# Why does light diffract?

AlanPartr
I was wondering if anyone knew why light (or any other particles) diffracts when it goes through an aperture?

Ambitwistor
Originally posted by AlanPartr
I was wondering if anyone knew why light (or any other particles) diffracts when it goes through an aperture?

If you want an answer to this question in terms of particles, rather than waves, you can do no better than to read Feynman's book QED: The Strange Theory of Light and Matter.

Staff Emeritus
Gold Member
This is due to the wave nature of light. It is was well documented in the 19th century. You may wish to research the work of Fresnel and Fraunhofer for the classical presentation.

For the particle aspect of it read QED by Richard Feynman.

Homework Helper
THe quantum explanation is currently the best explanation, and for that you would need to read Feynman's "QED" (it's a thin book that takes a long time to read). BUt before reading this book, you need a full-year of introduction to physics, including basic vectors (vector arrows are used when analyzing photons, but they are called "amplitudes" which actually means "probability.")

The best thing about Feynman's QED is that the same explanation also explains refraction, reflection and interference. This is the kind of simplification that Physics is all about.

In an undeniably insufficient explanation:
Photons will take a certain path that will require the least amount of time to reach a point at which there is a greater-than-zero probability of existing there. Due to the nature of all the possible paths for the photon, there will be points of higher and lower probability (that the photon will be there) Bright diffraction "fringes" indicate zones of high probability and dark "fringes" indicate regions of near zero probability.

IF you want the old explanation of "why diffraction of light" then you must follow the wave model. Do a google search for "Huygens' Principle." It's not easy either, and it's also incorrect for light (according to our current, most accepted explanation).

Homework Helper
Dang it!

Why can't I ever get the first word in?

Staff Emeritus
Gold Member
It is reassuring that we all posted the same reference!

Ambitwistor
Originally posted by Chi Meson
THe quantum explanation is currently the best explanation, and for that you would need to read Feynman's "QED" (it's a thin book that takes a long time to read). BUt before reading this book, you need a full-year of introduction to physics

I don't want to scare Alan off ... Feynman's book was based off of lectures intended for laymen, and I think you can absorb a great deal from it without prior exposure to physics or vectors.

AlanPartr
cheers

cheers

looks like I've got some reading to do