What causes a lifting force on an airplane's wing design?

[ChloeYip]

As I know, lifting force is because the surface of upper side of the airfoil is longer than the bottom side, causing the pressure of down side larger and produce lefting force...
this is a picture from my lecture notes, which the instructor didn't explained but seems contradict to what I knew...
Anything wrong with my mindset? can anyone help to explain the picture?
Thanks.

 

[billy_joule]

There's no contradiction.
The image is nothing to do with how a wing generates lift, it shows deflection and stresses due to a distributed load (which happens to come from the lift force).
You'll find many similar (albeit mostly upside down) images if you google 'beam deflection'.

 

[rcgldr]

A lifting force coexists with downwards deflection of the air flow relative to the wing. In the case of a thin wing, the upper and lower surfaces have the same length, and they don't need to be curved. A thin flat wing at some angle of attack can generate lift (at the cost of increased drag compared to more efficient wing shapes).

As posted by billy_joule, the image shows an exaggerated example of a wing flexing under load, as viewed from behind (or in front) of an aircraft.