What causes light passing through a hole to change direction? On diagrams showing light passing through a hole, the wave of light appears to change direction when it emerges from the hole.
What causes that change of direction? Is it maybe the walls of the hole imparting a pulling force or the sudden absence of light next to the emerging beam causes the light to spread?
Or maybe light does this all the time and we only notice when we put a wall with a hole in the way.
Please explain this to like I'm a five year old.
 A: It might be useful to first ask a different question. Namely, why does light not scatter (change direction) when passing through a bulk medium such as glass (assuming no impurities, etc.)? One answer is the Ewald-Oseen extinction theorem which gives a rigorous mathematical account of how light propagates through matter. The atoms in a material absorb the incident light and then re-radiate it in different directions. The extinction theorem guarantees that the light radiated by the atoms will interfere with the incident beam in such a way that the light continues through the medium without changing direction. What happens, then, if a hole is carved out of that medium? The atoms on the edge of the hole are free to radiate light in all directions without this cancellation effect taking place.
One might also ask why atoms do not radiate light in all directions in the case of specular reflection (i.e. reflection off of a smooth surface). Again, the extinction theorem shines light on this issue because Snell's law and the law of reflection are derived from it.
A: Light is a wave , at the edge of the hole the oscillation spreads in all directions but the highest intensity is forward.
A: Light is wave in the electromagnetic field and obeys a wave equation (3D) similar to a wave in a taut string (1D) or on a drum membrane (2D). You may imagine a "tension" whenever the field varies from place to place. This is simply a fundamental property of the EM field that occurs in the absence of any matter or charge. $$\frac{d^2u}{dt^2}=\nabla^2u,$$ where $u$ is any of the 6 components of the EM field (note that there are further constraints between the components). $\nabla^2u$ measures how much $u$ at a given point is bigger/smaller than $u$ at nearby points. $\frac{d^2u}{dt^2}$ relates that to how $u$ changes in time.
In the interior of a light beam, the EM field only varies in the direction of propagation, so the motion only occurs in that direction. At the edges of a finite beam, the field is strong towards the inside and weak on the outside, so the light leaks out of the beam. This is diffraction. A hole in a wall is an easy way to create a finite beam, but in the end the spreading happens afterwards and is a property of the light itself. E.g. if you could somehow create a finite segment of a plane EM wave just in empty space, it would still diffract as if it had just passed through an aperture.
You can also consider disturbances in the EM field as always "flowing" in all directions. In the interior of a light beam, the flows perpendicular to the propagation direction cancel out, since in each such plane the field is constant, while the flows along the propagation actually do something. But at the edges, the natural flow of light from the inside to the outside is not balanced, so we have diffraction. (This point of view is the Huygens-Fresnel principle.)
So no, the atoms/material of the wall have nothing to do with diffraction (no "pulling") outside of the act of destroying some of the light. Light knows how to diffract all on its own.
A: Photons, including single photons interact with single edges. The effect is more noticeable when the edge is sharp. Photons are pulled around and behind the edge But photons also scatter away from the Edge. A single slit is created with two sharp edges. Each edge is diffracting and scattering photons on their way to the detection screen. On the screen you have four Single edge patterns overlapping to create a single slit interference pattern. See “Single Edge Certainty” at billalsept.com
