Public Lab Research note

Troubleshooting: White light instead of emission spectra

by lionheadrex | June 22, 2014 21:32 22 Jun 21:32 | #10603 | #10603

What I want to do

Fix my spectrometer (I only see a white light instead of an emission spectra)

My attempt and results

Built it, observed no emission spectra and instead saw a bright white light; tried to block out light getting into the box, that did nothing; tried letting more light in the box (from the sides instead of the slit), strangely, that lets me see what looks like an emission spectra at an angle.

Questions and next steps

Why do I see a bright light? Based on a question from Liz Barry in a google groups post I'm guessing I messed up the DVD insertion step, but I'm not sure what the next step is in fixing it.

Why I'm interested

Because I want to fix my spectrometer.


You may notice in the image that my box looks like it can't completely close. It doesn't (whoops). It does get really close, and I wrap the fabric from a pair of heavy black jeans all the way around the box (except the opening where I want light to come in!), and it does seem to block all residual light.

More Pictures





The grating (DVD) bends the light as it separates it into colors. So the grating has to be at an angle to the light coming from the entrance slit. Note the ~45° angle of the web cam and DVD piece in the photo in step 5. So just rotate your webcam and it might work. The concentric grooves on the DVD should be perpendicular to the incoming light.

Reply to this comment...

Hmmm... when I do this, I can get green and blue lines running the right way - until I close my box. Then they're gone completely and still all I can see is the white light.

Reply to this comment...

This is your box:

|o> |=| |__||

Your pointing your camera directly at the double slit - you instead need to point it at the black of the wall (at a 45° angle as mentioned above) so its like this...

|O」..........|=| | * .| | |________||

This ^ - is where you point to get the rainbow. :)

Hope this ASCII doesn't screw up. Amy

Reply to this comment...

Hmm, can you upload a photo of the box with the new angled layout?

Is this a question? Click here to post it to the Questions page.

Reply to this comment...

Here you go. I've tried a variety of angles, and I can see some emmission spectra, but once I close the box it goes away and all I can see is dark or possibly the line of light moving in the box. I'm certain I'm doing something wrong, I just don't know what. :-(


Reply to this comment...

That looks like a good arrangement. The DVD grating might have to be rotated. The circular grooves in the DVD (concentric to the outer edge) should be vertical in that photo.

Reply to this comment...

THANK YOU!!! I'm terribly impressed that you could tell the circular grooves were horizontal. I'm going to have to tinker a little bit to make the spectra exactly linear where I want it, but I think this fixed it!

For anyone else looking for the solution - I basically I had two problems. One, my webcam was not angled properly. It shouldn't be looking directly at the light source but angled 45° from it. Two, my dvd needed to be rotated - it was 90° off.

Thanks to everyone for their help!

Reply to this comment...

Login to comment.