Why isn't 'Qt changing the opacity of window?

I'm trying to change the opacity of the window of my program in Qt and using C++11. In other words, I want to open the main Windows slowly by adding some instructions to main.cpp. You see those instructons below:

MainWindow a;
a.show();

for(int Counter = 0; Counter <= 100; Counter++)
{
    a.SetWindowOpacity(Counter);
    for(Wait = 0; Wait < 100; Wait++);
}

But when I build and run my project, I don't see any changes and it normally runs. This is whole source code, main.cpp.

#include "mainwindow.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;

    w.setWindowTitle("Add a new student");
    w.show();

    for(int Counter = 0; Counter <= 100; Counter++)
    {
        w.setWindowOpacity(Counter);
        for(int Wait = 0; Wait < 100; Wait++);
    }

    return a.exec();
}

So, why don't Qt change the opacity of the window of my program?