How do diffraction gratings work?

histronic

Hey guys,

I'm working on Spectrophotometry in chemistry right now and I don't understand how a diffraction grating works at all. I understand how a prism works, but not at all how a diffraction grating is able to separate light at different wavelengths. I also don't understand how orders of magnitude work.

Any information would be much appreciated because I really want to know how this stuff works, its really interesting.

Infidel

Physics isn't my main area but I know a bit. Do you understand wave theory at all? Constructive and destructive interference?

A way to describe what is happening (I'll try it) is that the grating divides the light source into what function like many individual point sources (along one axis, against the grating) that will mostly cause destructive interference and most points will cancel each other out on the target (other side of the grating.)

But at certain spots, the points will be in phase and add up together constructively, and you'll see the light visibly. The distance between these spots are directly related to the wavelength of the light, and so if you have multiple wavelengths in a light source you will separate them since where each wavelength will meet up with all of its parts will be different from the other wavelengths.

Mojo_Jojo

Diffraction gratings work on the principle that the degree of diffraction light experiences passing through a narrow slit is determined by its wavelength (and the slit dimensions). So different colours behave differently.

histronic wrote: »

I also don't understand how orders of magnitude work.

This scares me slightly, and I don't feel comfortable carrying on about actual science if you're missing such a basic block. Still, an order of magnitude is a pretty straight forward concept. We generally only use it to refer to things in base 10 (although you could scale things by other amounts and still be justified in using the term).

So, we have the number 1. An order of magnitude more, just means we multiply it by ten. So we have 10.

An order of magnitude less than 1, is 0.1.

Six orders of magnitude more than 1 is 1,000,000. A million.

Now, for numbers with lots of zeroes we use standard form, so
1 = 1 * 10^0
10 = 1 * 10^1
0.1 = 1 * 10^-1
1000000 = 1 * 10^6

That is literally all there is to it.

Edit: And as everybody else seems to be saying "I'm not a physicist." I'll qualify this with "I am a physicist".

Ogotai

I haven't looked at this materal in a while, I need to though for some of my own work, I think I remember some of the basics. I'm a anatomist, not a physicist, so I am better with application than the theory behind it.


The whole system works off the idea of light as a wave. You have one beam of light hitting the grating, that contains multiple wavelengths. Each slit in the grating acts a as a point source of a new beam of light. The light waves from each of these sources interact on the far side of the grating. When they do, based on the wavelengths and angles they meet at, the interaction can be disruptive or constructive. So if you know what your doing, you can figure out for a give wavelength of light, what angle that wavelength would under go constructive interference and there for show a spot of that wavelent on a surface at a given distance from the grating. Each differnt wavelength would have a different angle as the different wavelength changes the geometry of the situation, producing the same overall effect as a prism. Then if your building a spectrometer, you put a intensity sensor at the locations each wavelength would show a spot at a given distance.

edit: ah, beaten.

histronic

Mojo_Jojo wrote: »

Diffraction gratings work on the principle that the degree of diffraction light experiences passing through a narrow slit is determined by its wavelength (and the slit dimensions). So different colours behave differently.

histronic wrote: »

I also don't understand how orders of magnitude work.

This scares me slightly, and I don't feel comfortable carrying on about actual science if you're missing such a basic block. Still, an order of magnitude is a pretty straight forward concept. We generally only use it to refer to things in base 10 (although you could scale things by other amounts and still be justified in using the term).

So, we have the number 1. An order of magnitude more, just means we multiply it by ten. So we have 10.

An order of magnitude less than 1, is 0.1.

Six orders of magnitude more than 1 is 1,000,000. A million.

Now, for numbers with lots of zeroes we use standard form, so
1 = 1 * 10^0
10 = 1 * 10^1
0.1 = 1 * 10^-1
1000000 = 1 * 10^6

That is literally all there is to it.

Edit: And as everybody else seems to be saying "I'm not a physicist." I'll qualify this with "I am a physicist".

Sorry, I don't mean orders of magnitude like that, I mean orders of light magnitude. Like light with a wavelength of 900 nm has a 2nd order of magnitude at 450 nm, and a 3rd order of magnitude at 300 nm... and so on

Druhim

I assume you mean the magnitude of energy



But I don't really get how you don't get how orders of magnitude work in this sense, if that is what you're referring to. You don't understand how wavelength is related to energy?

Infidel

histronic wrote: »

Mojo_Jojo wrote: »

Diffraction gratings work on the principle that the degree of diffraction light experiences passing through a narrow slit is determined by its wavelength (and the slit dimensions). So different colours behave differently.

histronic wrote: »

I also don't understand how orders of magnitude work.

This scares me slightly, and I don't feel comfortable carrying on about actual science if you're missing such a basic block. Still, an order of magnitude is a pretty straight forward concept. We generally only use it to refer to things in base 10 (although you could scale things by other amounts and still be justified in using the term).

So, we have the number 1. An order of magnitude more, just means we multiply it by ten. So we have 10.

An order of magnitude less than 1, is 0.1.

Six orders of magnitude more than 1 is 1,000,000. A million.

Now, for numbers with lots of zeroes we use standard form, so
1 = 1 * 10^0
10 = 1 * 10^1
0.1 = 1 * 10^-1
1000000 = 1 * 10^6

That is literally all there is to it.

Edit: And as everybody else seems to be saying "I'm not a physicist." I'll qualify this with "I am a physicist".

Sorry, I don't mean orders of magnitude like that, I mean orders of light magnitude. Like light with a wavelength of 900 nm has a 2nd order of magnitude at 450 nm, and a 3rd order of magnitude at 300 nm... and so on

You are confusing the issue by using the word magnitude.

I think you are talking about first order, second order, etc.

First order 400nm has a second order at 800nm, etc.

The 400nm light will contaminate at the 800nm, as 400nm second order / 800nm first order overlap.

Again, this is because we're talking waves and they repeat, so you have to separate things that are in phase at certain points but have different periods.



This picture isn't entirely appropriate but my point is if you have a wavelength of say 300nm, you'll have constructive interference with the 900nm wave. As you go up in order, the intensity decreases, so this 300nm fourth order wave will "contaminate" in the 900nm and 450nm spectrum.

Plutonium

In layman's terms, what a diffraction grating does is "widen" the incoming light spectrum, allowing you to isolate a more pure wavelength of light through a slit of the same size as the entrance slit.

Here's a diagram of a simple monochromator consisting of an entrance slit, an exit slit, and a diffraction grating. Here we have a large wavelength band of light coming in through the entrance slit in red, and a smaller slice of that light exiting through the exit slit in green.



You can adjust the purity of your light by changing the size of your exit slit, and the wavelength that you're selecting for by changing the angle of the diffraction grating.