Register to reply

Reflection grating

by sliorbra
Tags: grating, reflection
Share this thread:
sliorbra
#1
Nov13-12, 05:08 PM
P: 10
hi,

I am bothered by some question for a long time.
From basic optics, i know that the the angle of incidence equals the angle of reflection [where we define the incident angle as the angle between the beam and the normal to the surface of course].

What I can't understand is why when a polychromatic beam reflects from the surface of a reflection grating [for example, with a periodic structure] the reflection angle of the beam depends on the wavelength? Why the reflection angle should not be the same for all the wavelengths that compose the polychromatic beam and equal the incident angle [which is, in my opinion, the same for the different wavelengths that compose the beam]??

I hope my question is clear.
Phys.Org News Partner Physics news on Phys.org
Step lightly: All-optical transistor triggered by single photon promises advances in quantum applications
The unifying framework of symmetry reveals properties of a broad range of physical systems
What time is it in the universe?
sophiecentaur
#2
Nov13-12, 05:29 PM
Sci Advisor
Thanks
PF Gold
sophiecentaur's Avatar
P: 12,167
Wouldn't the main beam (main maximum) be the same for a plane mirror as for a plane reflection grating? The path differences for all wavelengths would surely all be zero in the direction of the classical main beam (isn't that all about Fermat's Principle?).
sliorbra
#3
Nov14-12, 03:10 AM
P: 10
I am not sure that I understood your answer. Can you please detail more?

Philip Wood
#4
Nov14-12, 03:41 AM
PF Gold
P: 953
Reflection grating

(a) A crude model of a plane reflection grating would be reflecting strips alternating with absorbing strips. The distance from centre of reflecting strip to centre of next reflecting strip (the grating element) is a few wavelengths. Thus each reflecting strip is, perhaps, two or three wavelengths wide. Such narrow reflectors don't reflect light approaching (in a plane of incidence at right angles to the strip lengths) according to the laws of reflection. There is a wide spread of angles for the light leaving the strip; we have scattering rather than 'specular reflection'. Even quite crude drawings of 'wavelets' arising from different points across the width of the strip will demonstrate this. The phenomenon is very similar to diffraction.

(b) What determines the angles at which light leaves the grating as a whole is interference between light scattered from the different strips.

(c) In practice, the intensity of the beams leaving the grating is 'tweaked' by modifying the simple grating I described at the beginning.
sophiecentaur
#5
Nov14-12, 04:04 AM
Sci Advisor
Thanks
PF Gold
sophiecentaur's Avatar
P: 12,167
Quote Quote by sliorbra View Post
I am not sure that I understood your answer. Can you please detail more?
Have you looked up Fermat's Principle yet?
Have you looked up The Reflection Grating?

Where does the zeroth order beam emerge from a reflection grating? It will be in a direction where there is no path difference for any of the wavelets produced by the grating. What direction will this be?
Philip Wood
#6
Nov15-12, 10:32 AM
PF Gold
P: 953
Sliorbra. Did you follow my attempt to answer your question?


Register to reply

Related Discussions
Diffraction grating & young double slit -light not perpendicular to grating Introductory Physics Homework 1
How does a Reflection diffraction grating work? General Physics 10
Reflection grating Introductory Physics Homework 4
Interference using a reflection grating: how to determine the slit width? General Physics 0
Specular Reflection from a Reflection Diffraction Grating General Physics 1