I checked by a program also, there is a intensity difference of around ~15 (8-bit images, intensity ranging from 0 to 255). By naked eye, one can quite easily observe this.
Well, I used a program to check the pixel intensities of the marked portions in (2) and (3). They were exactly same! However, these images are artificially created.
If we use a camera to capture the two scene about which I am talking about, even in the images the shadow appears to get darker...
What I have understood after reading other answers as well: "less brighter objects surrounded by brighter area, appear darker and if the brightness of the brighter area increases further, then the less bright objects appear more dark."
Am I right? The same thing also happens when we take photos...
I observed following phenomena today: in my room with some lightings, there is a stuff toy placed in front of a wall. The ambient lighting doesn't cause any shadow on the wall currently. Now, I switched ON a torch and pointed it to a stuff toy which was kept in front of a wall and a shadow...