Following upon the recommendations by Tamás, you might benefit from looking at Introduction to Fourier Optics by J. W. Goodman. The section on the Huygens-Fresnel principle is particularly useful for building physical intuition about diffraction. The idea is that each point along the wavefront of a wave acts as a discrete source of spherical waves. Each of these conceptual point sources, moreover, oscillates at the frequency, amplitude and phase of the wave at that point. The coherent superposition of the waves emanating from those conceptual point sources creates the three-dimensional wave that propagates onward.

In a plane wave, for example, the superposition of spherical waves from all of the points along the wavefront has precisely the phase relationship needed to recreate the next planar wavefront in the train of wavefronts.

The presence of an obstacle such as the corner of a wall eliminates some of those conceptual point sources. The spherical wave emanating from the point source right at the corner therefore propagates outward as a spherical wave, and thus propagates around the corner.

Dear bro lauri,

When the ray of light encounters a edge ( slit like opening ) the atoms on the edges absorb and re emit the light wave in the form of a cylindrical wave front.

Its called extinction shift effect. This simple principle states that a wave of light interacting with any interfering medium is immediately extinguished and replaced by a new wave. Thus when a ray of light enters a continuous medium it will be continuously absorbed and re emitted. The diffraction phenomenon can be termed as extinction shift effect

Please click following link

http://www.extinctionshift.com/DOUBLE_SLIT_Topic/DOUBLE_SLIT_topic.htm

I hope my answer and demonstration some what better understanding to your question.

with regards

V.Jayaramakrishnan

Well, it is not that accurate that light is absorbed by the atoms at the slit boarder and re-emitted. In fact this is wrong. It is only based on the wave-character of light.

The Introduction in Wikipedia is a good one and for a better understanding go to the little Java-apps linked there. They show you how light behaves at slits and other obstacles and you can play with the parameters and see the outcome.

http://en.wikipedia.org/wiki/Diffraction

I hope this is a little bit of help.

Following upon the recommendations by Tamás, you might benefit from looking at Introduction to Fourier Optics by J. W. Goodman. The section on the Huygens-Fresnel principle is particularly useful for building physical intuition about diffraction. The idea is that each point along the wavefront of a wave acts as a discrete source of spherical waves. Each of these conceptual point sources, moreover, oscillates at the frequency, amplitude and phase of the wave at that point. The coherent superposition of the waves emanating from those conceptual point sources creates the three-dimensional wave that propagates onward.

In a plane wave, for example, the superposition of spherical waves from all of the points along the wavefront has precisely the phase relationship needed to recreate the next planar wavefront in the train of wavefronts.

The presence of an obstacle such as the corner of a wall eliminates some of those conceptual point sources. The spherical wave emanating from the point source right at the corner therefore propagates outward as a spherical wave, and thus propagates around the corner.

A light plane wave can be seen as a superposition of many point light sources disposed along a moving line (wave front). In this case, the wave vector only has a component parallel to the incidence direction. When light founds an opaque body, some of these point sources are removed from the front. Then, the front after passing the body will have a non-zero wave vector component normal to the incidence direction. Now, the wave vector is not parallel to the former one. Thus, light would have bent.

Thanks! Now I have some new leads to look into. Does the transmitting media influence the phenomenon or is it caused by light and the edge alone? In other words, is a diffraction pattern identical a) in air, b) in water (denser), c) in space (less dense)?

@Lauri: The overall scale of the diffraction pattern due to an edge depends on the refractive index of the medium because the wavelength of light is inversely proportional to the wavelength of light. A medium with higher refractive index yields a diffraction pattern with larger-scale features. This assumes that the medium is linear (wavelength independent of intensity), homogeneous (wavelength independent of position) and isotropic (wavelength independent of direction).

Light is electromagnetic wave, like X-rays and gamma-rays, infrared (IR) and ultraviolet (UV) rays, or like radio waves and microwaves. Bending of light around an obstacle or a gap (opening, aperture, slit) in a barrier (or deviation from its straight line propagation) is known as diffraction. This bending is due to wave nature of light and can be explained by Huygens’s Principle, which states that every point on a propagation wave front acts as the source of spherical secondary wavelets. Phenomenon of diffraction occurs with all waves, including sound waves, water waves and electromagnetic waves, and small moving particles such as atoms, neutrons, and electrons, which show wavelike properties.

