''Photography is said to be a medium of modern art. What is your opinion about this?''
It all depends on the way in which photography is used. Its natural purpose and common use is documentary; the more exact and precise it is, the more useful it is.
But undeniably, there are artists who use photography as a medium of expression. But then what they do is no longer an exact copy of Nature, it is an arrangement of forms and colours intended to express something else which is usually hidden by physical appearances.