It doesn't. Light can bend when it encounters a change in the index of refraction, e.g. air to glass or vice-versa. https://en.wikipedia.org/wiki/Refraction#Explanation Light travels in a straight line after it has entered the glass, and again after it exits. Only the angle and the difference in refractive index matters. That's why you can build a Fresnel lens which is, so to speak, all surface. https://en.wikipedia.org/wiki/Fresnel_lens#Description

that's really interesting. apparently I need to read up

The shape of the lens puts the surface of the lens at the correct angle. You can make a thin lens by splitting it into many discontinuous parts, this is how a Fresnel lens works. See e.g. these two pictures:

http://upload.wikimedia.org/wikipedia/commons/9/97/Lens3b.sv... http://en.wikipedia.org/wiki/File:Fresnel_lens.svg

OP above is correct. The light will only bend at the interfaces between two mediums (air, glass).