reasonably yellow. No surprise that her face is dark in comparison.
However, if you carefully photographed just the region where her face is located and you would make sure that the exposure time is reasonable for a human face and you used flash (and no undesirable nonlinear transformations of the brightness inspired by the bright Sun would be applied, all such things may occur in digital cameras etc.), the face would look fine.
This won't be a very detailed explanation, but this is because of the concept of a dynamic range.
The reason the person appears so dark against the sun is because the brightness of the two objects differs vastly and the camera determines exposure according to the overall brightness of the light illuminating the sensor. Since there is one main contributor (i.e. the Sun), which is much brighter than other objects, it will always look much brighter in a photo.
You may ask why do we see it differently than the sensor does that. It is because our sensors in the eyes (the rods and cones) are adapted to do a much better job. As I understand it (I haven't read about it, it's just my opinion), it's either the brain doing its job and reducing the apparent brightness of very bright objects, or it's some safety mechanism which turn on on particular cones and rods which makes parts of the whole image dimmer.
If you want to make photos with higher dynamic range, consider about HDR technology, or overexposing the sensor, but then you'll lose detail in the background.