Let a small rectangle be x in length and y in width, then xy = 1.

Area of shadow part = area of triangular shadow in the first vertical rectangle+area of trapezoidal shadow in the second vertical rectangle+area of triangular shadow in the last two series.

= 2xy× 12+(2x+3x)y× 12+3x×2y× 12

= 132xy=6.5xy=6.5。

So the answer is: 6.5.