Diffraction is most noticeable when the obstacle or gap size is about the same as the wavelength of the wave.

Light is electromagnetic wave, like X-rays and gamma-rays, infrared (IR) and ultraviolet (UV) rays, or like radio waves and microwaves. Bending of light around an obstacle or a gap (opening, aperture, slit) in a barrier (or deviation from its straight line propagation) is known as diffraction. This bending is due to wave nature of light and can be explained by Huygens’s Principle, which states that every point on a propagation wave front acts as the source of spherical secondary wavelets. Phenomenon of diffraction occurs with all waves, including sound waves, water waves and electromagnetic waves, and small moving particles such as atoms, neutrons, and electrons, which show wavelike properties.

Diffraction is most noticeable when the obstacle or gap size is about the same as the wavelength of the wave.

Thank you for your answers. Based on them, I did some digging, and here is a summary of the findings:

- absorb - re-emit -theory is wrong

- pinball theory is wrong

- Huygens-Fresnel is not entirely correct, but is simple and often usable

- wave superposition is often correct, but has trouble explaining the single photon slit experiment

- quantum optics can explain all observed phenomena.

So back to my original question: What causes light to bend around edges?

1. Light moves as a probability function, taking all the possible paths at once. The alternate paths cancel themselves out, resulting in a particle-like combined motion.

2. When relatively close to an edge, some paths will collide in the edge and their opposing pairs are thus not annihilated and continue at bent trajectories around the edge.

A simple description can be found in the scattering theory:

I think that light can bend around edges because it is scattered by the medium (theory of scattering is difficult but complete) in which the experiment is done:

The scattering between light and particles, both of the medium and the edge produce deflection on the incoming radiation. In case of vacuum

collision, scattering and interference occur only between light it self or between the light and the edges

I think it helps to rephrase the question, since light doesn't really "bend". That can be a useful way to think about it for some applications (GRIN lenses for example), but even in quantum mechanics light has wave properties that result in constructive and destructive interference. Where constructive interference occurs, amplitudes add and there is light - where destructive interference occurs, amplitudes substract and there is no light. I like to use the water wave analogy (which is imperfect but helps build intuition) - light appears in the geometric shadow of an object for the same basic reasons that the Persian Gulf was hit by the 2004 Indian Ocean tsunami, http://en.wikipedia.org/wiki/File:2004_Indonesia_Tsunami_Complete.gif.

The amount (extent) of diffraction depends on the wavelength (“lambda”) of wave and the size of the obstacle or gap the wave is travelling (passing) through:

- Sound waves can diffract (“bend”) through a doorway.

- Radio waves can diffract (“bend”) around buildings, hills and mountains or even around the whole planet.

- Light waves can diffract (“bend”) through tiny (narrow) slits or small openings in a barrier and around corners (edges) of an object.

- X-rays can diffract (“bend”) around atoms.

- Water waves can diffract (“bend”) when passing through a gap in a harbor wall.

I agree, all the waves bend around an obstacle - not only light. The velocity of a wave depends on the properties of the material and thus differs from material to material. This is the Physics origin of all the diffraction phenomena. Huygens-Fresnel approach is a nice way to explain wave phenomena. Solution of the wave equation in two media is the mathematical approach (Maxwell equations for light if necessary). There are different mathematical methods for solving the wave equation such as Fourier methods.

If someone desires to go deeper in Physics should ask why the velocity of a wave is different in different media. The velocity of sound in a material depends on its density and its elestic constants whereas for electromagnetic waves the refractive index depends on the electric susceptibility of the medium (continuum, for discreet approach scattering theory). The velocity of a wave depends on its wavelength and thus no linear phenomena can arise.

If somebody would like to discuss wave-particle duality the photo-electric effect is a good starting point.

If you are interested in the light propagation in the fibre it is all related to the concept of total internal reflection. As it has been come up in the discussion already, the change in the reflective index of the materials that light propagates through them result in variation in the light speed. Subsequently, the direction changes according to the medium reflective index. It can be said that controlling the angle of the propagating light will be entirely depends on the medium’s reflective index and of course the initial optical launch power.

Arash,

Instead of ‘reflective index’: ‘refractive index’ or ‘index of refraction’.

One answer to the "why" can be the pilot wave (https://www.youtube.com/watch?v=WIyTZDHuarQ). The pilot wave of a photon hits the edges and changes shape. This may cause the foton to alter trajectory.