Bending of light Why does bending of light(diffraction) occur?
 A: Essentially, the smaller the aperture gets the more it starts acting like a point source. As mentioned by dominecf, a point source creates circular wavefronts therefore resulting in a "bending" of light.
A quantum mechanical explanation of the phenomenon has to do with the Heisenberg uncertainty principle which relates the uncertainties in position and momentum to one another.
$\delta x \delta p = h/4\pi$    where $h$ is the planck constant

In a single slit the vertical position of a photon is extremely well defined which means that the uncertainty in vertical momentum increases proportionally.
A: Light is at first a wave, and like any wave the simplest description of his propagation is the Huygens-Fresnel principle : at any point a new wave is re-emitted in every direction. When you sum the contributions, in free space the only location that does not vanish trough interferences is the next location in straight line direction. But if there is a hole or an obstacle, it changes the cancelation pattern, thus the bending, betraying the "every direction" local reemitting.
A: In the case of refraction we have to know that electromagnetic waves travel at a different velocity in different mediums, for example they propagates faster in air that they do in water. So, if we have a ray of light which is emitted from below the surface of water it will accelerate when it reaches the surface of the water. Now, If the ray hits the surface at exactly 90 Degrees all goes well and the light will continue straight up into the air without being bent (refracted), it will simply pick up speed when it transitions from water to air. But what happens if the ray hits the surface at an angle, lets say 120 Degrees? The left side of the wavefront will hit the surface before the Right, and exactly as a car who's left wheels would suddenly pick up speed while its right wheels would remain at the previous, slower speed, it will curve to the right. That explains how refraction bends light when it changes from one medium to another. It is interesting to note that if the ray of light is bent enough so that its new trajectory would have it re-enter the water, it will be perfectly reflected by the surface instead of being refracted (bent). This phenomenon is called total internal reflection.
Also, different wavelengths of electromagnetic waves will propagate at different velocities in a given medium, this is why a prism separates white light into its component colors...
