Each wave front is comprised of an infinite number of point sources, each of which produce wavelets with the same wavelength as the wave itself. The superposition of the wavelets produce a new wave front tangential to the wavelets, thus propagating the wave front.
Now, according to the same principle, when a wave diffracts through a slit there are an infinite number of point sources between the slits as the wave is passing through. These points create wavelets which superimpose, forming the 'new' diffracted wave front, which should appear tangential to the wavelets, as shown by the red line in the diagram above.
However, in text books and educational websites the diffracted wave front is drawn as below, labelled as "Diffraction through a wide gap":
If the wave fronts are 'bent' around the edges, then they cannot be tangential to the wavelets. Why, then, are they drawn like this? And if there is a reason I am unaware of which explains why they are 'bent', then why are normal, non-diffracted wave fronts drawn as straight lines?
Note: I'm a high school student, so while I appreciate the complex mathematical proofs offered by the users here, could any answers also include a simpler explanation I would understand? Thanks.