How to determine wavelength? 
Posted: 27 September 2011 01:41 AM   [ Ignore ]
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Hello,

This may sound silly, but I do not know the wavelength of the IR LEDs I’m using. The store I purchased them from are kinda hopeless, and even asking the voltage seemed to create a challenge. I could guess, but was wondering if there is a effective way of determining this.

I was thinking along the lines of incrementally layers floppy disk material over the camera, and perhaps the IR light will appear brighter with various layering setups. But I may be wrong wink (probably)

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Posted: 27 September 2011 02:27 AM   [ Ignore ]   [ # 1 ]
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If you can, try get access to an few different IR bandpass filters of various wavelengths such as 780 830 850 etc and keep trying them untill you get light. I’m not sure who could do that where you are. Maybe a specialist shop that deals with IR equipment

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Posted: 27 September 2011 02:31 AM   [ Ignore ]   [ # 2 ]
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Yeah, sounds like a plan. Was thinking that I should perhaps visit a company that sell security cameras, as those normally have Night Vision modes that use IR light. Perhaps they have some testing equipment. Thank you!

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Posted: 27 September 2011 01:40 PM   [ Ignore ]   [ # 3 ]
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Hm, I guess I do have uses of the Physics from secondary school

Well there is a way to calculate wavelength quite exactly by using something called a Double-slit.

There is a simple formula:

nλ=d∙sin(α_n)

Where d is the distance between the slits.
Where n is the index of the wave top.
Where λ is the wavelength of the light.
Where a is the angle to the n’th maxpoint of the light.

So some repositioning results in:
λ=(d∙sin(α_n))/n

To read more: Double-slit experiment

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Posted: 28 September 2011 12:46 AM   [ Ignore ]   [ # 4 ]
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@Misz! Thank you very much. If I can get this to work, I’ll be super stoked. smile

However, after having read that Wiki, I realise that it’s highly unlikely that I could replicate an experiment like this without special equipment. I do like that it exposes just a little bit of the physics we are using to make our MTs work. I’ve kinda taken that for granted up until now…

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Posted: 28 September 2011 01:59 AM   [ Ignore ]   [ # 5 ]
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I feel stupid, I’m studying physics, I have exams in a month I should know this!!! haha

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Posted: 28 September 2011 01:00 PM   [ Ignore ]   [ # 6 ]
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@Ryuman, Special Equipment is needed for high precision, like when you are doing quantum physics.

It should be sufficient enough for the need of MT.

There should be a page on the NUI Wiki, with all formulas that might be useful when doing MT.

@Auzeras
Which level are physics are you studying?
I learned the double slit stuff in the second course of physics in secondary school.

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Posted: 28 September 2011 01:46 PM   [ Ignore ]   [ # 7 ]
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NCEA Lvl 2, which is the NZ Secondary schools standard. So I guess thats about the same as 1st course. 5th form do LVL1 6th lvl2 and 7th lvl3

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Posted: 29 September 2011 12:15 AM   [ Ignore ]   [ # 8 ]
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@misz You’ve given me enough confidence that I might give it a shot. You say it’s pretty easy to setup? I’m not too worried about the Math, if I don’t succeed I’m surrounded by clever people wink

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Posted: 29 September 2011 02:57 AM   [ Ignore ]   [ # 9 ]
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Well what you need is a double slit. Which is a black piece of glass with two slits in it.

I have been using the technique with lasers, which have focused light, so I guess when using a LED you might need something to filter out the light from the LED that does not pass through the double slit. I recommend something black to absorb the light instead of reflecting it.

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Posted: 29 September 2011 08:05 AM   [ Ignore ]   [ # 10 ]
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I found this site about the diffraction of light, not sure if it’s the same experiment. Nice JAVA app though to visualize the experiment.

http://micro.magnet.fsu.edu/primer/java/scienceopticsu/diffraction/basicdiffraction/index.html

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Posted: 29 September 2011 08:58 AM   [ Ignore ]   [ # 11 ]
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Here is another classroom type experiment not really related to wavelength, but rather colour temperature. Very cool.

Herscel - Colour Temperature Variation Experiment

And this page better describes how to put the experiment together. (Youngs Experiment)

Neat test…

You can actually do the single slit experiment wherever you are right now! Hold two of your fingers very close together; there should be only the tiniest little gap between them that you can barely see through. Look towards a light source, light a light bulb, through the gap in your fingers. In the gap between your fingers you shold see very faint gray lines that run parallel to your fingers… these are the destructive interference “dark” fringes!

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Posted: 27 December 2011 10:19 PM   [ Ignore ]   [ # 12 ]
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You need a simple way, which is not precise but easy to apply.
1. power on your led.
2. observe
(1) if you or someone of your friends(depends) could observe a tight red ------ <850nm
(2)nothing, but red via your phone-carried-camera ------ 940nm
(3) nothing red, even with every cameras you could found ------ 1030nm or farther

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LI, Jinpeng wink

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Posted: 23 February 2012 01:41 PM   [ Ignore ]   [ # 13 ]
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Hey,
You can even measure the wavelength with a single slit, it gives you a similar interference pattern as in the double slit experiment. But I think, its easier create a single slit, with a measurable width, than two of them. It also gives you the better results, because in the double slit experiment you basicaly have two single slit interference patterns, that interfere again.

This experiment works quite well, if you can do it under good conditions. Do it in total darkness, and observe the interfernce pattern with a camera that senses IR. Try to make the slit as small as possible, the smaller the slit is, the better you can see the interference pattern. If the slit is too large, there will only be light bending on the edges. Unfortuantely a small slit reduces light intensity.

Try to choose the distance between slit and the projection surface as long as possible.

Tried this experiment last week, and it worked out pretty good.

Good Luck!

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