And yet, another rainbow

[colinbm]

I am sorry if I don't know what I am talking about, we didn't have anything like this when I was at school, I have been a tradesman at a few trades, but no other formal education & have only learnt about all this since being retired early injured & worn out.

[Stefano]

Colin, don't worry, I too learned this recently.

[Andy Perrin]

A better spectrum:

 

Please ignore the constantly-spaced "ripples", I don't know what causes them, they can be seen with the naked eye too.

 

Part of the diffraction pattern I think. See the graph at the bottom here. It shows similar small ripples:

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/gratint.html

[Stefano]

So the "rainbow" is caused by diffraction, and the ripples by interference. Sunlight is not coherent, but interference can be observed with incoherent light too, for example the colors of a soap bubble or the coatings on camera lenses we hate so much.

[Andy Perrin]

No to all that.

1) there is not much of a difference between diffraction and interference. Both caused by phase differences. It’s historical accident that we have two words for one phenomenon. Typically it’s “diffraction” if light is going past an edge or through a single slit, but “interference” if there is more than one slit.

 

2) incoherent vs coherent light is not a hard distinction. There is a coherence length that can be calculated and for features smaller than the coherence length, “incoherent” light will behave as coherent. In the case of the grating, the rulings are more closely spaced than the coherence length. Same with soap bubbles etc.

 

3) Actual rainbows (the kind on the sky) are caused by refraction, like with a prism, not diffraction/interference.

[Stefano]

I remember in high school when our physics teacher shined a red laser through some slits (they were more than one I think), and the pattern formed on the wall had a lot of closely-spaced peaks but with an intensity variation (like when you have a wave whose intensity is modulated by another wave with much longer wavelength), and he told us the little peaks were caused by interference and the bigger "intensity wave" was caused by diffraction. I hope I remembered correctly.

 

Yes, I called it a rainbow, but I know the difference. Prisms are also more linear I think, with diffraction gratings above a certain wavelength (equal to the spacing, in my case 1 µm) the angle distance is infinite.

[Andy Perrin]

You remember fine, but your teacher maybe didn’t understand that it’s all one phenomenon? You can calculate the whole pattern at once as a single equation. It’s not two separate phenomenon happening at once. Whether you want to call it “diffraction” for the central hump (which could be observed with a single slit) and interference for the smaller bumps is a language issue not a physics issue.

[Stefano]

Thanks Andy, yes it's always constructive/destructive interference, so actually the same thing.

[colinbm]

Here is where rainbows come from....

[Andrea B.]

Hey, Stefano, way to go, man ! You got the rainbows !

 

I have fun trying to shoot the rainbowshere in UV & Vis & IR. I have a knack for misplacing the camera so that I get the Vis and the IR, but the UV can't be recorded because most of it is behind a building or a tree or something.

 

*****

 

Isn't that Yosemite Falls rainbow the coolest thing? Gazillions of people show up there to try to capture that.