# Homework Help: Estimating IR wavelength

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1. Nov 6, 2014

### XXZZ

1. The problem statement, all variables and given/known data

Hi,
I have to do lab experiment - estimating infrared wavelength (from remote control). My experimental setup includes CD, remote control, webcam (without IR filter, so I can see the infrared radiation), sheet of paper (I will see diffracted light spots on it) with hole for infrared diode (opposite to the CD). I will obtain angle theta from distance between CD and diode (l) and between first order spot and hole (x). CD grooves spacing length (d) is known. So, I have no problem with theory behind this experiment.

2. Relevant equations

3. The attempt at a solution
But when I tried to do that, something went wrong - independently of length between CD and sheet of paper (from a few centimeters to approx. one meter), I couldn't see the diffracted light! I could only see shining IR diode but nothing else. What I did wrong? Previously, I did similar experiment with laser pointer and everything was OK.

2. Nov 6, 2014

### haruspex

I don't understand what equipment in your set-up is going to turn IR into something visible.

3. Nov 7, 2014

4. Nov 7, 2014

### collinsmark

I think you might have trouble using the CD to measure the diffraction or interference pattern of the LED. Unlike a laser, LEDs are not necessarily coherent light sources, and its that coherence that you want to produce the diffraction and/or interference patterns.

Not to mention that unlike a focused laser, the LED in the remote will shine in many directions -- not a focused beam -- so you won't be able to see the bright dot/pattern on a piece of paper. You'll have to somehow angle the CD such that the CD reflects the light -- similar to how a mirror reflects light -- from the LED, reflecting off of the CD, and into the webcam lens.

The CD will reflect different frequencies of light at different angles in a way similar to how a prism separates the colors of light. You can use that to your advantage.

You might start by setting up your equipment such that a white light shines through the hole in the paper, and the CD reflects that into the webcam. You should see a rainbow sort of image, taken by the webcam. [Edit: if you don't adjust the angle of the CD until a rainbow appears in the image.] You can use that as a reference: you know the wavelength of the different colors of visible light, and you can measure their respective angles via the image from the webcam.

Now here is the important thing. Without moving the piece of paper (with the hole in it) and without touching the position of the CD, replace the white light source with the IR LED source. (Again, do not change the position or orientation of the paper with the hole in it, and do not change the location or orientation of the CD! [or the position of the webcam!]) Then take the image using the webcam. In the image you should see some sort of bright spot. Compare the location of that bright spot to the previous image that contains the rainbow. With that, you should be able to obtain a rough estimate of the wavelength of the IR light. [Edit: by extrapolating.]

At least that's my guess on how you can do this experiment.

Last edited: Nov 7, 2014