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# How to Determine the Wavelength of your Laser with a CD!!

#### styropyro

##### New member
In celebration of pi day, I decided to go a bit more "mathy" with my newest video. Here I use a CD to determine the wavelength of a laser!

#### Alex2893

##### Active member
that light blue pen laser is like my dream laser. Its so pretty !!! I also just realized i need to go back to college. Thats probably easy math more most on here but its over my head. :cryyy:

Pretty amazing video none the less

#### BowtieGuy

##### Well-known member
LPF Site Supporter
This is probably a dumb question, but is the distance between slits on the particular CD that you are using a "standard" width?
In other words, would a different CD have the same distance between slits?

#### styropyro

##### New member
that light blue pen laser is like my dream laser. Its so pretty !!! I also just realized i need to go back to college. Thats probably easy math more most on here but its over my head. :cryyy:

Pretty amazing video none the less
That blue laser is definitely my all time favorite laser that I've owned...the actual wavelength is probably near 480nm considering the spectrometer tests that were done on this type of laser diode.

If you are ever wanting to brush up or learn more math, the internet is full of great resources! I have learned a lot from places like "Paul's Online Notes". Spending a couple hours here and there with a pencil, paper, and a good math resource can be very enlightening. :beer:

This is probably a dumb question, but is the distance between slits on the particular CD that you are using a "standard" width?
In other words, would a different CD have the same distance between slits?
I would assume this would be a standard thing, and according to some websites is 1.6 microns. If that's true, my measurement (~1.5 micron) was close, but definitely not perfect. Oh well, this experiment isn't meant to be perfect, just to show that cool science can be done with household items.

#### Mattronium

##### Active member
Good video!

I doubt you can ever do a DIY wavelength measurement with with greater certainty than +/- 1nm, even that would be very difficult.

Once I tried to measure the WL of all my lasers using a diffraction grating I measured to be 261.5 lines/mm. I had 4.159m from the grating to the wall.
Unfortunately, a shift of one mm for the measurement of the dot locations on the wall equaled a shift of ~0.9nm of the wavelength calculated and one mm shift in the distance from the grating to the wall equaled a shift of ~0.2nm. Since I doubt I could measure any better than +/- 2mm the MOST accurate I could say the wavelength for my 515nm laser that I tested would be, is 513.8nm +/- 2nm. And that's not considering that if the laser passes through a transmission diffraction grating at an angle the effective line spacing would change too.

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#### SteveT

##### Member
Another great vid; might have to give this a go later. Need to dust off my Pythagorus and trig skills before I do though, haven't used them since 1998

#### paul1598419

##### Well-known member
I've done it with a known diffraction grating, but never with a CD. That is a very interesting way to use common items to measure something that one would not normally think of. At least I haven't. Nice presentation of the set up and math. Kudos!

Styro, I went back and checked your math and found you have some typos in your notes. Just thought I'd mention it since it is on YouTube and people pick up on stuff like that. Still, very nice!

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#### styropyro

##### New member
Are you referring to the absence of the factorial sign for the mclauren series expansion of the sine function? If so, I saw that mistake while uploading it and I added an annotation to fix it.

#### paul1598419

##### Well-known member
Yeah. But, also, ( 1.4964 X 10^-6M ) Sin ( 18.5576 degrees ) = 4.76 X 10^-7 = 476nm.
No big deal.

I'm sorry. I made a mistake here, not you.

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#### CelticLaser1

##### New member
Does the distance from the cd to the wall change anything? Obviously if it was closer to the wall the dots would be closer together. Keep the videos coming. They're very informative.

#### styropyro

##### New member
This is where the trig comes in. Yes the dots would be closer together if the cd was closer to the wall, but the angle between the beams will be the same no matter the distance. You only need that angle to plug into the formula, so theoretically you could just use a protractor if you could somehow be precise enough.

##### Active member
OMG....look at all the burn marks on the wall behind you Styro!!! heheheh

Naawww...that's not a laser lab!

-G

#### Shakenawake

##### Well-known member
What is a CD?:crackup:

nice vid there